[cctalk] Re: Identifying a Failed Diode in a Rainbow H7842 Power Supply

[Peter Coghlan via cctalk]

>
> I think that a good thing is to ramp up the input voltage slowly. Use a
> bench PSU to supply the Vstart voltage and then use a variac with
> insulation transformer to feed the rest if the supply. Use some small
> loads on the 5V and 12V outputs. Now you can safely probe the PSU and
> monitor base and collector voltages of the main switch transistor and
> see that everything looks fine.  Check output voltages. Stay below 50VAC
> input and very little harm can be done. Could be good idea to have an
> amp-meter inline with input AC from the variac to find out if there is 
> short somewhere.
>

I'm sure I recall we had a discussion about this a some time ago and I
think we came to the conclusion that a variac is not a good tool for this
sort of work, mainly because it doesn't limit current.

A series light bulb bulb on the other hand will not allow the power supply
to draw more current than the maximum current drawn by the light bulb, even
if the power supply is a complete short circuit.  This avoids the need to
monitor an ammeter and react quickly to switch off if a high current is
noticed.  It also avoids having to aquire an AC ammeter which can accurately
measure the odd current waveforms likely to be drawn by a switch mode power
supply.

An isolation transformer can be added if it turns out to be necessary to
probe the primary side of the chopper transformer.  Usually this won't
be necessary as the light bulb will give a good indication of what is
happening there.  If the bulb lights at normal brilliance and the power
supply does nothing, there is likely to be a serious short on the input
side.  If the bulb barely glows and the power supply produces reasonable
output voltages into small loads, then all is probably well.  If the bulb
glows somewhat and one or more outputs are missing, then check the
associated output rectifiers and smoothing networks for shorts.  If the
bulb pulses, the power supply could be tripping in response to an overload
somewhere.  If the bulb doesn't light and the power supply does nothing,
either something is open circuit or the chopper control circuit is not
working.

I should probably add that the bulb needs to be a mains bulb.  A 100W
bulb will limit the current to less than half an Amp (in this part of
world anyway).

Remember to check that the reservoir capacitors for the rectified mains
have discharged before handling that part of the power supply.

Regards,
Peter Coghlan.

[cctalk] Re: Identifying a Failed Diode in a Rainbow H7842 Power Supply

[John Robertson via cctalk]

On 2022/11/27 1:21 a.m., Rob Jarratt via cctalk wrote:

I have done a little more probing around. I have found that the 7812 regulator 
that drives Vstart on sheet 1 of Tony Duell’s schematic is shorted, so I will 
have to replace this too. I have not found anything else that looks obviously 
suspicious. I can’t test the output rectifiers for shorts without desoldering 
them, which I would rather avoid. I guess the next step is to replace the 
broken parts and use the light bulb current-limiter method to power on the PSU.

Regards
Rob


My experience with blown 7812s is that there was a surge on the 
unregulated side that went over the maximum input rating for the device. 
You may want to add a Transient Suppression Diode on the input if this 
is a future possibility or a suitable line protection MOV on the input 
just after the line fuse.


John :-#)#

  


From: Rob Jarratt 
Sent: 24 November 2022 21:45
To: 'Mattis Lind' ; r...@jarratt.me.uk; 'General Discussion: 
On-Topic and Off-Topic Posts' 
Subject: RE: [cctalk] Identifying a Failed Diode in a Rainbow H7842 Power Supply

  


Thanks for the suggestion Mattis. The UF4007 has a PIV of 1000V, I had a 
suggestion that the PIV should be 200V. Not sure what rating I should be going 
for here?

  


Regards

  


Rob

  


From: Mattis Lind mailto:mattisl...@gmail.com> >
Sent: 22 November 2022 07:54
To: r...@jarratt.me.uk  ; General Discussion: On-Topic and 
Off-Topic Posts mailto:cctalk@classiccmp.org> >
Subject: Re: [cctalk] Identifying a Failed Diode in a Rainbow H7842 Power Supply

  

  


Hello Rob!

  



Given that before the transistor blew up there had clearly been another
failure somewhere else, I tried to find the original failure. There were no
obviously damaged parts, so I just probed around near the transistor for any
parts that were open circuit or short circuit. I found a diode connected to
the base of the transistor that appeared to be short circuit. So, I decided
to lift one end to check it. As I de-soldered one of the leads, the diode
broke in two. So clearly the diode was either damaged by the failure of the
transistor, or it was the cause of the failure. This is the diode:
https://rjarratt.files.wordpress.com/2022/11/img_20221120_165913.jpg.

  

  


DEC used a lot of A114x diodes in their PSUs. They looked exactly like that 
one. Those are fast recovery diodes. 
https://pdf2.alldatasheet.com/datasheet-pdf/view/7563180/2074/A114F.html

  


I would replace it with a UF4007 or something similar. 
https://www.mouser.se/datasheet/2/849/uf4001-2578577.pdf

  

  

  



I can't quite make out the markings on the diode to know what to replace it
with. I think it says "D610". Would that be the right designation? If so,
can anyone suggest a suitable replacement please?



The diode seems to connect an inductor to the base of the switching
transistor and the collector of the transistor is connected to a
transformer. Should I be looking for other failed parts? Not sure if the
diode failed first and then caused the transistor to fail? Or if something
else has failed which caused these parts to fail?

  

  


Also check all other semiconductors. Also on the outputs. If there is a 1 ohm 
fusible resistor in the base drive circuit check that one as well. In the VT100 
PSUs it happens that it blows.

  

  





I do know that there are no shorts in the Rainbow itself, because I have a
spare PSU that still works fine in the same machine.



I blogged this here (it repeats most of that I have said above):
https://robs-old-computers.com/2022/11/20/dec-rainbow-h7842-power-supply-fai 

lure/

  


/Mattis


Thanks



Rob



--
 John's Jukes Ltd.
7 - 3979 Marine Way, Burnaby, BC, Canada V5J 5E3
Call (604)872-5757 (Pinballs, Jukes, Video Games)
 flippers.com
 "Old pinballers never die, they just flip out"

[cctalk] Re: Identifying a Failed Diode in a Rainbow H7842 Power Supply

[Mattis Lind via cctalk]

> On 27 Nov 2022, at 10:21, Rob Jarratt  wrote:
> 
> 
> I have done a little more probing around. I have found that the 7812 
> regulator that drives Vstart on sheet 1 of Tony Duell’s schematic is shorted, 
> so I will have to replace this too. I have not found anything else that looks 
> obviously suspicious. I can’t test the output rectifiers for shorts without 
> desoldering them, which I would rather avoid. I guess the next step is to 
> replace the broken parts and use the light bulb current-limiter method to 
> power on the PSU.

I think that a good thing is to ramp up the input voltage slowly. Use a bench 
PSU to supply the Vstart voltage and then use a variac with insulation 
transformer to feed the rest if the supply. Use some small loads on the 5V and 
12V outputs. Now you can safely probe the PSU and monitor base and collector 
voltages of the main switch transistor and see that everything looks fine.  
Check output voltages. Stay below 50VAC input and very little harm can be done. 
Could be good idea to have an amp-meter inline with input AC from the variac to 
find out if there is short somewhere. 

