Message: 11
Date: Sun, 30 Nov 2025 12:56:45 -0000
From: "Rob Jarratt"<[email protected]>
Subject: [cctalk] Re: Hot Video Shift Register on VT100
To: "'General Discussion: On-Topic and Off-Topic Posts'"
<[email protected]>
Cc: 'Hans-Ulrich Hölscher'<[email protected]>
Message-ID:<[email protected]>
Content-Type: text/plain; charset="iso-8859-1"
-----Original Message-----
From: Hans-Ulrich Hölscher via cctalk<[email protected]>
Sent: 29 November 2025 09:17
To: General Discussion: On-Topic and Off-Topic Posts
<[email protected]>
Cc: Hans-Ulrich Hölscher<[email protected]>
Subject: [cctalk] Re: Hot Video Shift Register on VT100
Hi Rob,
this is not intended to indicate the cause or solution, but merely to help
analyze how critical your IC temperature is: Did you actually measure the
temperature of the IC? I recently bought an inexpensive infrared
thermometer (like:https://www.ebay.co.uk/itm/156810933790) that can
measure the temperature of small areas relatively accurately. I made the
experience that I cannot judge temperatures very well with my fingers.
According to Wikipedia, TTL ICs are specified for temperatures up to
70 °C. This
temperature should obviously be avoided, but my fingers already perceive
anything above about 50 °C as "hot".
I hope this can help a litte!
Yes, I have an IR Thermometer and the chip gets to about 40 celsius. I
realise that this is well below the maximum rating, but I can smell the heat
and it worries me a bit. If I knew this was expected then I would be OK with
it, but it feels anomalous and I have felt the chip to be hotter than this
sometimes. The observation about the negative voltages also concerns me a
bit, although I do have spares for the 74S299 should I need them.
It looks now like the DC011 is just producing a DOT CLK output with negative
spikes. Maybe this is why later revisions introduced an inductor or maybe it
is just a bad DC011 or something is wrong with what is around the DC011.
I am going to look at the DC011 inputs to see if anything looks amiss. The
other thing I would like to do is to see what happens if I can add an
inductor to the output of the DC011. But I don't know how to read the spec
on the printset to know what I might need. The printset says "BEAD, FERRITE
.1380DX.380LG", can anyone tell me what this might mean in terms of an
inductor that I could try?
Thanks
Rob
Ulli
Am Sa., 29. Nov. 2025 um 09:44 Uhr schrieb Henk Gooijen via cctalk <
[email protected]>:
Rob,
I have a few silly thoughts, but maybe they can be useful.
The negative going signal is something I would not be happy with, as
it may damage the IC (over time).
TTL does not like negative voltages on their pins.
Is there between the DC011 (signal generating) and the LS299 (signal
"user") wiring, or are they on the same PCB?
In case of wiring, capacitive coupling may cause negative going
voltages.
Could a clamping diode with an as low as possible Vf (< 0.3 V) help to
reduce the negative voltage? It is not a permanent solution, because
the
DC011 might be a bit unhappy with that diode, but for a test it might
help to draw a conclusion.
If all does not help, in getting the LS299 running cooler, consider
some temperature conductive paste and a small "IC heatsink". Tie the
heatsink onto the IC with a piece of waxed lacing. The IC can radiate
more efficiently the generated heat, so that it can run cooler. This
could drop temperature with some 10 degrees.
Success hunting down this issue,
Henk
-----Oorspronkelijk bericht-----
Van: Rob Jarratt via cctalk<[email protected]>
Verzonden: zaterdag 29 november 2025 08:11
Aan: 'Doug Jackson'<[email protected]>;[email protected]
CC: 'General Discussion: On-Topic and Off-Topic Posts' <
[email protected]>; Rob Jarratt <[email protected]>
Onderwerp: [cctalk] Re: Hot Video Shift Register on VT100
Yes, it is a negative voltage. I measured this by attaching the ground
lead to the ground connection on the 74S299.
The DC011 that is producing the CLK signal is socketed, but the one on
my
VT102 isnt. I would try swapping them to see what happens, but I
dont really want to touch a fully working board.
Thanks
Rob
From: Doug Jackson<[email protected]>
Sent: 28 November 2025 22:59
To:[email protected]
Cc: General Discussion: On-Topic and Off-Topic Posts <
[email protected]>
Subject: Re: [cctalk] Re: Hot Video Shift Register on VT100
Is that a negative excursion at the bottom of the waveform? now low -
looks like -0.3v to me - With '"logic, that will likely turn on the
protection diodes on the inputs, making them warm.
