30 Nov
2025
30 Nov
'25
2:40 p.m.
Rob,
What is behind this stupidity of quoting the entire listserver replies
(well, it seems like it anyway) on your messages????? Quoted below is
what I am talking about. Maybe it is past time for you to learn about
editing!
Marvin
> Message: 11
> Date: Sun, 30 Nov 2025 12:56:45 -0000
> From: "Rob Jarratt"<robert.jarratt(a)ntlworld.com>
> Subject: [cctalk] Re: Hot Video Shift Register on VT100
> To: "'General Discussion: On-Topic and Off-Topic Posts'"
> <cctalk(a)classiccmp.org>
> Cc: 'Hans-Ulrich Hölscher'<vaxorcist(a)googlemail.com>
> Message-ID:<013e01dc61f8$c8d2e3c0$5a78ab40$@ntlworld.com>
> Content-Type: text/plain; charset="iso-8859-1"
>
>> -----Original Message-----
>> From: Hans-Ulrich Hölscher via cctalk<cctalk(a)classiccmp.org>
>> Sent: 29 November 2025 09:17
>> To: General Discussion: On-Topic and Off-Topic Posts
> <cctalk(a)classiccmp.org>
>> Cc: Hans-Ulrich Hölscher<vaxorcist(a)googlemail.com>
>> 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 <
>> cctalk(a)classiccmp.org>gt;:
>>
>>> 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<cctalk(a)classiccmp.org>
>>> Verzonden: zaterdag 29 november 2025 08:11
>>> Aan: 'Doug Jackson'<doug@doughq.com>;rob@jarratt.me.uk
>>> CC: 'General Discussion: On-Topic and Off-Topic Posts' <
>>> cctalk(a)classiccmp.org>gt;; Rob Jarratt <robert.jarratt(a)ntlworld.com>
>>> 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<doug(a)doughq.com>
>>> Sent: 28 November 2025 22:59
>>> To:rob@jarratt.me.uk
>>> Cc: General Discussion: On-Topic and Off-Topic Posts <
>>> cctalk(a)classiccmp.org>
>>> 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:doug@doughq.com <mailto:doug@doughq.com>
>>>
>>> ph: 0414 986878
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>> On Sat, 29 Nov 2025 at 08:52, Rob Jarratt <robert.jarratt(a)ntlworld.com
>>> <mailto:robert.jarratt@ntlworld.com> > 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 <doug(a)doughq.com <mailto:doug@doughq.com> >
>>> Sent: 25 November 2025 01:15
>>> To:rob@jarratt.me.uk <mailto:rob@jarratt.me.uk>
>>> Cc: General Discussion: On-Topic and Off-Topic Posts <
>>> cctalk(a)classiccmp.org <mailto:cctalk@classiccmp.org> >
>>> 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:doug@doughq.com <mailto:doug@doughq.com>
>>>
>>> ph: 0414 986878
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>> On Tue, 25 Nov 2025 at 10:48, Rob Jarratt <robert.jarratt(a)ntlworld.com
>>> <mailto:robert.jarratt@ntlworld.com> > wrote:
>>>
>>>
>>>
>>>> -----Original Message-----
>>>> From: Doug Jackson via cctalk <cctalk(a)classiccmp.org <mailto:
>>> cctalk(a)classiccmp.org> >
>>>> Sent: 24 November 2025 23:04
>>>> To: General Discussion: On-Topic and Off-Topic Posts <
>>> cctalk(a)classiccmp.org <mailto:cctalk@classiccmp.org> >
>>>> Cc: Doug Jackson <doug(a)doughq.com <mailto:doug@doughq.com>
>
>>>> 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:doug@doughq.com <mailto:doug@doughq.com>
>>>> ph: 0414 986878
>>>>
>>>>
>>>>
>>>>
>>>> On Tue, 25 Nov 2025 at 09:52, Peter Coghlan via cctalk <
>>>> cctalk(a)classiccmp.org <mailto:cctalk@classiccmp.org> > 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"<robert.jarratt(a)ntlworld.com>
> Subject: [cctalk] Re: Hot Video Shift Register on VT100