If this works then try with full input. Remove any scope probes…


/Mattis

>  
> Regards
>  
> Rob
>  
> From: Rob Jarratt  
> Sent: 24 November 2022 21:45
> To: 'Mattis Lind' ; r...@jarratt.me.uk; 'General 
> Discussion: On-Topic and Off-Topic Posts' 
> Subject: RE: [cctalk] Identifying a Failed Diode in a Rainbow H7842 Power 
> Supply
>  
> Thanks for the suggestion Mattis. The UF4007 has a PIV of 1000V, I had a 
> suggestion that the PIV should be 200V. Not sure what rating I should be 
> going for here?
>  
> Regards
>  
> Rob
>  
> From: Mattis Lind  
> Sent: 22 November 2022 07:54
> To: r...@jarratt.me.uk; General Discussion: On-Topic and Off-Topic Posts 
> 
> Subject: Re: [cctalk] Identifying a Failed Diode in a Rainbow H7842 Power 
> Supply
>  
>  
> Hello Rob!
>  
> 
> Given that before the transistor blew up there had clearly been another
> failure somewhere else, I tried to find the original failure. There were no
> obviously damaged parts, so I just probed around near the transistor for any
> parts that were open circuit or short circuit. I found a diode connected to
> the base of the transistor that appeared to be short circuit. So, I decided
> to lift one end to check it. As I de-soldered one of the leads, the diode
> broke in two. So clearly the diode was either damaged by the failure of the
> transistor, or it was the cause of the failure. This is the diode:
> https://rjarratt.files.wordpress.com/2022/11/img_20221120_165913.jpg.
> 
>  
>  
> DEC used a lot of A114x diodes in their PSUs. They looked exactly like that 
> one. Those are fast recovery diodes. 
> https://pdf2.alldatasheet.com/datasheet-pdf/view/7563180/2074/A114F.html
>  
> I would replace it with a UF4007 or something similar. 
> https://www.mouser.se/datasheet/2/849/uf4001-2578577.pdf
>  
>  
>  
> 
> I can't quite make out the markings on the diode to know what to replace it
> with. I think it says "D610". Would that be the right designation? If so,
> can anyone suggest a suitable replacement please?
> 
> 
> 
> The diode seems to connect an inductor to the base of the switching
> transistor and the collector of the transistor is connected to a
> transformer. Should I be looking for other failed parts? Not sure if the
> diode failed first and then caused the transistor to fail? Or if something
> else has failed which caused these parts to fail?
>  
>  
> Also check all other semiconductors. Also on the outputs. If there is a 1 ohm 
> fusible resistor in the base drive circuit check that one as well. In the 
> VT100 PSUs it happens that it blows.
>  
>  
> 
> 
> 
> I do know that there are no shorts in the Rainbow itself, because I have a
> spare PSU that still works fine in the same machine.
> 
> 
> 
> I blogged this here (it repeats most of that I have said above):
> https://robs-old-computers.com/2022/11/20/dec-rainbow-h7842-power-supply-fai
> lure/
> 
>  
> /Mattis 
> 
> Thanks
> 
> 
> 
> Rob

[cctalk] Re: Identifying a Failed Diode in a Rainbow H7842 Power Supply

[Rob Jarratt via cctalk]

I have done a little more probing around. I have found that the 7812 regulator 
that drives Vstart on sheet 1 of Tony Duell’s schematic is shorted, so I will 
have to replace this too. I have not found anything else that looks obviously 
suspicious. I can’t test the output rectifiers for shorts without desoldering 
them, which I would rather avoid. I guess the next step is to replace the 
broken parts and use the light bulb current-limiter method to power on the PSU.

 

Regards

 

Rob

 

From: Rob Jarratt  
Sent: 24 November 2022 21:45
To: 'Mattis Lind' ; r...@jarratt.me.uk; 'General 
Discussion: On-Topic and Off-Topic Posts' 
Subject: RE: [cctalk] Identifying a Failed Diode in a Rainbow H7842 Power Supply

 

Thanks for the suggestion Mattis. The UF4007 has a PIV of 1000V, I had a 
suggestion that the PIV should be 200V. Not sure what rating I should be going 
for here?

 

Regards

 

Rob

 

From: Mattis Lind mailto:mattisl...@gmail.com> > 
Sent: 22 November 2022 07:54
To: r...@jarratt.me.uk  ; General Discussion: 
On-Topic and Off-Topic Posts mailto:cctalk@classiccmp.org> >
Subject: Re: [cctalk] Identifying a Failed Diode in a Rainbow H7842 Power Supply

 

 

Hello Rob!

 


Given that before the transistor blew up there had clearly been another
failure somewhere else, I tried to find the original failure. There were no
obviously damaged parts, so I just probed around near the transistor for any
parts that were open circuit or short circuit. I found a diode connected to
the base of the transistor that appeared to be short circuit. So, I decided
to lift one end to check it. As I de-soldered one of the leads, the diode
broke in two. So clearly the diode was either damaged by the failure of the
transistor, or it was the cause of the failure. This is the diode:
https://rjarratt.files.wordpress.com/2022/11/img_20221120_165913.jpg.

 

 

DEC used a lot of A114x diodes in their PSUs. They looked exactly like that 
one. Those are fast recovery diodes. 
https://pdf2.alldatasheet.com/datasheet-pdf/view/7563180/2074/A114F.html

 

I would replace it with a UF4007 or something similar. 
https://www.mouser.se/datasheet/2/849/uf4001-2578577.pdf

 

 

 


I can't quite make out the markings on the diode to know what to replace it
with. I think it says "D610". Would that be the right designation? If so,
can anyone suggest a suitable replacement please?



The diode seems to connect an inductor to the base of the switching
transistor and the collector of the transistor is connected to a
transformer. Should I be looking for other failed parts? Not sure if the
diode failed first and then caused the transistor to fail? Or if something
else has failed which caused these parts to fail?

 

 

Also check all other semiconductors. Also on the outputs. If there is a 1 ohm 
fusible resistor in the base drive circuit check that one as well. In the VT100 
PSUs it happens that it blows.

 

 




I do know that there are no shorts in the Rainbow itself, because I have a
spare PSU that still works fine in the same machine.



I blogged this here (it repeats most of that I have said above):
https://robs-old-computers.com/2022/11/20/dec-rainbow-h7842-power-supply-fai 

 
lure/

 

/Mattis 


Thanks



Rob

[cctalk] Re: Identifying a Failed Diode in a Rainbow H7842 Power Supply

[Rob Jarratt via cctalk]

> -Original Message-
> From: Will Cooke via cctalk 
> Sent: 26 November 2022 19:12
> To: Rob Jarratt via cctalk 
> Cc: wrco...@wrcooke.net
> Subject: [cctalk] Re: Identifying a Failed Diode in a Rainbow H7842 Power
> Supply
> 
> 
> I'm not at all familiar with either this circuit or any of the mentioned 
> diodes.
> However, I would point out that a diode's forward voltage drop varies with
> current through it.  Usually, the datasheet will list the "max" forward drop, 
> at
> the rated current and typically at the lowest rated temp (the drop decreases
> as temp rises.)  So it is entirely possible that your tester is putting a 
> very small
> current through a high-current diode and getting that 0.5V.   It might be
> useful to feed it something close to its rated current and measure the drop
> for a more accurate estimate.

Thanks, I didn't know this, there is still so much I have to learn! I had 
noticed that the datasheets say "max" forward voltage but wasn't sure of the 
significance.

> 
> Will

[cctalk] Re: Identifying a Failed Diode in a Rainbow H7842 Power Supply

[Will Cooke via cctalk]

> On 11/26/2022 12:47 PM CST Rob Jarratt via cctalk  
> wrote:
> 
> 
> Just trying to decide what to replace the failed diode with, and looking at 
> the UF400x series, as suggested by Mattis. It seems to me that as long as the 
> PIV is 200V or higher it should be fine from that point of view, the 
> switching speed is never higher than 70ns, while the original A114x (assuming 
> it *is* an A114x) has a switching speed of 200ns (possibly even 200us from 
> the datasheet).
> 
> However, I am wondering about the forward voltage drop. The datasheets 
> suggest that the A114x parts have a 1.3V forward voltage drop. I have a spare 
> H7842 that was working (until I messed it up today, another story), so I 
> tested the diode in that, its forward voltage appears to be 0.5V, using a 
> little tester I have. The UF400x have ratings of either 1.0V or 1.7V.
> 
> How sensitive is the circuit going to be to the forward voltage on the diode? 
> Given that the forward voltage of the suggested replacement is higher, would 
> it slow down the speed with which the transistor is switched off too much and 
> cause it to be overloaded and fail?
> 
> Thanks
> 
> Rob
> 
> 
I'm not at all familiar with either this circuit or any of the mentioned 
diodes.  However, I would point out that a diode's forward voltage drop varies 
with current through it.  Usually, the datasheet will list the "max" forward 
drop, at the rated current and typically at the lowest rated temp (the drop 
decreases as temp rises.)  So it is entirely possible that your tester is 
putting a very small current through a high-current diode and getting that 
0.5V.   It might be useful to feed it something close to its rated current and 
measure the drop for a more accurate estimate.

Will

[cctalk] Re: Identifying a Failed Diode in a Rainbow H7842 Power Supply

[Rob Jarratt via cctalk]

Just trying to decide what to replace the failed diode with, and looking at the 
UF400x series, as suggested by Mattis. It seems to me that as long as the PIV 
is 200V or higher it should be fine from that point of view, the switching 
speed is never higher than 70ns, while the original A114x (assuming it *is* an 
A114x) has a switching speed of 200ns (possibly even 200us from the datasheet).

