Re: Analogue monitor board repair

[Adrian Graham]

Hi folks,

Replying to myself here since I've had a bit of a breakthrough but not in
the way I'd have liked. A fellow collector who also got one of these
machines at the same time opened his up and there was an extra little
bridgeboard piggybacked off some resistors next to the teletext display chip
that goes to a 7-pin socket on the rear of the case.

The board only has a small cap and a 74LS244 buffer chip on it so I traced
it all out and built my own to produce composite RGB + sync which I plugged
into an old Microvitec CUB monitor, the workhorse of the BBC Micro.

This is the picture I got:

http://www.binarydinosaurs.co.uk/STCExecutelScreenOutput.jpg

Funnily enough that's EXACTLY what I get on the little B display, the
green background explains why I thought the brightness/contrast wasn't
correct, and it's scrolling in the same way.

This means of course that the analogue board is fine and it's the INPUT to
this display chip that isn't correct, I know the chip itself is OK because I
have a Tandata TD2500 teletext terminal that uses the same device and it
works in that. 

I might have to admit defeat on this one, documentation on this system is
non-existent. I've dumped the ROMs so I know they're OK and I can get the
datasheet for the 8085a CPU to check clock/address/data lines etc, RAM is
all 4116-2 so one of them could be bad as well. But there my knowledge ends.

Cheers!


On 31/10/2016 22:29, "tony duell"  wrote:

> 
>> Before I replaced the failed potentiometer (new one seen top left) the
>> display looked like its horizonal hold had gone so I reasoned that's what
> 
> OK... The TDA1180 is the horizontal oscillator, etc, IC. It's well-known.
> 
> Start by getting its data sheet. Indentify the horizontal oscillator
> components
> connected to pins 12, 13, 14, 15 of that IC. Typically you will find a pot
> that controls the DC voltage on pin 15 (slider of pot to pin 15 through a
> resistor). That sets the free-running horizontal frequency

That's the pot I replaced, it goes through a 22ohm resistor.

> Now try adjusting it. If you can get the oscillator to run both too fast
> and too slow (lines sloping both ways) that that part is most likely fine
> and the fault is in the sync circuit. If not, then the oscillator components
> have problems.

Yep, lines sloping both ways is exactly what happens. Perhaps bizarrely this
also seems to affect the contrast.

> Are you getting a sync pulse at pin 8 of the IC? If not, trace back from
> there to the connector to the logic board and if necessary to the video
> IC.

Pin 8 goes right back to a 74LS04 up near the RAM/ROM section of the
motherboard. I'm going to see if it's possible to assemble the whole thing
outside of the big plastic housing and still hold the screen as securely as
possible, it's a complete pain to get the screen in its swivel top and
connect up everything without risk of breakage.

> Are you getting a flyback pulse at pin 6? The horizontal control circuit is
> basically a phase-locked loop comparing the incoming sync pulses with
> flyback signal from the horizontal output stage.

Watch this space :)

-- 
Adrian/Witchy
Binary Dinosaurs creator/curator
Www.binarydinosaurs.co.uk - the UK's biggest private home computer
collection?

-- End of Forwarded Message

Re: Analogue monitor board repair

[Adrian Graham]

On 31/10/2016 23:14, "drlegendre ."  wrote:

> "Hence the question - am I wasting my time without a schematic for this
> analogue board?"
> 
> Might be helpful if you mentioned exactly what is or isn't wrong with the
> unit.. describe the symptom(s), etc.

I thought I'd ask before ploughing on with a full description. Even just
typing the original question out has made me realise what the display driver
chip is.

See my replies to TonyD, there's more pictures there as well.

Cheers!