> To:<rob@jarratt.me.uk>, "'General Discussion: On-Topic and
> Off-Topic Posts'"<cctalk(a)classiccmp.org>
> Cc: 'Hans-Ulrich Hölscher'<vaxorcist(a)googlemail.com>
> Message-ID:<014901dc6205$efd87090$cf8951b0$@ntlworld.com>
> 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<cctalk(a)classiccmp.org>
>> Sent: 30 November 2025 12:57
>> To: 'General Discussion: On-Topic and Off-Topic Posts'
>> <cctalk(a)classiccmp.org>
>> Cc: 'Hans-Ulrich Hölscher'<vaxorcist(a)googlemail.com>il.com>; Rob Jarratt
>> <robert.jarratt(a)ntlworld.com>
>> Subject: [cctalk] Re: Hot Video Shift Register on VT100
>>
>>> -----Original Message-----
>>> From: Hans-Ulrich Hölscher via cctalk<cctalk(a)classiccmp.org>
>>> Sent: 29 November 2025 09:17
>>> To: General Discussion: On-Topic and Off-Topic Posts
>> <cctalk(a)classiccmp.org>
>>> Cc: Hans-Ulrich Hölscher<vaxorcist(a)googlemail.com>
>>> 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 <
>>> cctalk(a)classiccmp.org>gt;:
>>>
>>>> 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<cctalk(a)classiccmp.org>
>>>> Verzonden: zaterdag 29 november 2025 08:11
>>>> Aan: 'Doug Jackson'<doug@doughq.com>;rob@jarratt.me.uk
>>>> CC: 'General Discussion: On-Topic and Off-Topic Posts' <
>>>> cctalk(a)classiccmp.org>gt;; Rob Jarratt
<robert.jarratt(a)ntlworld.com>
>>>> 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<doug(a)doughq.com>
>>>> Sent: 28 November 2025 22:59
>>>> To:rob@jarratt.me.uk
>>>> Cc: General Discussion: On-Topic and Off-Topic Posts <
>>>> cctalk(a)classiccmp.org>
>>>> 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:doug@doughq.com <mailto:doug@doughq.com>
>>>>
>>>> ph: 0414 986878
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>> On Sat, 29 Nov 2025 at 08:52, Rob Jarratt
>>>> <robert.jarratt(a)ntlworld.com
<mailto:robert.jarratt@ntlworld.com> >
>> 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 <doug(a)doughq.com <mailto:doug@doughq.com>
>
>>>> Sent: 25 November 2025 01:15
>>>> To:rob@jarratt.me.uk <mailto:rob@jarratt.me.uk>
>>>> Cc: General Discussion: On-Topic and Off-Topic Posts <
>>>> cctalk(a)classiccmp.org <mailto:cctalk@classiccmp.org> >
>>>> 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:doug@doughq.com <mailto:doug@doughq.com>
>>>>
>>>> ph: 0414 986878
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>> On Tue, 25 Nov 2025 at 10:48, Rob Jarratt
>>>> <robert.jarratt(a)ntlworld.com
<mailto:robert.jarratt@ntlworld.com> >
>> wrote:
>>>>
>>>>
>>>>> -----Original Message-----
>>>>> From: Doug Jackson via cctalk <cctalk(a)classiccmp.org <mailto:
>>>> cctalk(a)classiccmp.org> >
>>>>> Sent: 24 November 2025 23:04
>>>>> To: General Discussion: On-Topic and Off-Topic Posts <
>>>> cctalk(a)classiccmp.org <mailto:cctalk@classiccmp.org> >
>>>>> Cc: Doug Jackson <doug(a)doughq.com <mailto:doug@doughq.com>
>
>>>>> 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:doug@doughq.com <mailto:doug@doughq.com>
>>>>> ph: 0414 986878
>>>>>
>>>>>
>>>>>
>>>>>
>>>>> On Tue, 25 Nov 2025 at 09:52, Peter Coghlan via cctalk <
>>>>> cctalk(a)classiccmp.org <mailto:cctalk@classiccmp.org> >
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<charlesmorris800(a)protonmail.com>
> Subject: [cctalk] Re: Hot Video Shift Register on VT100
> To:cctalk@classiccmp.org
> Message-ID:<bf019ff7-a240-4ec5-b6a3-749e6cd9b3af@protonmail.com>