However, I am wondering about the forward voltage drop. The datasheets suggest 
that the A114x parts have a 1.3V forward voltage drop. I have a spare H7842 
that was working (until I messed it up today, another story), so I tested the 
diode in that, its forward voltage appears to be 0.5V, using a little tester I 
have. The UF400x have ratings of either 1.0V or 1.7V.

How sensitive is the circuit going to be to the forward voltage on the diode? 
Given that the forward voltage of the suggested replacement is higher, would it 
slow down the speed with which the transistor is switched off too much and 
cause it to be overloaded and fail?

Thanks

Rob


> -Original Message-
> From: Peter Coghlan via cctalk 
> Sent: 25 November 2022 09:44
> To: General Discussion: On-Topic and Off-Topic Posts
> 
> Cc: Peter Coghlan 
> Subject: [cctalk] Re: Identifying a Failed Diode in a Rainbow H7842 Power
> Supply
> 
> It is often possible to infer the component ratings needed from the other
> components around them.  A component in the base circuit of a transistor is
> likely to experience lower currents and voltages than one in the collector
> circuit.
> 
> In this case, we can see from Tony's diagram that there is a 2.7 Ohm resistor
> in parallel with the diode.  Suppose it is a 1W resistor.  This means that the
> from P = I squared R, the average current the resistor is likely to pass is 
> less
> than 1A.  From Ohm's law, V/I = R, this means the average voltage across the
> resistor is likely to be no more than 2.7 Volts.
> 
> It is possible that the peak current / voltage involved could be higher than
> the average for short periods of time but we have plenty of margin for error
> here so we don't need to think about that too much.  A diode with a PIV of
> 200V should be fine here.
> 
> Regards,
> Peter Coghlan
> 
> >
> > Tony Duell has reverse engineered the following schematic.
> >
> >
> >
> > http://www.bitsavers.org/pdf/dec/rainbow/duell_schematics/psu.pdf
> >
> >
> >
> > I will go with the 1000V as you suggest anyway.
> >
> >
> >
> > Thanks
> >
> >
> >
> > Rob
> >
> >
> >
> > From: Mattis Lind 
> > Sent: 25 November 2022 07:12
> > To: r...@jarratt.me.uk
> > Cc: General Discussion: On-Topic and Off-Topic Posts
> > 
> > Subject: Re: [cctalk] Identifying a Failed Diode in a Rainbow H7842
> > Power Supply
> >
> >
> >
> >
> >
> >
> >
> >
> >
> > On 24 Nov 2022, at 22:45, Rob Jarratt  <mailto:robert.jarr...@ntlworld.com> > wrote:
> >
> > 
> >
> > Thanks for the suggestion Mattis. The UF4007 has a PIV of 1000V, I had
> > a suggestion that the PIV should be 200V. Not sure what rating I
> > should be going for here?
> >
> >
> >
> > Given that I didn’t have a schematic and this is on the primary side I
> > went for the recommendation of 1000V. 200V may a bit low on the
> > primary side depending on the application of the diode. On the primary
> > there can be sustained voltages up to 400V and peaks that go even
> > higher. Using a diode with higher PIV almost never affects the
> > operation as long as other parameters stay the same. In this case the
> > most important parameter is the trr. It has to be a fast recovery
> > diode. In this case the UF4007 is slightly slower than the UF4004. But
> > I doubt it has a big significance. Actually the
> > A114 is much slower. 200 ns.
> >
> >
> >
> > :Mattis
> >
> >
> >
> >
> >
> > Regards
> >
> >
> >
> > Rob
> >
> >
> >
> > From: Mattis Lind mailto:mattisl...@gmail.com>
> > >
> > Sent: 22 November 2022 07:54
> > To: r...@jarratt.me.uk <mailto:r...@jarratt.me.uk> ;
> > General Discussion: On-Topic and Off-Topic Posts
> > mailto:cctalk@classiccmp.org> >
> > Subject: Re: [cctalk] Identifying a Failed Diode in a Rainbow H7842
> > Power Supply
> >
> >
> >
> >
> >
> > Hello Rob!
> >
> >
> >
> >
> > Given that before the transistor blew up there had clearly been
> > another failure somewhere else, I tried to 

[cctalk] Re: Identifying a Failed Diode in a Rainbow H7842 Power Supply

[Peter Coghlan via cctalk]

It is often possible to infer the component ratings needed from the other
components around them.  A component in the base circuit of a transistor is
likely to experience lower currents and voltages than one in the collector
circuit.

In this case, we can see from Tony's diagram that there is a 2.7 Ohm resistor
in parallel with the diode.  Suppose it is a 1W resistor.  This means that
the from P = I squared R, the average current the resistor is likely to pass
is less than 1A.  From Ohm's law, V/I = R, this means the average voltage
across the resistor is likely to be no more than 2.7 Volts.

It is possible that the peak current / voltage involved could be higher than
the average for short periods of time but we have plenty of margin for error
here so we don't need to think about that too much.  A diode with a PIV of
200V should be fine here.

Regards,
Peter Coghlan

>
> Tony Duell has reverse engineered the following schematic.
> 
>  
> 
> http://www.bitsavers.org/pdf/dec/rainbow/duell_schematics/psu.pdf
> 
>  
> 
> I will go with the 1000V as you suggest anyway.
> 
> 
>
> Thanks
>
> 
>
> Rob
>
> 
>
> From: Mattis Lind  
> Sent: 25 November 2022 07:12
> To: r...@jarratt.me.uk
> Cc: General Discussion: On-Topic and Off-Topic Posts 
> Subject: Re: [cctalk] Identifying a Failed Diode in a Rainbow H7842 Power 
> Supply
> 
>  
> 
>  
>
> 
> 
>
> 
> On 24 Nov 2022, at 22:45, Rob Jarratt   > wrote:
> 
> 
> 
> Thanks for the suggestion Mattis. The UF4007 has a PIV of 1000V, I had a
> suggestion that the PIV should be 200V. Not sure what rating I should be
> going for here?
> 
>  
>
> Given that I didn’t have a schematic and this is on the primary side I went
> for the recommendation of 1000V. 200V may a bit low on the primary side
> depending on the application of the diode. On the primary there can be
> sustained voltages up to 400V and peaks that go even higher. Using a diode
> with higher PIV almost never affects the operation as long as other
> parameters stay the same. In this case the most important parameter is the
> trr. It has to be a fast recovery diode. In this case the UF4007 is slightly
> slower than the UF4004. But I doubt it has a big significance. Actually the
> A114 is much slower. 200 ns. 
> 
>  
> 
> :Mattis
> 
> 
> 
>  
> 
> Regards
> 
>  
> 
> Rob
> 
>  
> 
> From: Mattis Lind mailto:mattisl...@gmail.com> > 
> Sent: 22 November 2022 07:54
> To: r...@jarratt.me.uk  ;
> General Discussion: On-Topic and Off-Topic Posts   >
> Subject: Re: [cctalk] Identifying a Failed Diode in a Rainbow H7842 Power 
> Supply
> 
>  
> 
>  
> 
> Hello Rob!
> 
>  
> 
> 
> Given that before the transistor blew up there had clearly been another
> failure somewhere else, I tried to find the original failure. There were no
> obviously damaged parts, so I just probed around near the transistor for any
> parts that were open circuit or short circuit. I found a diode connected to
> the base of the transistor that appeared to be short circuit. So, I decided
> to lift one end to check it. As I de-soldered one of the leads, the diode
> broke in two. So clearly the diode was either damaged by the failure of the
> transistor, or it was the cause of the failure. This is the diode:
> https://rjarratt.files.wordpress.com/2022/11/img_20221120_165913.jpg.
> 
> 
> 
> 
>  
> 
>  
> 
> DEC used a lot of A114x diodes in their PSUs. They looked exactly like that
> one. Those are fast recovery diodes.
> https://pdf2.alldatasheet.com/datasheet-pdf/view/7563180/2074/A114F.html
> 
>  
> 
> I would replace it with a UF4007 or something similar.
> https://www.mouser.se/datasheet/2/849/uf4001-2578577.pdf
> 
>  
> 
>  
> 
>  
> 
> 
> I can't quite make out the markings on the diode to know what to replace it
> with. I think it says "D610". Would that be the right designation? If so,
> can anyone suggest a suitable replacement please?
> 
> 
> 
> The diode seems to connect an inductor to the base of the switching
> transistor and the collector of the transistor is connected to a
> transformer. Should I be looking for other failed parts? Not sure if the
> diode failed first and then caused the transistor to fail? Or if something
> else has failed which caused these parts to fail?
> 
>  
> 
>  
> 
> Also check all other semiconductors. Also on the outputs. If there is a 1 ohm
> fusible resistor in the base drive circuit check that one as well. In the
> VT100 PSUs it happens that it blows.
> 
>  
> 
>  
> 
> 
> 
> 
> I do know that there are no shorts in the Rainbow itself, because I have a
> spare PSU that still works fine in the same machine.
> 
> 
> 
> I blogged this here (it repeats most of that I have said above):
> https://robs-old-computers.com/2022/11/20/dec-rainbow-h7842-power-supply-failure/
> 
> 
> 
> 
> 
>  
> 
> /Mattis 
> 
> 
> Thanks
> 
> 
> 
> Rob
> 

[cctalk] Re: Identifying a Failed Diode in a Rainbow H7842 Power Supply

[Rob Jarratt via cctalk]

Tony Duell has reverse engineered the following schematic.