Double check where the ground actually is on the CRO display.
It's also interesting that the input doesn't even reach 4v, let alone
5v.
Whats driving that?
Kindest regards,
Doug Jackson
em:[email protected] <mailto:[email protected]>
ph: 0414 986878
On Sat, 29 Nov 2025 at 08:52, Rob Jarratt <[email protected]
<mailto:[email protected]> > wrote:
I checked the CLK input for the 74S299 with the 74S299 removed and the
signal is still spiky and looks like this:
https://rjarratt.wordpress.com/wp-content/uploads/2025/11/vt102-74s299
-clk-signal.png
I removed the other load on this signal, the DC012, and the CLK signal
still looked spiky. I also put back the 74S299 (now socketed) with
DC012 removed and it still gets hot. I tried another new 74S299 just
in case I had a bad one and that also got hot without a load on the
output.
I am thinking that it is either the spiky input or it is just expected
to get this hot.
Regards
Rob
From: Doug Jackson <[email protected] <mailto:[email protected]> >
Sent: 25 November 2025 01:15
To:[email protected] <mailto:[email protected]>
Cc: General Discussion: On-Topic and Off-Topic Posts <
[email protected] <mailto:[email protected]> >
Subject: Re: [cctalk] Re: Hot Video Shift Register on VT100
The oscilloscope will show you the actual driving waveform.
You can then see the level of the spikes
Kindest regards,
Doug Jackson
em:[email protected] <mailto:[email protected]>
ph: 0414 986878
On Tue, 25 Nov 2025 at 10:48, Rob Jarratt <[email protected]
<mailto:[email protected]> > wrote:
-----Original Message-----
From: Doug Jackson via cctalk <[email protected] <mailto:
[email protected]> >
Sent: 24 November 2025 23:04
To: General Discussion: On-Topic and Off-Topic Posts <
[email protected] <mailto:[email protected]> >
Cc: Doug Jackson <[email protected] <mailto:[email protected]> >
Subject: [cctalk] Re: Hot Video Shift Register on VT100
Another test would be to disconnect the output and see if that runs
the device cooler. Maybe the load, while not a short, is simply too
great
for the
device.
I think I can try this, the DC012 it drives is socketed.
But I am leaning towards the inputs containing spikes that need to
be dissipated as heat in clamping diodes. A decent CRO on the
device input
pins
while it is out of circuit will show that as well.
I guess CRO means Cathode Ray Oscilloscope? Not sure what you are
suggesting I try here?
Kindest regards,
Doug Jackson
em:[email protected] <mailto:[email protected]>
ph: 0414 986878
On Tue, 25 Nov 2025 at 09:52, Peter Coghlan via cctalk <
[email protected] <mailto:[email protected]> > wrote:
Sorry if I wasn't clear in my earlier reply, but there were some
answers inline. The summary is that Vcc looks fine, there does
not seem to be any short on the outputs of the shift register
and the chip seems to be
working
correctly. It just gets hot and I wonder if this could be due to
the CLK input being a bit spikier on the VT100 than on my VT102?
Could this be
why
later revisions of the VT100 introduced an inductor on the DOT
CLK output from the DC011?
Perhaps one way to test the hypothesis would be to insert an
inductor like those used on the later revisions in the DOT CLK
output and check whether
a) everything still works and b) the shift register runs cooler?
Regards,
Peter Coghlan.
------------------------------
Message: 12
Date: Sun, 30 Nov 2025 14:30:54 -0000
From: "Rob Jarratt"<[email protected]>
Subject: [cctalk] Re: Hot Video Shift Register on VT100
To:<[email protected]>, "'General Discussion: On-Topic and
Off-Topic Posts'"<[email protected]>
Cc: 'Hans-Ulrich Hölscher'<[email protected]>
Message-ID:<[email protected]>
Content-Type: text/plain; charset="iso-8859-1"
I have traced the clock signal back. The MAS CLK input on the DC011 also has
the negative spikes. This is coming from a 74S157. The input to that is an
oscillator and that too has the negative spikes.
So, it seems the 74S157 is just passing through the inputs.
There is a capacitor across pins 4 and 2 of the oscillator (Vcc and Gnd),
would a failure there cause this kind of negative spike? It seems unlikely
to me, but would be happy to be corrected.
The oscillator is rated at 24.0734MHz. I can find lots rated at exactly
24MHz and I guess the difference in frequency is small enough not to matter,
right?