> 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<cctalk(a)classiccmp.org>
>>> Sent: 30 November 2025 12:57
>>> To: 'General Discussion: On-Topic and Off-Topic Posts'
>>> <cctalk(a)classiccmp.org>
>>> Cc: 'Hans-Ulrich Hölscher'<vaxorcist(a)googlemail.com>il.com>; Rob
Jarratt
>>> <robert.jarratt(a)ntlworld.com>
>>> Subject: [cctalk] Re: Hot Video Shift Register on VT100
>>>
>>>> -----Original Message-----
>>>> From: Hans-Ulrich Hölscher via cctalk<cctalk(a)classiccmp.org>
>>>> Sent: 29 November 2025 09:17
>>>> To: General Discussion: On-Topic and Off-Topic Posts
>>> <cctalk(a)classiccmp.org>
>>>> Cc: Hans-Ulrich Hölscher<vaxorcist(a)googlemail.com>
>>>> 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 <
>>>> cctalk(a)classiccmp.org>gt;:
>>>>
>>>>> 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<cctalk(a)classiccmp.org>
>>>>> Verzonden: zaterdag 29 november 2025 08:11
>>>>> Aan: 'Doug Jackson'<doug@doughq.com>;rob@jarratt.me.uk
>>>>> CC: 'General Discussion: On-Topic and Off-Topic Posts' <
>>>>> cctalk(a)classiccmp.org>gt;; Rob Jarratt
<robert.jarratt(a)ntlworld.com>
>>>>> 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<doug(a)doughq.com>
>>>>> Sent: 28 November 2025 22:59
>>>>> To:rob@jarratt.me.uk
>>>>> Cc: General Discussion: On-Topic and Off-Topic Posts <
>>>>> cctalk(a)classiccmp.org>
>>>>> 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:doug@doughq.com <mailto:doug@doughq.com>
>>>>>
>>>>> ph: 0414 986878
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>> On Sat, 29 Nov 2025 at 08:52, Rob Jarratt
>>>>> <robert.jarratt(a)ntlworld.com
<mailto:robert.jarratt@ntlworld.com> >
>>> 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 <doug(a)doughq.com
<mailto:doug@doughq.com> >
>>>>> Sent: 25 November 2025 01:15
>>>>> To:rob@jarratt.me.uk <mailto:rob@jarratt.me.uk>
>>>>> Cc: General Discussion: On-Topic and Off-Topic Posts <
>>>>> cctalk(a)classiccmp.org <mailto:cctalk@classiccmp.org> >
>>>>> 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:doug@doughq.com <mailto:doug@doughq.com>
>>>>>
>>>>> ph: 0414 986878
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>> On Tue, 25 Nov 2025 at 10:48, Rob Jarratt
>>>>> <robert.jarratt(a)ntlworld.com
<mailto:robert.jarratt@ntlworld.com> >
>>> wrote:
>>>>>
>>>>>> -----Original Message-----
>>>>>> From: Doug Jackson via cctalk <cctalk(a)classiccmp.org
<mailto:
>>>>> cctalk(a)classiccmp.org> >
>>>>>> Sent: 24 November 2025 23:04
>>>>>> To: General Discussion: On-Topic and Off-Topic Posts <
>>>>> cctalk(a)classiccmp.org <mailto:cctalk@classiccmp.org> >
>>>>>> Cc: Doug Jackson <doug(a)doughq.com
<mailto:doug@doughq.com> >
>>>>>> 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:doug@doughq.com <mailto:doug@doughq.com>
>>>>>> ph: 0414 986878
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>> On Tue, 25 Nov 2025 at 09:52, Peter Coghlan via cctalk <
>>>>>> cctalk(a)classiccmp.org <mailto:cctalk@classiccmp.org> >
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"<robert.jarratt(a)ntlworld.com>
> Subject: [cctalk] Re: Hot Video Shift Register on VT100
> To: "'General Discussion: On-Topic and Off-Topic Posts'"
> <cctalk(a)classiccmp.org>
> Cc: 'Charles Morris'<charlesmorris800(a)protonmail.com>
> Message-ID:<014a01dc6215$b2b40610$181c1230$@ntlworld.com>