 

http://www.bitsavers.org/pdf/dec/rainbow/duell_schematics/psu.pdf

 

I will go with the 1000V as you suggest anyway.

 

Thanks

 

Rob

 

From: Mattis Lind  
Sent: 25 November 2022 07:12
To: r...@jarratt.me.uk
Cc: General Discussion: On-Topic and Off-Topic Posts 
Subject: Re: [cctalk] Identifying a Failed Diode in a Rainbow H7842 Power Supply

 

 





On 24 Nov 2022, at 22:45, Rob Jarratt mailto:robert.jarr...@ntlworld.com> > wrote:



Thanks for the suggestion Mattis. The UF4007 has a PIV of 1000V, I had a 
suggestion that the PIV should be 200V. Not sure what rating I should be going 
for here?

 

Given that I didn’t have a schematic and this is on the primary side I went for 
the recommendation of 1000V. 200V may a bit low on the primary side depending 
on the application of the diode. On the primary there can be sustained voltages 
up to 400V and peaks that go even higher. Using a diode with higher PIV almost 
never affects the operation as long as other parameters stay the same. In this 
case the most important parameter is the trr. It has to be a fast recovery 
diode. In this case the UF4007 is slightly slower than the UF4004. But I doubt 
it has a big significance. Actually the A114 is much slower. 200 ns. 

 

:Mattis



 

Regards

 

Rob

 

From: Mattis Lind mailto:mattisl...@gmail.com> > 
Sent: 22 November 2022 07:54
To: r...@jarratt.me.uk  ; General Discussion: 
On-Topic and Off-Topic Posts mailto:cctalk@classiccmp.org> >
Subject: Re: [cctalk] Identifying a Failed Diode in a Rainbow H7842 Power Supply

 

 

Hello Rob!

 


Given that before the transistor blew up there had clearly been another
failure somewhere else, I tried to find the original failure. There were no
obviously damaged parts, so I just probed around near the transistor for any
parts that were open circuit or short circuit. I found a diode connected to
the base of the transistor that appeared to be short circuit. So, I decided
to lift one end to check it. As I de-soldered one of the leads, the diode
broke in two. So clearly the diode was either damaged by the failure of the
transistor, or it was the cause of the failure. This is the diode:
https://rjarratt.files.wordpress.com/2022/11/img_20221120_165913.jpg.




 

 

DEC used a lot of A114x diodes in their PSUs. They looked exactly like that 
one. Those are fast recovery diodes. 
https://pdf2.alldatasheet.com/datasheet-pdf/view/7563180/2074/A114F.html

 

I would replace it with a UF4007 or something similar. 
https://www.mouser.se/datasheet/2/849/uf4001-2578577.pdf

 

 

 


I can't quite make out the markings on the diode to know what to replace it
with. I think it says "D610". Would that be the right designation? If so,
can anyone suggest a suitable replacement please?



The diode seems to connect an inductor to the base of the switching
transistor and the collector of the transistor is connected to a
transformer. Should I be looking for other failed parts? Not sure if the
diode failed first and then caused the transistor to fail? Or if something
else has failed which caused these parts to fail?

 

 

Also check all other semiconductors. Also on the outputs. If there is a 1 ohm 
fusible resistor in the base drive circuit check that one as well. In the VT100 
PSUs it happens that it blows.

 

 




I do know that there are no shorts in the Rainbow itself, because I have a
spare PSU that still works fine in the same machine.



I blogged this here (it repeats most of that I have said above):
https://robs-old-computers.com/2022/11/20/dec-rainbow-h7842-power-supply-fai 

 
lure/




 

/Mattis 


Thanks



Rob

[cctalk] Re: Identifying a Failed Diode in a Rainbow H7842 Power Supply

[Mattis Lind via cctalk]

> On 24 Nov 2022, at 22:45, Rob Jarratt  wrote:
> 
> 
> Thanks for the suggestion Mattis. The UF4007 has a PIV of 1000V, I had a 
> suggestion that the PIV should be 200V. Not sure what rating I should be 
> going for here?

Given that I didn’t have a schematic and this is on the primary side I went for 
the recommendation of 1000V. 200V may a bit low on the primary side depending 
on the application of the diode. On the primary there can be sustained voltages 
up to 400V and peaks that go even higher. Using a diode with higher PIV almost 
never affects the operation as long as other parameters stay the same. In this 
case the most important parameter is the trr. It has to be a fast recovery 
diode. In this case the UF4007 is slightly slower than the UF4004. But I doubt 
it has a big significance. Actually the A114 is much slower. 200 ns. 

:Mattis
>  
> Regards
>  
> Rob
>  
> From: Mattis Lind  
> Sent: 22 November 2022 07:54
> To: r...@jarratt.me.uk; General Discussion: On-Topic and Off-Topic Posts 
> 
> Subject: Re: [cctalk] Identifying a Failed Diode in a Rainbow H7842 Power 
> Supply
>  
>  
> Hello Rob!
>  
> 
> Given that before the transistor blew up there had clearly been another
> failure somewhere else, I tried to find the original failure. There were no
> obviously damaged parts, so I just probed around near the transistor for any
> parts that were open circuit or short circuit. I found a diode connected to
> the base of the transistor that appeared to be short circuit. So, I decided
> to lift one end to check it. As I de-soldered one of the leads, the diode
> broke in two. So clearly the diode was either damaged by the failure of the
> transistor, or it was the cause of the failure. This is the diode:
> https://rjarratt.files.wordpress.com/2022/11/img_20221120_165913.jpg.
> 
> 
>  
>  
> DEC used a lot of A114x diodes in their PSUs. They looked exactly like that 
> one. Those are fast recovery diodes. 
> https://pdf2.alldatasheet.com/datasheet-pdf/view/7563180/2074/A114F.html
>  
> I would replace it with a UF4007 or something similar. 
> https://www.mouser.se/datasheet/2/849/uf4001-2578577.pdf
>  
>  
>  
> 
> I can't quite make out the markings on the diode to know what to replace it
> with. I think it says "D610". Would that be the right designation? If so,
> can anyone suggest a suitable replacement please?
> 
> 
> 
> The diode seems to connect an inductor to the base of the switching
> transistor and the collector of the transistor is connected to a
> transformer. Should I be looking for other failed parts? Not sure if the
> diode failed first and then caused the transistor to fail? Or if something
> else has failed which caused these parts to fail?
>  
>  
> Also check all other semiconductors. Also on the outputs. If there is a 1 ohm 
> fusible resistor in the base drive circuit check that one as well. In the 
> VT100 PSUs it happens that it blows.
>  
>  
> 
> 
> 
> I do know that there are no shorts in the Rainbow itself, because I have a
> spare PSU that still works fine in the same machine.
> 
> 
> 
> I blogged this here (it repeats most of that I have said above):
> https://robs-old-computers.com/2022/11/20/dec-rainbow-h7842-power-supply-fai
> lure/
> 
> 
>  
> /Mattis 
> 
> Thanks
> 
> 
> 
> Rob

[cctalk] Re: Identifying a Failed Diode in a Rainbow H7842 Power Supply

[Don R via cctalk]

You’re right,  I superimposed the number.