> On Mon, Oct 31, 2016 at 1:44 PM, Adrian Graham > wrote:
> 
>> Folks,
>> 
>> I think I know the answer to this before I even ask, and that answer will
>> be
>> 'got a schematic' to which the answer's 'no and I doubt one exists any
>> more'
>> but...
>> 
>> My recent Executel addition has a 5" screen with associated analogue board
>> that seems to be powered from a display chip I can't find any info on, and
>> at least one of the adjustment potentiometers has suffered metal fatigue
>> and
>> broken:
>> 
>> http://www.binarydinosaurs.co.uk/STCExecutelScreenPot.jpg
>> http://www.binarydinosaurs.co.uk/STCExecutelAnalogueBoard.jpg
>> http://www.binarydinosaurs.co.uk/STCExecutelDisplayChip.jpg
>> 
>> I've replaced it with a seemingly common 10K modern part that I had in my
>> spares box but it's unknown whether the display worked at all prior to it
>> being put in very damp storage, also the rating of the failed pot isn't
>> known but the capacitor next to it is 50V.
>> 
>> I've also removed, checked and replaced out of spec capacitors, one of them
>> was a 680nF 50V radial. I could only find a 63V PET version on cpc's
>> website
>> and did much reading on differing capacitor types, concluding that it
>> SHOULD
>> be OK.
>> 
>> Power supply is known good because it's one of my working ones, the
>> original
>> is still dead.
>> 
>> Hence the question - am I wasting my time without a schematic for this
>> analogue board?
>> 
>> Cheers!
>> 
>> --
>> Adrian/Witchy
>> Binary Dinosaurs creator/curator
>> Www.binarydinosaurs.co.uk - the UK's biggest private home computer
>> collection?
>> 
>> 
>> 

-- 
Adrian/Witchy
Binary Dinosaurs creator/curator
Www.binarydinosaurs.co.uk - the UK's biggest private home computer
collection?

Re: Analogue monitor board repair

[Adrian Graham]

On 31/10/2016 22:22, "Peter Coghlan"  wrote:

>> 
>> Before I replaced the failed potentiometer (new one seen top left) the
>> display looked like its horizonal hold had gone so I reasoned that's what
>> the pot controlled. I can *nearly* get a steady picture but the brightness
>> is out as well, despite there being working pots for that and contrast. The
>> brightness seemed more constant before I replaced the radial cap with the
>> PET one which is why I wondered if the choice was wrong.
>> 
> 
> Did you measure the resistance of the track on the old potentiometer
> and use something with a similar value for the replacement?
> 
> Is it possible that you misread or misinterpreted the value of the capacitor
> that you replaced? You could try putting the old one back and seeing if there
> is an improvement.

It says 10K on what's left of the wiper, there's a pic in an earlier message
from tonight :) I can't put it back because it's physically rotted
unfortunately.

> What is PET in this context? I am assuming it is nothing to do with Commodore.

PET as in the plastic housing, PET Film cap. I figured my replacement must
be OK since there was one other on the board in that style already.

> Can you take a photograph of what you are seeing on the screen?

Once I've put it all back together, yes. Might be difficult to capture with
a camera though but I'll have a go.

Cheers!

-- 
Adrian/Witchy
Binary Dinosaurs creator/curator
Www.binarydinosaurs.co.uk - the UK's biggest private home computer
collection?

Re: Analogue monitor board repair

[Adrian Graham]

On 31/10/2016 22:29, "tony duell"  wrote:

> 
>> Before I replaced the failed potentiometer (new one seen top left) the
>> display looked like its horizonal hold had gone so I reasoned that's what
> 
> OK... The TDA1180 is the horizontal oscillator, etc, IC. It's well-known.
> 
> Start by getting its data sheet. Indentify the horizontal oscillator
> components
> connected to pins 12, 13, 14, 15 of that IC. Typically you will find a pot
> that controls the DC voltage on pin 15 (slider of pot to pin 15 through a
> resistor). That sets the free-running horizontal frequency

That's the pot I replaced, it goes through a 22ohm resistor.

> Now try adjusting it. If you can get the oscillator to run both too fast
> and too slow (lines sloping both ways) that that part is most likely fine
> and the fault is in the sync circuit. If not, then the oscillator components
> have problems.

Yep, lines sloping both ways is exactly what happens. Perhaps bizarrely this
also seems to affect the contrast.

> Are you getting a sync pulse at pin 8 of the IC? If not, trace back from
> there to the connector to the logic board and if necessary to the video
> IC.

Pin 8 goes right back to a 74LS04 up near the RAM/ROM section of the
motherboard. I'm going to see if it's possible to assemble the whole thing
outside of the big plastic housing and still hold the screen as securely as
possible, it's a complete pain to get the screen in its swivel top and
connect up everything without risk of breakage.

> Are you getting a flyback pulse at pin 6? The horizontal control circuit is
> basically a phase-locked loop comparing the incoming sync pulses with
> flyback signal from the horizontal output stage.