I will order the part and also a new capacitor for it in case that is where
the fault lies and see.
Thanks
Rob
-----Original Message-----
From: Rob Jarratt via cctalk<[email protected]>
Sent: 30 November 2025 12:57
To: 'General Discussion: On-Topic and Off-Topic Posts'
<[email protected]>
Cc: 'Hans-Ulrich Hölscher'<[email protected]>; Rob Jarratt
<[email protected]>
Subject: [cctalk] Re: Hot Video Shift Register on VT100
-----Original Message-----
From: Hans-Ulrich Hölscher via cctalk<[email protected]>
Sent: 29 November 2025 09:17
To: General Discussion: On-Topic and Off-Topic Posts
<[email protected]>
Cc: Hans-Ulrich Hölscher<[email protected]>
Subject: [cctalk] Re: Hot Video Shift Register on VT100
Hi Rob,
this is not intended to indicate the cause or solution, but merely to
help analyze how critical your IC temperature is: Did you actually
measure the temperature of the IC? I recently bought an inexpensive
infrared thermometer (like:https://www.ebay.co.uk/itm/156810933790)
that can measure the temperature of small areas relatively accurately.
I made the experience that I cannot judge temperatures very well with my
fingers.
According to Wikipedia, TTL ICs are specified for temperatures up to
70 °C. This
temperature should obviously be avoided, but my fingers already
perceive anything above about 50 °C as "hot".
I hope this can help a litte!
Yes, I have an IR Thermometer and the chip gets to about 40 celsius. I
realise
that this is well below the maximum rating, but I can smell the heat and
it
worries me a bit. If I knew this was expected then I would be OK with it,
but it
feels anomalous and I have felt the chip to be hotter than this sometimes.
The observation about the negative voltages also concerns me a bit,
although
I do have spares for the 74S299 should I need them.
It looks now like the DC011 is just producing a DOT CLK output with
negative
spikes. Maybe this is why later revisions introduced an inductor or maybe
it is
just a bad DC011 or something is wrong with what is around the DC011.
I am going to look at the DC011 inputs to see if anything looks amiss. The
other thing I would like to do is to see what happens if I can add an
inductor
to the output of the DC011. But I don't know how to read the spec on the
printset to know what I might need. The printset says "BEAD, FERRITE
.1380DX.380LG", can anyone tell me what this might mean in terms of an
inductor that I could try?
Thanks
Rob
Ulli
Am Sa., 29. Nov. 2025 um 09:44 Uhr schrieb Henk Gooijen via cctalk <
[email protected]>:
Rob,
I have a few silly thoughts, but maybe they can be useful.
The negative going signal is something I would not be happy with, as
it may damage the IC (over time).
TTL does not like negative voltages on their pins.
Is there between the DC011 (signal generating) and the LS299 (signal
"user") wiring, or are they on the same PCB?
In case of wiring, capacitive coupling may cause negative going
voltages.
Could a clamping diode with an as low as possible Vf (< 0.3 V) help
to reduce the negative voltage? It is not a permanent solution,
because the
DC011 might be a bit unhappy with that diode, but for a test it
might help to draw a conclusion.
If all does not help, in getting the LS299 running cooler, consider
some temperature conductive paste and a small "IC heatsink". Tie the
heatsink onto the IC with a piece of waxed lacing. The IC can
radiate more efficiently the generated heat, so that it can run
cooler. This could drop temperature with some 10 degrees.
Success hunting down this issue,
Henk
-----Oorspronkelijk bericht-----
Van: Rob Jarratt via cctalk<[email protected]>
Verzonden: zaterdag 29 november 2025 08:11
Aan: 'Doug Jackson'<[email protected]>;[email protected]
CC: 'General Discussion: On-Topic and Off-Topic Posts' <
[email protected]>; Rob Jarratt <[email protected]>
Onderwerp: [cctalk] Re: Hot Video Shift Register on VT100
Yes, it is a negative voltage. I measured this by attaching the
ground lead to the ground connection on the 74S299.
The DC011 that is producing the CLK signal is socketed, but the one
on my
VT102 isnt. I would try swapping them to see what happens, but I
dont really want to touch a fully working board.
Thanks
Rob
From: Doug Jackson<[email protected]>
Sent: 28 November 2025 22:59
To:[email protected]
Cc: General Discussion: On-Topic and Off-Topic Posts <
[email protected]>
Subject: Re: [cctalk] Re: Hot Video Shift Register on VT100
Is that a negative excursion at the bottom of the waveform? now low
- looks like -0.3v to me - With '"logic, that will likely turn on
the protection diodes on the inputs, making them warm.