That certainly doesn’t help at all.

Don Resor

Sent from someone's iPhone

> On Nov 23, 2022, at 10:50 PM, Don R via cctalk  wrote:
> 
> The data sheet lists H5892 through H5899 and H5900 through H5911.
> 
> Don Resor
> 
> Sent from someone's iPhone
> 
>> On Nov 23, 2022, at 4:32 PM, Peter Coghlan via cctalk 
>>  wrote:
>> 
>> Don,
>> 
>> Does the url suggest you may have searched for a H5901 rather than a H9501?
>> 
>> Regards,
>> Peter Coghlan.
>> 
>>> 
>>> NTE seems to turn up these possibilities for a H9501.
>>> 
>>> https://www.nteinc.com/search/search/search.php?ss360Query=H5901
>>> 
>>> Don Resor
>>> 
>>> Sent from someone's iPhone
>>> 
> On Nov 23, 2022, at 12:04 PM, Antonio Carlini via cctalk 
>  wrote:
 
 On 21/11/2022 21:45, Antonio Carlini wrote:
> 
> I have two more I can open up and look at, but I cannot get to them 
> tonight and I'm probably out tomorrow too. But I think I can get to those 
> other two supplies on Wednesday. Hopefully at least one of them will be 
> readable! Otherwise I can desolder the diode from one of the other two 
> and hopefully find a useful marking.
 
 
 Turns out I have three more PSUs ... and the diode markings are 
 unfortunately invisible on all but one. That one is this one:
 
 https://drive.google.com/file/d/1TeGTcBBv7KecMJ2CmNcR0O-tb3jZigy6/view?usp=share_link
 
 
 It's not really visible there either but with the PSU out and one end 
 desoldered I can see that it is marked H9501. I can also see that it 
 doesn't conduct either way, which might mean that this PSU is the 
 non-working one I know I have. Obviously the capacitor (820uF 250V 
 electrolytic) is going to need replacing (might as well do both). But 
 first I need to remove them and see what (if anything has happened) 
 underneath.
 
 
 This now goes back into my queue (behind the MicroVAXes and the H7868B PSU 
 modules) so if you fix yours before I get to mine, please let me know what 
 you did :-)
 
 
 Antonio
 
 
 -- 
 Antonio Carlini
 anto...@acarlini.com
 
>> 
>> 

[cctalk] Re: Identifying a Failed Diode in a Rainbow H7842 Power Supply

[Rob Jarratt via cctalk]

Thanks for the suggestion Mattis. The UF4007 has a PIV of 1000V, I had a 
suggestion that the PIV should be 200V. Not sure what rating I should be going 
for here?

 

Regards

 

Rob

 

From: Mattis Lind  
Sent: 22 November 2022 07:54
To: r...@jarratt.me.uk; General Discussion: On-Topic and Off-Topic Posts 

Subject: Re: [cctalk] Identifying a Failed Diode in a Rainbow H7842 Power Supply

 

 

Hello Rob!

 


Given that before the transistor blew up there had clearly been another
failure somewhere else, I tried to find the original failure. There were no
obviously damaged parts, so I just probed around near the transistor for any
parts that were open circuit or short circuit. I found a diode connected to
the base of the transistor that appeared to be short circuit. So, I decided
to lift one end to check it. As I de-soldered one of the leads, the diode
broke in two. So clearly the diode was either damaged by the failure of the
transistor, or it was the cause of the failure. This is the diode:
https://rjarratt.files.wordpress.com/2022/11/img_20221120_165913.jpg.



 

 

DEC used a lot of A114x diodes in their PSUs. They looked exactly like that 
one. Those are fast recovery diodes. 
https://pdf2.alldatasheet.com/datasheet-pdf/view/7563180/2074/A114F.html

 

I would replace it with a UF4007 or something similar. 
https://www.mouser.se/datasheet/2/849/uf4001-2578577.pdf

 

 

 


I can't quite make out the markings on the diode to know what to replace it
with. I think it says "D610". Would that be the right designation? If so,
can anyone suggest a suitable replacement please?



The diode seems to connect an inductor to the base of the switching
transistor and the collector of the transistor is connected to a
transformer. Should I be looking for other failed parts? Not sure if the
diode failed first and then caused the transistor to fail? Or if something
else has failed which caused these parts to fail?

 

 

Also check all other semiconductors. Also on the outputs. If there is a 1 ohm 
fusible resistor in the base drive circuit check that one as well. In the VT100 
PSUs it happens that it blows.

 

 




I do know that there are no shorts in the Rainbow itself, because I have a
spare PSU that still works fine in the same machine.



I blogged this here (it repeats most of that I have said above):
https://robs-old-computers.com/2022/11/20/dec-rainbow-h7842-power-supply-fai 

 
lure/



 

/Mattis 


Thanks



Rob

[cctalk] Re: Identifying a Failed Diode in a Rainbow H7842 Power Supply

[Don R via cctalk]

The data sheet lists H5892 through H5899 and H5900 through H5911.

Don Resor

Sent from someone's iPhone

> On Nov 23, 2022, at 4:32 PM, Peter Coghlan via cctalk  
> wrote:
> 
> Don,
> 
> Does the url suggest you may have searched for a H5901 rather than a H9501?
> 
> Regards,
> Peter Coghlan.
> 
>> 
>> NTE seems to turn up these possibilities for a H9501.
>> 
>> https://www.nteinc.com/search/search/search.php?ss360Query=H5901
>> 
>> Don Resor
>> 
>> Sent from someone's iPhone
>> 
 On Nov 23, 2022, at 12:04 PM, Antonio Carlini via cctalk 
  wrote:
>>> 
>>> On 21/11/2022 21:45, Antonio Carlini wrote:
 
 I have two more I can open up and look at, but I cannot get to them 
 tonight and I'm probably out tomorrow too. But I think I can get to those 
 other two supplies on Wednesday. Hopefully at least one of them will be 
 readable! Otherwise I can desolder the diode from one of the other two and 
 hopefully find a useful marking.
>>> 
>>> 
>>> Turns out I have three more PSUs ... and the diode markings are 
>>> unfortunately invisible on all but one. That one is this one:
>>> 
>>> https://drive.google.com/file/d/1TeGTcBBv7KecMJ2CmNcR0O-tb3jZigy6/view?usp=share_link
>>> 
>>> 
>>> It's not really visible there either but with the PSU out and one end 
>>> desoldered I can see that it is marked H9501. I can also see that it 
>>> doesn't conduct either way, which might mean that this PSU is the 
>>> non-working one I know I have. Obviously the capacitor (820uF 250V 
>>> electrolytic) is going to need replacing (might as well do both). But first 
>>> I need to remove them and see what (if anything has happened) underneath.
>>> 
>>> 
>>> This now goes back into my queue (behind the MicroVAXes and the H7868B PSU 
>>> modules) so if you fix yours before I get to mine, please let me know what 
>>> you did :-)
>>> 
>>> 
>>> Antonio
>>> 
>>> 
>>> -- 
>>> Antonio Carlini
>>> anto...@acarlini.com
>>> 
> 
> 

[cctalk] Re: Identifying a Failed Diode in a Rainbow H7842 Power Supply

[Peter Coghlan via cctalk]

Don,

Does the url suggest you may have searched for a H5901 rather than a H9501?

Regards,
Peter Coghlan.