Watch this space :)

-- 
Adrian/Witchy
Binary Dinosaurs creator/curator
Www.binarydinosaurs.co.uk - the UK's biggest private home computer
collection?

Re: Analogue monitor board repair

[drlegendre .]

"Hence the question - am I wasting my time without a schematic for this
analogue board?"

Might be helpful if you mentioned exactly what is or isn't wrong with the
unit.. describe the symptom(s), etc.

On Mon, Oct 31, 2016 at 1:44 PM, Adrian Graham  wrote:

> Folks,
>
> I think I know the answer to this before I even ask, and that answer will
> be
> 'got a schematic' to which the answer's 'no and I doubt one exists any
> more'
> but...
>
> My recent Executel addition has a 5" screen with associated analogue board
> that seems to be powered from a display chip I can't find any info on, and
> at least one of the adjustment potentiometers has suffered metal fatigue
> and
> broken:
>
> http://www.binarydinosaurs.co.uk/STCExecutelScreenPot.jpg
> http://www.binarydinosaurs.co.uk/STCExecutelAnalogueBoard.jpg
> http://www.binarydinosaurs.co.uk/STCExecutelDisplayChip.jpg
>
> I've replaced it with a seemingly common 10K modern part that I had in my
> spares box but it's unknown whether the display worked at all prior to it
> being put in very damp storage, also the rating of the failed pot isn't
> known but the capacitor next to it is 50V.
>
> I've also removed, checked and replaced out of spec capacitors, one of them
> was a 680nF 50V radial. I could only find a 63V PET version on cpc's
> website
> and did much reading on differing capacitor types, concluding that it
> SHOULD
> be OK.
>
> Power supply is known good because it's one of my working ones, the
> original
> is still dead.
>
> Hence the question - am I wasting my time without a schematic for this
> analogue board?
>
> Cheers!
>
> --
> Adrian/Witchy
> Binary Dinosaurs creator/curator
> Www.binarydinosaurs.co.uk - the UK's biggest private home computer
> collection?
>
>
>

RE: Analogue monitor board repair

[tony duell]

> Before I replaced the failed potentiometer (new one seen top left) the
> display looked like its horizonal hold had gone so I reasoned that's what

OK... The TDA1180 is the horizontal oscillator, etc, IC. It's well-known.

Start by getting its data sheet. Indentify the horizontal oscillator components
connected to pins 12, 13, 14, 15 of that IC. Typically you will find a pot
that controls the DC voltage on pin 15 (slider of pot to pin 15 through a 
resistor). That sets the free-running horizontal frequency

Now try adjusting it. If you can get the oscillator to run both too fast
and too slow (lines sloping both ways) that that part is most likely fine
and the fault is in the sync circuit. If not, then the oscillator components
have problems.

Are you getting a sync pulse at pin 8 of the IC? If not, trace back from
there to the connector to the logic board and if necessary to the video
IC.

Are you getting a flyback pulse at pin 6? The horizontal control circuit is
basically a phase-locked loop comparing the incoming sync pulses with 
flyback signal from the horizontal output stage.

-tony

Re: Analogue monitor board repair

[Peter Coghlan]

>
> Before I replaced the failed potentiometer (new one seen top left) the
> display looked like its horizonal hold had gone so I reasoned that's what
> the pot controlled. I can *nearly* get a steady picture but the brightness
> is out as well, despite there being working pots for that and contrast. The
> brightness seemed more constant before I replaced the radial cap with the
> PET one which is why I wondered if the choice was wrong.
>

Did you measure the resistance of the track on the old potentiometer
and use something with a similar value for the replacement?

Is it possible that you misread or misinterpreted the value of the capacitor
that you replaced? You could try putting the old one back and seeing if there
is an improvement.

What is PET in this context? I am assuming it is nothing to do with Commodore.

Can you take a photograph of what you are seeing on the screen?

Regards,
Peter Coghlan.