Double check where the ground actually is on the CRO display.
It's also interesting that the input doesn't even reach 4v, let
alone
5v.
Whats driving that?
Kindest regards,
Doug Jackson
em:[email protected] <mailto:[email protected]>
ph: 0414 986878
On Sat, 29 Nov 2025 at 08:52, Rob Jarratt
<[email protected] <mailto:[email protected]> >
wrote:
I checked the CLK input for the 74S299 with the 74S299 removed and
the signal is still spiky and looks like this:
https://rjarratt.wordpress.com/wp-content/uploads/2025/11/vt102-74s2
99
-clk-signal.png
I removed the other load on this signal, the DC012, and the CLK
signal still looked spiky. I also put back the 74S299 (now socketed)
with
DC012 removed and it still gets hot. I tried another new 74S299 just
in case I had a bad one and that also got hot without a load on the
output.
I am thinking that it is either the spiky input or it is just
expected to get this hot.
Regards
Rob
From: Doug Jackson <[email protected] <mailto:[email protected]> >
Sent: 25 November 2025 01:15
To:[email protected] <mailto:[email protected]>
Cc: General Discussion: On-Topic and Off-Topic Posts <
[email protected] <mailto:[email protected]> >
Subject: Re: [cctalk] Re: Hot Video Shift Register on VT100
The oscilloscope will show you the actual driving waveform.
You can then see the level of the spikes
Kindest regards,
Doug Jackson
em:[email protected] <mailto:[email protected]>
ph: 0414 986878
On Tue, 25 Nov 2025 at 10:48, Rob Jarratt
<[email protected] <mailto:[email protected]> >
wrote:
-----Original Message-----
From: Doug Jackson via cctalk <[email protected] <mailto:
[email protected]> >
Sent: 24 November 2025 23:04
To: General Discussion: On-Topic and Off-Topic Posts <
[email protected] <mailto:[email protected]> >
Cc: Doug Jackson <[email protected] <mailto:[email protected]> >
Subject: [cctalk] Re: Hot Video Shift Register on VT100
Another test would be to disconnect the output and see if that
runs the device cooler. Maybe the load, while not a short, is
simply too great
for the
device.
I think I can try this, the DC012 it drives is socketed.
But I am leaning towards the inputs containing spikes that need to
be dissipated as heat in clamping diodes. A decent CRO on the
device input
pins
while it is out of circuit will show that as well.
I guess CRO means Cathode Ray Oscilloscope? Not sure what you are
suggesting I try here?
Kindest regards,
Doug Jackson
em:[email protected] <mailto:[email protected]>
ph: 0414 986878
On Tue, 25 Nov 2025 at 09:52, Peter Coghlan via cctalk <
[email protected] <mailto:[email protected]> > wrote:
Sorry if I wasn't clear in my earlier reply, but there were
some answers inline. The summary is that Vcc looks fine, there
does not seem to be any short on the outputs of the shift
register and the chip seems to be
working
correctly. It just gets hot and I wonder if this could be due
to the CLK input being a bit spikier on the VT100 than on my
VT102?
Could this be
why
later revisions of the VT100 introduced an inductor on the DOT
CLK output from the DC011?
Perhaps one way to test the hypothesis would be to insert an
inductor like those used on the later revisions in the DOT CLK
output and check whether
a) everything still works and b) the shift register runs cooler?
Regards,
Peter Coghlan.
------------------------------
Message: 13
Date: Sun, 30 Nov 2025 15:24:11 +0000
From: Charles Morris<[email protected]>
Subject: [cctalk] Re: Hot Video Shift Register on VT100
To:[email protected]
Message-ID:<[email protected]>
Content-Type: text/plain; charset=utf-8
Keep in mind that ringing (artifact caused by the inductance of the
scope probe ground lead) can result in a false display of overshoots...
How short is your ground lead?
-Charles
On 11/30/25 08:30, Rob Jarratt via cctalk wrote:
I have traced the clock signal back. The MAS CLK input on the DC011 also has
the negative spikes. This is coming from a 74S157. The input to that is an
oscillator and that too has the negative spikes.
So, it seems the 74S157 is just passing through the inputs.
There is a capacitor across pins 4 and 2 of the oscillator (Vcc and Gnd),
would a failure there cause this kind of negative spike? It seems unlikely
to me, but would be happy to be corrected.