>
> NTE seems to turn up these possibilities for a H9501.
> 
> https://www.nteinc.com/search/search/search.php?ss360Query=H5901
> 
> Don Resor
> 
> Sent from someone's iPhone
> 
>> On Nov 23, 2022, at 12:04 PM, Antonio Carlini via cctalk 
>>  wrote:
>> 
>> On 21/11/2022 21:45, Antonio Carlini wrote:
>>> 
>>> I have two more I can open up and look at, but I cannot get to them tonight 
>>> and I'm probably out tomorrow too. But I think I can get to those other two 
>>> supplies on Wednesday. Hopefully at least one of them will be readable! 
>>> Otherwise I can desolder the diode from one of the other two and hopefully 
>>> find a useful marking.
>> 
>> 
>> Turns out I have three more PSUs ... and the diode markings are 
>> unfortunately invisible on all but one. That one is this one:
>> 
>> https://drive.google.com/file/d/1TeGTcBBv7KecMJ2CmNcR0O-tb3jZigy6/view?usp=share_link
>> 
>> 
>> It's not really visible there either but with the PSU out and one end 
>> desoldered I can see that it is marked H9501. I can also see that it doesn't 
>> conduct either way, which might mean that this PSU is the non-working one I 
>> know I have. Obviously the capacitor (820uF 250V electrolytic) is going to 
>> need replacing (might as well do both). But first I need to remove them and 
>> see what (if anything has happened) underneath.
>> 
>> 
>> This now goes back into my queue (behind the MicroVAXes and the H7868B PSU 
>> modules) so if you fix yours before I get to mine, please let me know what 
>> you did :-)
>> 
>> 
>> Antonio
>> 
>> 
>> -- 
>> Antonio Carlini
>> anto...@acarlini.com
>> 

[cctalk] Re: Identifying a Failed Diode in a Rainbow H7842 Power Supply

[Don R via cctalk]

This data sheet may prove to be more useful.

https://www.nteinc.com/specs/5800to5899/pdf/nte5892_99.pdf

Don Resor

Sent from someone's iPhone

> On Nov 23, 2022, at 1:00 PM, Don R  wrote:
> 
> NTE seems to turn up these possibilities for a H9501.
> 
> https://www.nteinc.com/search/search/search.php?ss360Query=H5901
> 
> Don Resor
> 
> Sent from someone's iPhone
> 
>>> On Nov 23, 2022, at 12:04 PM, Antonio Carlini via cctalk 
>>>  wrote:
>>> 
>> On 21/11/2022 21:45, Antonio Carlini wrote:
>>> 
>>> I have two more I can open up and look at, but I cannot get to them tonight 
>>> and I'm probably out tomorrow too. But I think I can get to those other two 
>>> supplies on Wednesday. Hopefully at least one of them will be readable! 
>>> Otherwise I can desolder the diode from one of the other two and hopefully 
>>> find a useful marking.
>> 
>> 
>> Turns out I have three more PSUs ... and the diode markings are 
>> unfortunately invisible on all but one. That one is this one:
>> 
>> https://drive.google.com/file/d/1TeGTcBBv7KecMJ2CmNcR0O-tb3jZigy6/view?usp=share_link
>> 
>> 
>> It's not really visible there either but with the PSU out and one end 
>> desoldered I can see that it is marked H9501. I can also see that it doesn't 
>> conduct either way, which might mean that this PSU is the non-working one I 
>> know I have. Obviously the capacitor (820uF 250V electrolytic) is going to 
>> need replacing (might as well do both). But first I need to remove them and 
>> see what (if anything has happened) underneath.
>> 
>> 
>> This now goes back into my queue (behind the MicroVAXes and the H7868B PSU 
>> modules) so if you fix yours before I get to mine, please let me know what 
>> you did :-)
>> 
>> 
>> Antonio
>> 
>> 
>> -- 
>> Antonio Carlini
>> anto...@acarlini.com
>> 

[cctalk] Re: Identifying a Failed Diode in a Rainbow H7842 Power Supply

[Don R via cctalk]

NTE seems to turn up these possibilities for a H9501.

https://www.nteinc.com/search/search/search.php?ss360Query=H5901

Don Resor

Sent from someone's iPhone

> On Nov 23, 2022, at 12:04 PM, Antonio Carlini via cctalk 
>  wrote:
> 
> On 21/11/2022 21:45, Antonio Carlini wrote:
>> 
>> I have two more I can open up and look at, but I cannot get to them tonight 
>> and I'm probably out tomorrow too. But I think I can get to those other two 
>> supplies on Wednesday. Hopefully at least one of them will be readable! 
>> Otherwise I can desolder the diode from one of the other two and hopefully 
>> find a useful marking.
> 
> 
> Turns out I have three more PSUs ... and the diode markings are unfortunately 
> invisible on all but one. That one is this one:
> 
> https://drive.google.com/file/d/1TeGTcBBv7KecMJ2CmNcR0O-tb3jZigy6/view?usp=share_link
> 
> 
> It's not really visible there either but with the PSU out and one end 
> desoldered I can see that it is marked H9501. I can also see that it doesn't 
> conduct either way, which might mean that this PSU is the non-working one I 
> know I have. Obviously the capacitor (820uF 250V electrolytic) is going to 
> need replacing (might as well do both). But first I need to remove them and 
> see what (if anything has happened) underneath.
> 
> 
> This now goes back into my queue (behind the MicroVAXes and the H7868B PSU 
> modules) so if you fix yours before I get to mine, please let me know what 
> you did :-)
> 
> 
> Antonio
> 
> 
> -- 
> Antonio Carlini
> anto...@acarlini.com
> 

[cctalk] Re: Identifying a Failed Diode in a Rainbow H7842 Power Supply

[Antonio Carlini via cctalk]

On 21/11/2022 21:45, Antonio Carlini wrote:


I have two more I can open up and look at, but I cannot get to them 
tonight and I'm probably out tomorrow too. But I think I can get to 
those other two supplies on Wednesday. Hopefully at least one of them 
will be readable! Otherwise I can desolder the diode from one of the 
other two and hopefully find a useful marking.



Turns out I have three more PSUs ... and the diode markings are 
unfortunately invisible on all but one. That one is this one:


https://drive.google.com/file/d/1TeGTcBBv7KecMJ2CmNcR0O-tb3jZigy6/view?usp=share_link


It's not really visible there either but with the PSU out and one end 
desoldered I can see that it is marked H9501. I can also see that it 
doesn't conduct either way, which might mean that this PSU is the 
non-working one I know I have. Obviously the capacitor (820uF 250V 
electrolytic) is going to need replacing (might as well do both). But 
first I need to remove them and see what (if anything has happened) 
underneath.



This now goes back into my queue (behind the MicroVAXes and the H7868B 
PSU modules) so if you fix yours before I get to mine, please let me 
know what you did :-)



Antonio


--
Antonio Carlini
anto...@acarlini.com

[cctalk] Re: Identifying a Failed Diode in a Rainbow H7842 Power Supply

[Matt Burke via cctalk]

On 20/11/2022 21:03, Rob Jarratt via cctalk wrote:
> The location of the diode is arrowed on this picture: 
> https://rjarratt.files.wordpress.com/2022/11/img_20221120_205802-arrowed.jpg
>
> You can also see the heatsink where the transistor used to be.

The component you refer to as an inductor is actually a transformer,
providing isolation between the control PCB (riser card) and the
switching transistor. As such the control circuitry should not have been
harmed by the failure of the transistor.

You can test the control circuitry by applying about 15V from a bench
supply to the purple capacitor shown in the picture. This is the 2200uF
capacitor shown on "PSU sheet 1" of Tony Duell's schematics (available
on Bitsavers). You will then be able to see the switching drive signal
on the left hand pin of the control PCB.

On "PSU sheet 2" I'm assuming that the diode in question is the one in
parallel with the 2.7R resistor. My guess is that it is there to provide
fast turn-off of the switching transistor. If it were to fail then that
would lead to slower turn-off and probably overheating of the
transistor. That may explain the failure. Other components to check are
the ones in the snubber circuit. This is the two 500R resistors and
associated diode and capacitor connected to the collector.

Matt

[cctalk] Re: Identifying a Failed Diode in a Rainbow H7842 Power Supply

[Mattis Lind via cctalk]

Hello Rob!


>
> Given that before the transistor blew up there had clearly been another
> failure somewhere else, I tried to find the original failure. There were no
> obviously damaged parts, so I just probed around near the transistor for
> any
> parts that were open circuit or short circuit. I found a diode connected to
> the base of the transistor that appeared to be short circuit. So, I decided
> to lift one end to check it. As I de-soldered one of the leads, the diode
> broke in two. So clearly the diode was either damaged by the failure of the
> transistor, or it was the cause of the failure. This is the diode:
> https://rjarratt.files.wordpress.com/2022/11/img_20221120_165913.jpg.
>
>
>

DEC used a lot of A114x diodes in their PSUs. They looked exactly like that
one. Those are fast recovery diodes.
https://pdf2.alldatasheet.com/datasheet-pdf/view/7563180/2074/A114F.html

I would replace it with a UF4007 or something similar.
https://www.mouser.se/datasheet/2/849/uf4001-2578577.pdf




>
> I can't quite make out the markings on the diode to know what to replace it
> with. I think it says "D610". Would that be the right designation? If so,
> can anyone suggest a suitable replacement please?
>
>
>
> The diode seems to connect an inductor to the base of the switching
> transistor and the collector of the transistor is connected to a
> transformer. Should I be looking for other failed parts? Not sure if the
> diode failed first and then caused the transistor to fail? Or if something
> else has failed which caused these parts to fail?
>


Also check all other semiconductors. Also on the outputs. If there is a 1
ohm fusible resistor in the base drive circuit check that one as well. In
the VT100 PSUs it happens that it blows.