Re: Analogue monitor board repair

[Adrian Graham]

On 31/10/2016 20:51, "tony duell"  wrote:

> 
>> My recent Executel addition has a 5" screen with associated analogue board
>> that seems to be powered from a display chip I can't find any info on, and
> 
> Well, it won't be _powered_ from that IC. My guess is that said IC provides
> the HSync and VSync signals, the video comes from other devices on the
> main PCB. But that is a guess without seeing the machine/

My earlier google-fu was hampered by getting the IC number wrong and taking
a macro photo of it has shown my error, it's actually a Plessey MR9735
Teletext/Viewdata chip that 'drives a 625 line Colour Television Receiver to
display the contents of the Page Store'

http://www.datasheetarchive.com/dlmain/4d6e79fdc0fb675d8dd857876f8d6bdd195f5
7/M/MR9735

>> Hence the question - am I wasting my time without a schematic for this
>> analogue board?
> 
> You haven't told us what the problem is.

http://www.binarydinosaurs.co.uk/STCExecutelAnalogueBoard2.jpg

Before I replaced the failed potentiometer (new one seen top left) the
display looked like its horizonal hold had gone so I reasoned that's what
the pot controlled. I can *nearly* get a steady picture but the brightness
is out as well, despite there being working pots for that and contrast. The
brightness seemed more constant before I replaced the radial cap with the
PET one which is why I wondered if the choice was wrong.
 
> Do you get anything on the screen? Is the CRT heater glowing? Have you
> measured the CRT pin voltages?

There's a recognisable picture, yes.
 
> I can see one IC on that 'analogue board'. Looks to have metal heatsink
> tabs so I am going to make a wild guess that it's a TDA1170 vertical
> deflection IC Are there any other ICs on the board? My guess is that
> the 'analogue board' could be reverse-engineered without too much
> work.

It is indeed a TDA1170, there's also a TDA1180P.

Here's the machine in all its glory:
http://www.binarydinosaurs.co.uk/STCExecutel01.jpg

Board:
http://www.binarydinosaurs.co.uk/STCExecutelBoard.jpg

Cheers!

-- 
Adrian/Witchy
Binary Dinosaurs creator/curator
Www.binarydinosaurs.co.uk - the UK's biggest private home computer
collection?

RE: Analogue monitor board repair

[tony duell]

> My recent Executel addition has a 5" screen with associated analogue board
> that seems to be powered from a display chip I can't find any info on, and

Well, it won't be _powered_ from that IC. My guess is that said IC provides
the HSync and VSync signals, the video comes from other devices on the
main PCB. But that is a guess without seeing the machine/

[...]

> I've also removed, checked and replaced out of spec capacitors, one of them
> was a 680nF 50V radial. I could only find a 63V PET version on cpc's website
> and did much reading on differing capacitor types, concluding that it SHOULD
> be OK.

I would be very surprised if that was not OK.

[...]

> Hence the question - am I wasting my time without a schematic for this
> analogue board?

You haven't told us what the problem is.

Do you get anything on the screen? Is the CRT heater glowing? Have you
measured the CRT pin voltages?

I can see one IC on that 'analogue board'. Looks to have metal heatsink 
tabs so I am going to make a wild guess that it's a TDA1170 vertical 
deflection IC Are there any other ICs on the board? My guess is that
the 'analogue board' could be reverse-engineered without too much
work.

Can you provide a clear picture of the component side of the
board with nothing else in the way?

-tony

Analogue monitor board repair

[Adrian Graham]

Folks,

I think I know the answer to this before I even ask, and that answer will be
'got a schematic' to which the answer's 'no and I doubt one exists any more'
but...

My recent Executel addition has a 5" screen with associated analogue board
that seems to be powered from a display chip I can't find any info on, and
at least one of the adjustment potentiometers has suffered metal fatigue and
broken:

http://www.binarydinosaurs.co.uk/STCExecutelScreenPot.jpg
http://www.binarydinosaurs.co.uk/STCExecutelAnalogueBoard.jpg
http://www.binarydinosaurs.co.uk/STCExecutelDisplayChip.jpg

I've replaced it with a seemingly common 10K modern part that I had in my
spares box but it's unknown whether the display worked at all prior to it
being put in very damp storage, also the rating of the failed pot isn't
known but the capacitor next to it is 50V.

I've also removed, checked and replaced out of spec capacitors, one of them
was a 680nF 50V radial. I could only find a 63V PET version on cpc's website
and did much reading on differing capacitor types, concluding that it SHOULD
be OK.

Power supply is known good because it's one of my working ones, the original
is still dead.

Hence the question - am I wasting my time without a schematic for this
analogue board?

Cheers!

-- 
Adrian/Witchy
Binary Dinosaurs creator/curator
Www.binarydinosaurs.co.uk - the UK's biggest private home computer
collection?