The oscillator is rated at 24.0734MHz. I can find lots rated at exactly
24MHz and I guess the difference in frequency is small enough not to matter,
right?
I will order the part and also a new capacitor for it in case that is where
the fault lies and see.
Thanks
Rob
-----Original Message-----
From: Rob Jarratt via cctalk<[email protected]>
Sent: 30 November 2025 12:57
To: 'General Discussion: On-Topic and Off-Topic Posts'
<[email protected]>
Cc: 'Hans-Ulrich Hölscher'<[email protected]>; Rob Jarratt
<[email protected]>
Subject: [cctalk] Re: Hot Video Shift Register on VT100
-----Original Message-----
From: Hans-Ulrich Hölscher via cctalk<[email protected]>
Sent: 29 November 2025 09:17
To: General Discussion: On-Topic and Off-Topic Posts
<[email protected]>
Cc: Hans-Ulrich Hölscher<[email protected]>
Subject: [cctalk] Re: Hot Video Shift Register on VT100
Hi Rob,
this is not intended to indicate the cause or solution, but merely to
help analyze how critical your IC temperature is: Did you actually
measure the temperature of the IC? I recently bought an inexpensive
infrared thermometer (like:https://www.ebay.co.uk/itm/156810933790)
that can measure the temperature of small areas relatively accurately.
I made the experience that I cannot judge temperatures very well with my
fingers.
According to Wikipedia, TTL ICs are specified for temperatures up to
70 °C. This
temperature should obviously be avoided, but my fingers already
perceive anything above about 50 °C as "hot".
I hope this can help a litte!
Yes, I have an IR Thermometer and the chip gets to about 40 celsius. I
realise
that this is well below the maximum rating, but I can smell the heat and
it
worries me a bit. If I knew this was expected then I would be OK with it,
but it
feels anomalous and I have felt the chip to be hotter than this sometimes.
The observation about the negative voltages also concerns me a bit,
although
I do have spares for the 74S299 should I need them.
It looks now like the DC011 is just producing a DOT CLK output with
negative
spikes. Maybe this is why later revisions introduced an inductor or maybe
it is
just a bad DC011 or something is wrong with what is around the DC011.
I am going to look at the DC011 inputs to see if anything looks amiss. The
other thing I would like to do is to see what happens if I can add an
inductor
to the output of the DC011. But I don't know how to read the spec on the
printset to know what I might need. The printset says "BEAD, FERRITE
.1380DX.380LG", can anyone tell me what this might mean in terms of an
inductor that I could try?
Thanks
Rob
Ulli
Am Sa., 29. Nov. 2025 um 09:44 Uhr schrieb Henk Gooijen via cctalk <
[email protected]>:
Rob,
I have a few silly thoughts, but maybe they can be useful.
The negative going signal is something I would not be happy with, as
it may damage the IC (over time).
TTL does not like negative voltages on their pins.
Is there between the DC011 (signal generating) and the LS299 (signal
"user") wiring, or are they on the same PCB?
In case of wiring, capacitive coupling may cause negative going
voltages.
Could a clamping diode with an as low as possible Vf (< 0.3 V) help
to reduce the negative voltage? It is not a permanent solution,
because the
DC011 might be a bit unhappy with that diode, but for a test it
might help to draw a conclusion.
If all does not help, in getting the LS299 running cooler, consider
some temperature conductive paste and a small "IC heatsink". Tie the
heatsink onto the IC with a piece of waxed lacing. The IC can
radiate more efficiently the generated heat, so that it can run
cooler. This could drop temperature with some 10 degrees.
Success hunting down this issue,
Henk
-----Oorspronkelijk bericht-----
Van: Rob Jarratt via cctalk<[email protected]>
Verzonden: zaterdag 29 november 2025 08:11
Aan: 'Doug Jackson'<[email protected]>;[email protected]
CC: 'General Discussion: On-Topic and Off-Topic Posts' <
[email protected]>; Rob Jarratt <[email protected]>
Onderwerp: [cctalk] Re: Hot Video Shift Register on VT100
Yes, it is a negative voltage. I measured this by attaching the
ground lead to the ground connection on the 74S299.
The DC011 that is producing the CLK signal is socketed, but the one
on my
VT102 isnt. I would try swapping them to see what happens, but I
dont really want to touch a fully working board.