>
>
>
> I do know that there are no shorts in the Rainbow itself, because I have a
> spare PSU that still works fine in the same machine.
>
>
>
> I blogged this here (it repeats most of that I have said above):
>
> https://robs-old-computers.com/2022/11/20/dec-rainbow-h7842-power-supply-fai
> lure/
> 
>
>
>
/Mattis

>
> Thanks
>
>
>
> Rob
>
>

[cctalk] Re: Identifying a Failed Diode in a Rainbow H7842 Power Supply

[Antonio Carlini via cctalk]

On 20/11/2022 21:03, Rob Jarratt wrote:

Thanks Antonio,

The location of the diode is arrowed on this picture: 
https://rjarratt.files.wordpress.com/2022/11/img_20221120_205802-arrowed.jpg

You can also see the heatsink where the transistor used to be.



This is mine: 
https://drive.google.com/file/d/1igM9AqrICX93t-KHH3N4gRCr1Z8H6tAY/view?usp=share_link, 
so as you can see, the pick-and-place machine decided to rotate the 
diode for almost maximum inconvenience!



I have two more I can open up and look at, but I cannot get to them 
tonight and I'm probably out tomorrow too. But I think I can get to 
those other two supplies on Wednesday. Hopefully at least one of them 
will be readable! Otherwise I can desolder the diode from one of the 
other two and hopefully find a useful marking.



Antonio


--
Antonio Carlini
anto...@acarlini.com

[cctalk] Re: Identifying a Failed Diode in a Rainbow H7842 Power Supply

[Peter Coghlan via cctalk]

Hi Rob,

>
> Thanks for the analysis. There doesn't seem to be anything further back from
> the diode, unless you mean further back behind the inductor?
>

I probably do.  I guess the base drive for the main switching transistor
comes from another smaller transistor via the inductor and the diode which
failed? Or maybe the diode is in parallel across the drive voltage?  Anyway,
it is this smaller transistor providing the drive that I'm suggesting
checking next as it may have got a belt of anything up to three hundred and
something volts DC when the main switching transistor shorted out an instant
before it exploded.  If that one has failed too, check further back to
whatever is driving it.

>
> I will check on the output side as you suggest.
>

The rectifiers on the output side may be tedious to figure out and test
due in part to the very low resistances of the transformer windings
feeding them unfortunately, especially if there are double rectifiers
(maybe in packages which look suspiciously like transistors) connected
to either side of centre tapped transformer windings.

>
> The circuit breaker did pop out when it failed. The onboard fuse is intact.
>

If you mean the house breaker as opposed to a breaker in the power supply,
I would guess it was probably rated at 25A or more and given the
destruction of the transistor, I would think the instantaneous current
was way higher than that.  Perhaps the fuse is a time delay one.  It
seems likely things could have been worse if the breaker wasn't able
to cut the power as soon as it did and you had to wait for the fuse
to blow :-(

>
> The house RCD triggered and cut the power to the whole house when the
> transistor exploded!
>

I guess that the transistor exploding produced a momentary cloud of
ionised gas which allowed arcing/tracking from the live transistor
terminals to something earthed (power supply case maybe?) which
triggered the RCD.  Or maybe the heatsink the switching transistor
is mounted on is earthed and the insulating washer between the
heatsink and the transistor failed causing it's destruction and
the heatsink then became the path to earth that triggered the RCD?

>
> Do you, or anyone else, have an idea what the diode could be so that I can
> find a replacement. Like I said, it seems to be marked D610, and there are
> some other ones that look to be the same elsewhere in the PSU.
>

Sorry, I have no idea what the diode is.  Hopefully Antonio will be able
to help with that.  Or if your spare power supply is the same design as
the failed one, can you look at the diode in that one?

It has dawned on me that another mechanism for the switching transistor
to have failed so spectacularly is for it to have been switched on hard
and held on for more than an instant by something driving it resulting
in it effectively shorting out the rectified mains supply through the
primary of the chopper transformer.

When you next try powering it up, it might be good to use the old
(filament) light bulb in series with the mains supply trick in case
whatever caused the initial problem is still lurking in there.  This
can be useful for locating shorted output rectifiers too.  You might
see a slight voltage rise from the outputs with working rectifiers when
power is applied through the light bulb and a lesser or no voltage rise
on an output which has a shorted rectifier or other issue that needs
closer inspection.  It also means there is no need to hide behind the
sofa when you switch it on :-)

Regards,
Peter.

> 
> Thanks
> 
> Rob 
> 
> > -Original Message-
> > From: Peter Coghlan via cctalk 
> > Sent: 20 November 2022 18:50
> > To: General Discussion: On-Topic and Off-Topic Posts
> > 
> > Cc: Peter Coghlan 
> > Subject: [cctalk] Re: Identifying a Failed Diode in a Rainbow H7842 Power
> > Supply
> > 
> > Hi Rob,
> > 
> > I'm only guessing here.  I think the sequence may have been that the main
> > switching transistor failed first as it would be under more stress than a
> diode
> > in the base circuit.  If the transistor shorted E-B-C then the HT would
> become
> > connected to the circuitry at it's base which would be compelely unable to
> > cope with voltages and currents involved.  This probably resulted in the
> > failure of the diode.  I think it may be worth looking at the components
> > further back the drive chain from the diode.
> > The inductor could be ok unless it is a very frail little thing but small
> signal
> > semiconductor components and/or resistors further back may not have
> > fared as well as it.
> > 
> > It might also be worthwhile checking for shorted rectifiers on the output
> side
> > in case this was the cause of the stress on the switching transistor.
> > However, th

[cctalk] Re: Identifying a Failed Diode in a Rainbow H7842 Power Supply

[Rob Jarratt via cctalk]

Thanks Antonio,

The location of the diode is arrowed on this picture: 
https://rjarratt.files.wordpress.com/2022/11/img_20221120_205802-arrowed.jpg

You can also see the heatsink where the transistor used to be.

Regards

Rob

> -Original Message-
> From: Antonio Carlini via cctalk 
> Sent: 20 November 2022 20:38
> To: Rob Jarratt via cctalk 
> Cc: Antonio Carlini 
> Subject: [cctalk] Re: Identifying a Failed Diode in a Rainbow H7842 Power
> Supply
> 
> On 20/11/2022 17:40, Rob Jarratt via cctalk wrote:
> > The H7842 PSU in my Rainbow failed yesterday. At first the machine
> > just powered down and there was a slight burning smell, I wasn't next
> > to the machine when this happened, so I didn't see or hear anything to
> > tell me where the problem might be. Not being sure if there was a
> > short in the machine or a problem in the PSU, I disconnected the fans,
> > FDD and HDD and, probably foolishly, I applied power again to see if the
> machine would work.
> > At this point there was a bang and a flash in the PSU.
> >
> >
> 
> >
> > I blogged this here (it repeats most of that I have said above):
> > https://robs-old-computers.com/2022/11/20/dec-rainbow-h7842-power-
> supp
> > ly-fai
> > lure/
> >
> 
> I have Rainbow PSUs H7842A, H78420 (which I suspect I may have misread!
> ...) and H7842D available. I can look tomorrow; if you can supply an overview
> picture and maybe circle the location of the offending parts that might help
> me identify them more quickly (always assuming that they are marked at all,
> of course).
> 
> 
> Antonio
> 
> 
> --
> Antonio Carlini
> anto...@acarlini.com

[cctalk] Re: Identifying a Failed Diode in a Rainbow H7842 Power Supply

[Antonio Carlini via cctalk]

On 20/11/2022 17:40, Rob Jarratt via cctalk wrote:

The H7842 PSU in my Rainbow failed yesterday. At first the machine just
powered down and there was a slight burning smell, I wasn't next to the
machine when this happened, so I didn't see or hear anything to tell me
where the problem might be. Not being sure if there was a short in the
machine or a problem in the PSU, I disconnected the fans, FDD and HDD and,
probably foolishly, I applied power again to see if the machine would work.
At this point there was a bang and a flash in the PSU.