> Content-Type: text/plain; charset="UTF-8"
>
>
>
>> -----Original Message-----
>> From: Charles Morris via cctalk<cctalk(a)classiccmp.org>
>> Sent: 30 November 2025 15:24
>> To:cctalk@classiccmp.org
>> Cc: Charles Morris<charlesmorris800(a)protonmail.com>
>> 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<cctalk(a)classiccmp.org>
>>>> Sent: 30 November 2025 12:57
>>>> To: 'General Discussion: On-Topic and Off-Topic Posts'
>>>> <cctalk(a)classiccmp.org>
>>>> Cc: 'Hans-Ulrich Hölscher'<vaxorcist(a)googlemail.com>il.com>; Rob
Jarratt
>>>> <robert.jarratt(a)ntlworld.com>
>>>> Subject: [cctalk] Re: Hot Video Shift Register on VT100
>>>>
>>>>> -----Original Message-----
>>>>> From: Hans-Ulrich Hölscher via cctalk<cctalk(a)classiccmp.org>
>>>>> Sent: 29 November 2025 09:17
>>>>> To: General Discussion: On-Topic and Off-Topic Posts
>>>> <cctalk(a)classiccmp.org>
>>>>> Cc: Hans-Ulrich Hölscher<vaxorcist(a)googlemail.com>
>>>>> 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
<
>>>>> cctalk(a)classiccmp.org>gt;:
>>>>>
>>>>>> 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<cctalk(a)classiccmp.org>
>>>>>> Verzonden: zaterdag 29 november 2025 08:11
>>>>>> Aan: 'Doug
Jackson'<doug@doughq.com>;rob@jarratt.me.uk
>>>>>> CC: 'General Discussion: On-Topic and Off-Topic Posts'
<
>>>>>> cctalk(a)classiccmp.org>gt;; Rob Jarratt
<robert.jarratt(a)ntlworld.com>
>>>>>> 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<doug(a)doughq.com>
>>>>>> Sent: 28 November 2025 22:59
>>>>>> To:rob@jarratt.me.uk
>>>>>> Cc: General Discussion: On-Topic and Off-Topic Posts <
>>>>>> cctalk(a)classiccmp.org>
>>>>>> 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:doug@doughq.com <mailto:doug@doughq.com>
>>>>>>
>>>>>> ph: 0414 986878
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>> On Sat, 29 Nov 2025 at 08:52, Rob Jarratt
>>>>>> <robert.jarratt(a)ntlworld.com
<mailto:robert.jarratt@ntlworld.com> >
>>>> 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 <doug(a)doughq.com
<mailto:doug@doughq.com>
>>>>>> Sent: 25 November 2025 01:15
>>>>>> To:rob@jarratt.me.uk <mailto:rob@jarratt.me.uk>
>>>>>> Cc: General Discussion: On-Topic and Off-Topic Posts <
>>>>>> cctalk(a)classiccmp.org <mailto:cctalk@classiccmp.org> >
>>>>>> 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:doug@doughq.com <mailto:doug@doughq.com>
>>>>>>
>>>>>> ph: 0414 986878
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>> On Tue, 25 Nov 2025 at 10:48, Rob Jarratt
>>>>>> <robert.jarratt(a)ntlworld.com
<mailto:robert.jarratt@ntlworld.com> >
>>>> wrote:
>>>>>>
>>>>>>> -----Original Message-----
>>>>>>> From: Doug Jackson via cctalk <cctalk(a)classiccmp.org
<mailto:
>>>>>> cctalk(a)classiccmp.org> >
>>>>>>> Sent: 24 November 2025 23:04
>>>>>>> To: General Discussion: On-Topic and Off-Topic Posts <
>>>>>> cctalk(a)classiccmp.org <mailto:cctalk@classiccmp.org> >
>>>>>>> Cc: Doug Jackson <doug(a)doughq.com
<mailto:doug@doughq.com> >
>>>>>>> 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:doug@doughq.com <mailto:doug@doughq.com>
>>>>>>> ph: 0414 986878
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> On Tue, 25 Nov 2025 at 09:52, Peter Coghlan via cctalk <
>>>>>>> cctalk(a)classiccmp.org <mailto:cctalk@classiccmp.org>
> 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.
>>>>>>>>
>>>>>>