Thanks
Rob
From: Doug Jackson<[email protected]>
Sent: 28 November 2025 22:59
To:[email protected]
Cc: General Discussion: On-Topic and Off-Topic Posts <
[email protected]>
Subject: Re: [cctalk] Re: Hot Video Shift Register on VT100
Is that a negative excursion at the bottom of the waveform? now low
- looks like -0.3v to me - With '"logic, that will likely turn on
the protection diodes on the inputs, making them warm.
Double check where the ground actually is on the CRO display.
It's also interesting that the input doesn't even reach 4v, let
alone
5v.
Whats driving that?
Kindest regards,
Doug Jackson
em:[email protected] <mailto:[email protected]>
ph: 0414 986878
On Sat, 29 Nov 2025 at 08:52, Rob Jarratt
<[email protected] <mailto:[email protected]> >
wrote:
I checked the CLK input for the 74S299 with the 74S299 removed and
the signal is still spiky and looks like this:
https://rjarratt.wordpress.com/wp-content/uploads/2025/11/vt102-74s2
99
-clk-signal.png
I removed the other load on this signal, the DC012, and the CLK
signal still looked spiky. I also put back the 74S299 (now socketed)
with
DC012 removed and it still gets hot. I tried another new 74S299 just
in case I had a bad one and that also got hot without a load on the
output.
I am thinking that it is either the spiky input or it is just
expected to get this hot.
Regards
Rob
From: Doug Jackson <[email protected] <mailto:[email protected]> >
Sent: 25 November 2025 01:15
To:[email protected] <mailto:[email protected]>
Cc: General Discussion: On-Topic and Off-Topic Posts <
[email protected] <mailto:[email protected]> >
Subject: Re: [cctalk] Re: Hot Video Shift Register on VT100
The oscilloscope will show you the actual driving waveform.
You can then see the level of the spikes
Kindest regards,
Doug Jackson
em:[email protected] <mailto:[email protected]>
ph: 0414 986878
On Tue, 25 Nov 2025 at 10:48, Rob Jarratt
<[email protected] <mailto:[email protected]> >
wrote:
-----Original Message-----
From: Doug Jackson via cctalk <[email protected] <mailto:
[email protected]> >
Sent: 24 November 2025 23:04
To: General Discussion: On-Topic and Off-Topic Posts <
[email protected] <mailto:[email protected]> >
Cc: Doug Jackson <[email protected] <mailto:[email protected]> >
Subject: [cctalk] Re: Hot Video Shift Register on VT100
Another test would be to disconnect the output and see if that
runs the device cooler. Maybe the load, while not a short, is
simply too great
for the
device.
I think I can try this, the DC012 it drives is socketed.
But I am leaning towards the inputs containing spikes that need to
be dissipated as heat in clamping diodes. A decent CRO on the
device input
pins
while it is out of circuit will show that as well.
I guess CRO means Cathode Ray Oscilloscope? Not sure what you are
suggesting I try here?
Kindest regards,
Doug Jackson
em:[email protected] <mailto:[email protected]>
ph: 0414 986878
On Tue, 25 Nov 2025 at 09:52, Peter Coghlan via cctalk <
[email protected] <mailto:[email protected]> > wrote:
Sorry if I wasn't clear in my earlier reply, but there were
some answers inline. The summary is that Vcc looks fine, there
does not seem to be any short on the outputs of the shift
register and the chip seems to be
working
correctly. It just gets hot and I wonder if this could be due
to the CLK input being a bit spikier on the VT100 than on my
VT102?
Could this be
why
later revisions of the VT100 introduced an inductor on the DOT
CLK output from the DC011?
Perhaps one way to test the hypothesis would be to insert an
inductor like those used on the later revisions in the DOT CLK
output and check whether
a) everything still works and b) the shift register runs cooler?
Regards,
Peter Coghlan.
------------------------------
Message: 14
Date: Sun, 30 Nov 2025 16:23:44 -0000
From: "Rob Jarratt"<[email protected]>
Subject: [cctalk] Re: Hot Video Shift Register on VT100
To: "'General Discussion: On-Topic and Off-Topic Posts'"
<[email protected]>
Cc: 'Charles Morris'<[email protected]>
Message-ID:<[email protected]>
Content-Type: text/plain; charset="UTF-8"
-----Original Message-----
From: Charles Morris via cctalk<[email protected]>
Sent: 30 November 2025 15:24
To:[email protected]
Cc: Charles Morris<[email protected]>
Subject: [cctalk] Re: Hot Video Shift Register on VT100
Keep in mind that ringing (artifact caused by the inductance of the scope
probe ground lead) can result in a false display of overshoots...