  




I blogged this here (it repeats most of that I have said above):
https://robs-old-computers.com/2022/11/20/dec-rainbow-h7842-power-supply-fai
lure/



I have Rainbow PSUs H7842A, H78420 (which I suspect I may have misread! 
...) and H7842D available. I can look tomorrow; if you can supply an 
overview picture and maybe circle the location of the offending parts 
that might help me identify them more quickly (always assuming that they 
are marked at all, of course).



Antonio


--
Antonio Carlini
anto...@acarlini.com

[cctalk] Re: Identifying a Failed Diode in a Rainbow H7842 Power Supply

[Rob Jarratt via cctalk]

Hello Peter,

Thanks for the analysis. There doesn't seem to be anything further back from
the diode, unless you mean further back behind the inductor?

I will check on the output side as you suggest.

The circuit breaker did pop out when it failed. The onboard fuse is intact.
The house RCD triggered and cut the power to the whole house when the
transistor exploded!

Do you, or anyone else, have an idea what the diode could be so that I can
find a replacement. Like I said, it seems to be marked D610, and there are
some other ones that look to be the same elsewhere in the PSU.

Thanks

Rob 

> -Original Message-
> From: Peter Coghlan via cctalk 
> Sent: 20 November 2022 18:50
> To: General Discussion: On-Topic and Off-Topic Posts
> 
> Cc: Peter Coghlan 
> Subject: [cctalk] Re: Identifying a Failed Diode in a Rainbow H7842 Power
> Supply
> 
> Hi Rob,
> 
> I'm only guessing here.  I think the sequence may have been that the main
> switching transistor failed first as it would be under more stress than a
diode
> in the base circuit.  If the transistor shorted E-B-C then the HT would
become
> connected to the circuitry at it's base which would be compelely unable to
> cope with voltages and currents involved.  This probably resulted in the
> failure of the diode.  I think it may be worth looking at the components
> further back the drive chain from the diode.
> The inductor could be ok unless it is a very frail little thing but small
signal
> semiconductor components and/or resistors further back may not have
> fared as well as it.
> 
> It might also be worthwhile checking for shorted rectifiers on the output
side
> in case this was the cause of the stress on the switching transistor.
> However, the power supply might have an overcurrent trip to reduce the
> possibility of this sort of damage.  If there is an overcurrent trip or
thermal
> trip, this may have been reset after the power supply was powered off for
a
> while and when it was powered on again, the already damaged transistor
> could have been teed up to fail more spectacularly?  Like I said, just
guessing
> here.
> 
> Were there no fuses failed or cutouts cut out?  Does it look like there
should
> have been? I would think a shorted switching transistor should have caused
> some safety device to operate.  Or is it the case of the old adage that
the
> faster acting transistor managed to sacrifice itself in time to protect
the quick-
> blow fuse from blowing?
> 
> Regards,
> Peter Coghlan.
> 
> >
> > The H7842 PSU in my Rainbow failed yesterday. At first the machine
> > just powered down and there was a slight burning smell, I wasn't next
> > to the machine when this happened, so I didn't see or hear anything to
> > tell me where the problem might be. Not being sure if there was a
> > short in the machine or a problem in the PSU, I disconnected the fans,
> > FDD and HDD and, probably foolishly, I applied power again to see if the
> machine would work.
> > At this point there was a bang and a flash in the PSU.
> >
> >
> >
> > On opening up the H7842 power supply I found that one of the
> > transistors had completely disintegrated. It looks to be the main
> > switching transistor, here is a picture of it:
> > https://rjarratt.files.wordpress.com/2022/11/img_20221120_165850.jpg.
> > I have identified a source for this transistor, but if anyone can
> > suggest a modern replacement that would be useful too. However, that
> > is not my main problem.
> >
> >
> >
> > Given that before the transistor blew up there had clearly been
> > another failure somewhere else, I tried to find the original failure.
> > There were no obviously damaged parts, so I just probed around near
> > the transistor for any parts that were open circuit or short circuit.
> > I found a diode connected to the base of the transistor that appeared
> > to be short circuit. So, I decided to lift one end to check it. As I
> > de-soldered one of the leads, the diode broke in two. So clearly the
> > diode was either damaged by the failure of the transistor, or it was the
> cause of the failure. This is the diode:
> > https://rjarratt.files.wordpress.com/2022/11/img_20221120_165913.jpg.
> >
> >
> >
> > I can't quite make out the markings on the diode to know what to
> > replace it with. I think it says "D610". Would that be the right
> > designation? If so, can anyone suggest a suitable replacement please?
> >
> >
> >
> > The diode seems to connect an inductor to the base of the switching
> > transistor and the collector of the transistor is connected to a
> > transformer. S

[cctalk] Re: Identifying a Failed Diode in a Rainbow H7842 Power Supply

[Peter Coghlan via cctalk]

Hi Rob,

I'm only guessing here.  I think the sequence may have been that the
main switching transistor failed first as it would be under more stress
than a diode in the base circuit.  If the transistor shorted E-B-C then
the HT would become connected to the circuitry at it's base which would
be compelely unable to cope with voltages and currents involved.  This
probably resulted in the failure of the diode.  I think it may be worth
looking at the components further back the drive chain from the diode.
The inductor could be ok unless it is a very frail little thing but small
signal semiconductor components and/or resistors further back may not
have fared as well as it.

It might also be worthwhile checking for shorted rectifiers on the output
side in case this was the cause of the stress on the switching transistor.
However, the power supply might have an overcurrent trip to reduce the
possibility of this sort of damage.  If there is an overcurrent trip or
thermal trip, this may have been reset after the power supply was powered
off for a while and when it was powered on again, the already damaged
transistor could have been teed up to fail more spectacularly?  Like I
said, just guessing here.

Were there no fuses failed or cutouts cut out?  Does it look like there
should have been? I would think a shorted switching transistor should
have caused some safety device to operate.  Or is it the case of the
old adage that the faster acting transistor managed to sacrifice itself
in time to protect the quick-blow fuse from blowing?

Regards,
Peter Coghlan.

> 
> The H7842 PSU in my Rainbow failed yesterday. At first the machine just
> powered down and there was a slight burning smell, I wasn't next to the
> machine when this happened, so I didn't see or hear anything to tell me
> where the problem might be. Not being sure if there was a short in the
> machine or a problem in the PSU, I disconnected the fans, FDD and HDD and,
> probably foolishly, I applied power again to see if the machine would work.
> At this point there was a bang and a flash in the PSU.
> 
> 
> 
> On opening up the H7842 power supply I found that one of the transistors had
> completely disintegrated. It looks to be the main switching transistor, here
> is a picture of it:
> https://rjarratt.files.wordpress.com/2022/11/img_20221120_165850.jpg.  I
> have identified a source for this transistor, but if anyone can suggest a
> modern replacement that would be useful too. However, that is not my main
> problem.
> 
>  
> 
> Given that before the transistor blew up there had clearly been another
> failure somewhere else, I tried to find the original failure. There were no
> obviously damaged parts, so I just probed around near the transistor for any
> parts that were open circuit or short circuit. I found a diode connected to
> the base of the transistor that appeared to be short circuit. So, I decided
> to lift one end to check it. As I de-soldered one of the leads, the diode
> broke in two. So clearly the diode was either damaged by the failure of the
> transistor, or it was the cause of the failure. This is the diode:
> https://rjarratt.files.wordpress.com/2022/11/img_20221120_165913.jpg.
> 
>  
> 
> I can't quite make out the markings on the diode to know what to replace it
> with. I think it says "D610". Would that be the right designation? If so,
> can anyone suggest a suitable replacement please?
> 
>  
> 
> The diode seems to connect an inductor to the base of the switching
> transistor and the collector of the transistor is connected to a
> transformer. Should I be looking for other failed parts? Not sure if the
> diode failed first and then caused the transistor to fail? Or if something
> else has failed which caused these parts to fail?
> 
>  
> 
> I do know that there are no shorts in the Rainbow itself, because I have a
> spare PSU that still works fine in the same machine.
> 
>  
> 
> I blogged this here (it repeats most of that I have said above):
> https://robs-old-computers.com/2022/11/20/dec-rainbow-h7842-power-supply-fai
> lure/
> 
>  
> 
> Thanks
> 
>  
>
> Rob
>