How short is your ground lead?
Quite short. But what I saw is the exact same shape as the input to the chip
that gets hot and measuring the same chip on a VT102 I don't see this. So I am
fairly sure this is real.
-Charles
On 11/30/25 08:30, Rob Jarratt via cctalk wrote:
I have traced the clock signal back. The MAS CLK input on the DC011
also has the negative spikes. This is coming from a 74S157. The input
to that is an oscillator and that too has the negative spikes.
So, it seems the 74S157 is just passing through the inputs.
There is a capacitor across pins 4 and 2 of the oscillator (Vcc and
Gnd), would a failure there cause this kind of negative spike? It
seems unlikely to me, but would be happy to be corrected.
The oscillator is rated at 24.0734MHz. I can find lots rated at
exactly 24MHz and I guess the difference in frequency is small enough
not to matter, right?
I will order the part and also a new capacitor for it in case that is
where the fault lies and see.
Thanks
Rob
-----Original Message-----
From: Rob Jarratt via cctalk<[email protected]>
Sent: 30 November 2025 12:57
To: 'General Discussion: On-Topic and Off-Topic Posts'
<[email protected]>
Cc: 'Hans-Ulrich Hölscher'<[email protected]>; Rob Jarratt
<[email protected]>
Subject: [cctalk] Re: Hot Video Shift Register on VT100
-----Original Message-----
From: Hans-Ulrich Hölscher via cctalk<[email protected]>
Sent: 29 November 2025 09:17
To: General Discussion: On-Topic and Off-Topic Posts
<[email protected]>
Cc: Hans-Ulrich Hölscher<[email protected]>
Subject: [cctalk] Re: Hot Video Shift Register on VT100
Hi Rob,
this is not intended to indicate the cause or solution, but merely
to help analyze how critical your IC temperature is: Did you
actually measure the temperature of the IC? I recently bought an
inexpensive infrared thermometer
(like:https://www.ebay.co.uk/itm/156810933790)
that can measure the temperature of small areas relatively accurately.
I made the experience that I cannot judge temperatures very well
with my
fingers.
According to Wikipedia, TTL ICs are specified for temperatures up to
70 °C. This
temperature should obviously be avoided, but my fingers already
perceive anything above about 50 °C as "hot".
I hope this can help a litte!
Yes, I have an IR Thermometer and the chip gets to about 40 celsius.
I
realise
that this is well below the maximum rating, but I can smell the heat
and
it
worries me a bit. If I knew this was expected then I would be OK with
it,
but it
feels anomalous and I have felt the chip to be hotter than this sometimes.
The observation about the negative voltages also concerns me a bit,
although
I do have spares for the 74S299 should I need them.
It looks now like the DC011 is just producing a DOT CLK output with
negative
spikes. Maybe this is why later revisions introduced an inductor or
maybe
it is
just a bad DC011 or something is wrong with what is around the DC011.
I am going to look at the DC011 inputs to see if anything looks
amiss. The other thing I would like to do is to see what happens if I
can add an
inductor
to the output of the DC011. But I don't know how to read the spec on
the printset to know what I might need. The printset says "BEAD,
FERRITE .1380DX.380LG", can anyone tell me what this might mean in
terms of an inductor that I could try?
Thanks
Rob
Ulli
Am Sa., 29. Nov. 2025 um 09:44 Uhr schrieb Henk Gooijen via cctalk <
[email protected]>:
Rob,
I have a few silly thoughts, but maybe they can be useful.
The negative going signal is something I would not be happy with,
as it may damage the IC (over time).
TTL does not like negative voltages on their pins.
Is there between the DC011 (signal generating) and the LS299
(signal
"user") wiring, or are they on the same PCB?
In case of wiring, capacitive coupling may cause negative going
voltages.
Could a clamping diode with an as low as possible Vf (< 0.3 V) help
to reduce the negative voltage? It is not a permanent solution,
because the
DC011 might be a bit unhappy with that diode, but for a test it
might help to draw a conclusion.
If all does not help, in getting the LS299 running cooler, consider
some temperature conductive paste and a small "IC heatsink". Tie
the heatsink onto the IC with a piece of waxed lacing. The IC can
radiate more efficiently the generated heat, so that it can run
cooler. This could drop temperature with some 10 degrees.
Success hunting down this issue,
Henk
-----Oorspronkelijk bericht-----
Van: Rob Jarratt via cctalk<[email protected]>
Verzonden: zaterdag 29 november 2025 08:11
Aan: 'Doug Jackson'<[email protected]>;[email protected]
CC: 'General Discussion: On-Topic and Off-Topic Posts' <
[email protected]>; Rob Jarratt <[email protected]>
Onderwerp: [cctalk] Re: Hot Video Shift Register on VT100
Yes, it is a negative voltage. I measured this by attaching the
ground lead to the ground connection on the 74S299.
The DC011 that is producing the CLK signal is socketed, but the one
on my
VT102 isn’t. I would try swapping them to see what happens, but I
don’t really want to touch a fully working board.
Thanks
Rob
From: Doug Jackson<[email protected]>
Sent: 28 November 2025 22:59
To:[email protected]
Cc: General Discussion: On-Topic and Off-Topic Posts <
[email protected]>
Subject: Re: [cctalk] Re: Hot Video Shift Register on VT100
Is that a negative excursion at the bottom of the waveform? now
low
- looks like -0.3v to me - With '"logic, that will likely turn on
the protection diodes on the inputs, making them warm.
Double check where the ground actually is on the CRO display.
It's also interesting that the input doesn't even reach 4v, let
alone
5v.
Whats driving that?
Kindest regards,
Doug Jackson
em:[email protected] <mailto:[email protected]>
ph: 0414 986878
On Sat, 29 Nov 2025 at 08:52, Rob Jarratt
<[email protected] <mailto:[email protected]> >
wrote:
I checked the CLK input for the 74S299 with the 74S299 removed and
the signal is still spiky and looks like this:
https://rjarratt.wordpress.com/wp-content/uploads/2025/11/vt102-74s
2
99
-clk-signal.png
I removed the other load on this signal, the DC012, and the CLK
signal still looked spiky. I also put back the 74S299 (now
socketed) with
DC012 removed and it still gets hot. I tried another new 74S299
just in case I had a bad one and that also got hot without a load
on the
output.
I am thinking that it is either the spiky input or it is just
expected to get this hot.
Regards
Rob
From: Doug Jackson <[email protected] <mailto:[email protected]>
Sent: 25 November 2025 01:15
To:[email protected] <mailto:[email protected]>
Cc: General Discussion: On-Topic and Off-Topic Posts <
[email protected] <mailto:[email protected]> >
Subject: Re: [cctalk] Re: Hot Video Shift Register on VT100
The oscilloscope will show you the actual driving waveform.
You can then see the level of the spikes
Kindest regards,
Doug Jackson
em:[email protected] <mailto:[email protected]>
ph: 0414 986878
On Tue, 25 Nov 2025 at 10:48, Rob Jarratt
<[email protected] <mailto:[email protected]> >
wrote:
-----Original Message-----
From: Doug Jackson via cctalk <[email protected] <mailto:
[email protected]> >
Sent: 24 November 2025 23:04
To: General Discussion: On-Topic and Off-Topic Posts <
[email protected] <mailto:[email protected]> >
Cc: Doug Jackson <[email protected] <mailto:[email protected]> >
Subject: [cctalk] Re: Hot Video Shift Register on VT100
Another test would be to disconnect the output and see if that
runs the device cooler. Maybe the load, while not a short, is
simply too great
for the
device.
I think I can try this, the DC012 it drives is socketed.
But I am leaning towards the inputs containing spikes that need to
be dissipated as heat in clamping diodes. A decent CRO on the
device input
pins
while it is out of circuit will show that as well.
I guess CRO means Cathode Ray Oscilloscope? Not sure what you are
suggesting I try here?
Kindest regards,
Doug Jackson
em:[email protected] <mailto:[email protected]>
ph: 0414 986878
On Tue, 25 Nov 2025 at 09:52, Peter Coghlan via cctalk <
[email protected] <mailto:[email protected]> > wrote:
Sorry if I wasn't clear in my earlier reply, but there were some
answers inline. The summary is that Vcc looks fine, there does
not seem to be any short on the outputs of the shift register
and the chip seems to be
working
correctly. It just gets hot and I wonder if this could be due to
the CLK input being a bit spikier on the VT100 than on my
VT102?
Could this be
why
later revisions of the VT100 introduced an inductor on the DOT
CLK output from the DC011?
Perhaps one way to test the hypothesis would be to insert an
inductor like those used on the later revisions in the DOT CLK
output and check whether
a) everything still works and b) the shift register runs cooler?
Regards,
Peter Coghlan.
