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&quot;&lt;robert.jarratt(a)ntlworld.com&gt; > Subject: [cctalk] Re: Hot Video Shift Register on VT100 > To: "'General Discussion: On-Topic and Off-Topic Posts'" > &lt;cctalk(a)classiccmp.org&gt; > Cc: 'Hans-Ulrich Hölscher&#39;&lt;vaxorcist(a)googlemail.com&gt; > Message-ID:<013e01dc61f8$c8d2e3c0$5a78ab40$@ntlworld.com> > Content-Type: text/plain; charset="iso-8859-1" > >> -----Original Message----- >> From: Hans-Ulrich Hölscher via cctalk&lt;cctalk(a)classiccmp.org&gt; >> Sent: 29 November 2025 09:17 >> To: General Discussion: On-Topic and Off-Topic Posts > &lt;cctalk(a)classiccmp.org&gt; >> Cc: Hans-Ulrich Hölscher&lt;vaxorcist(a)googlemail.com&gt; >> 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;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&lt;cctalk(a)classiccmp.org&gt; >>> 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;gt;; Rob Jarratt &lt;robert.jarratt(a)ntlworld.com&gt; >>> 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&lt;doug(a)doughq.com&gt; >>> Sent: 28 November 2025 22:59 >>> To:rob@jarratt.me.uk >>> Cc: General Discussion: On-Topic and Off-Topic Posts < >>> cctalk(a)classiccmp.org&gt; >>> 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 &lt;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 &lt;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 &lt;robert.jarratt(a)ntlworld.com >>> <mailto:robert.jarratt@ntlworld.com> > wrote: >>> >>> >>> >>>> -----Original Message----- >>>> From: Doug Jackson via cctalk &lt;cctalk(a)classiccmp.org <mailto: >>> cctalk(a)classiccmp.org&gt; > >>>> 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 &lt;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&quot;&lt;robert.jarratt(a)ntlworld.com&gt; > Subject: [cctalk] Re: Hot Video Shift Register on VT100 > To:<rob@jarratt.me.uk>, "'General Discussion: On-Topic and > Off-Topic Posts&#39;&quot;&lt;cctalk(a)classiccmp.org&gt; > Cc: 'Hans-Ulrich Hölscher&#39;&lt;vaxorcist(a)googlemail.com&gt; > 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&lt;cctalk(a)classiccmp.org&gt; >> Sent: 30 November 2025 12:57 >> To: 'General Discussion: On-Topic and Off-Topic Posts' >> &lt;cctalk(a)classiccmp.org&gt; >> Cc: 'Hans-Ulrich Hölscher&#39;&lt;vaxorcist(a)googlemail.com&gt;il.com>; Rob Jarratt >> &lt;robert.jarratt(a)ntlworld.com&gt; >> Subject: [cctalk] Re: Hot Video Shift Register on VT100 >> >>> -----Original Message----- >>> From: Hans-Ulrich Hölscher via cctalk&lt;cctalk(a)classiccmp.org&gt; >>> Sent: 29 November 2025 09:17 >>> To: General Discussion: On-Topic and Off-Topic Posts >> &lt;cctalk(a)classiccmp.org&gt; >>> Cc: Hans-Ulrich Hölscher&lt;vaxorcist(a)googlemail.com&gt; >>> 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;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&lt;cctalk(a)classiccmp.org&gt; >>>> 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;gt;; Rob Jarratt &lt;robert.jarratt(a)ntlworld.com&gt; >>>> 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&lt;doug(a)doughq.com&gt; >>>> Sent: 28 November 2025 22:59 >>>> To:rob@jarratt.me.uk >>>> Cc: General Discussion: On-Topic and Off-Topic Posts < >>>> cctalk(a)classiccmp.org&gt; >>>> 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 >>>> &lt;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 &lt;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 >>>> &lt;robert.jarratt(a)ntlworld.com <mailto:robert.jarratt@ntlworld.com> > >> wrote: >>>> >>>> >>>>> -----Original Message----- >>>>> From: Doug Jackson via cctalk &lt;cctalk(a)classiccmp.org <mailto: >>>> cctalk(a)classiccmp.org&gt; > >>>>> 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 &lt;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&lt;charlesmorris800(a)protonmail.com&gt; > 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&lt;cctalk(a)classiccmp.org&gt; >>> Sent: 30 November 2025 12:57 >>> To: 'General Discussion: On-Topic and Off-Topic Posts' >>> &lt;cctalk(a)classiccmp.org&gt; >>> Cc: 'Hans-Ulrich Hölscher&#39;&lt;vaxorcist(a)googlemail.com&gt;il.com>; Rob Jarratt >>> &lt;robert.jarratt(a)ntlworld.com&gt; >>> Subject: [cctalk] Re: Hot Video Shift Register on VT100 >>> >>>> -----Original Message----- >>>> From: Hans-Ulrich Hölscher via cctalk&lt;cctalk(a)classiccmp.org&gt; >>>> Sent: 29 November 2025 09:17 >>>> To: General Discussion: On-Topic and Off-Topic Posts >>> &lt;cctalk(a)classiccmp.org&gt; >>>> Cc: Hans-Ulrich Hölscher&lt;vaxorcist(a)googlemail.com&gt; >>>> 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;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&lt;cctalk(a)classiccmp.org&gt; >>>>> 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;gt;; Rob Jarratt &lt;robert.jarratt(a)ntlworld.com&gt; >>>>> 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&lt;doug(a)doughq.com&gt; >>>>> Sent: 28 November 2025 22:59 >>>>> To:rob@jarratt.me.uk >>>>> Cc: General Discussion: On-Topic and Off-Topic Posts < >>>>> cctalk(a)classiccmp.org&gt; >>>>> 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 >>>>> &lt;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 &lt;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 >>>>> &lt;robert.jarratt(a)ntlworld.com <mailto:robert.jarratt@ntlworld.com> > >>> wrote: >>>>> >>>>>> -----Original Message----- >>>>>> From: Doug Jackson via cctalk &lt;cctalk(a)classiccmp.org <mailto: >>>>> cctalk(a)classiccmp.org&gt; > >>>>>> 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 &lt;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&quot;&lt;robert.jarratt(a)ntlworld.com&gt; > Subject: [cctalk] Re: Hot Video Shift Register on VT100 > To: "'General Discussion: On-Topic and Off-Topic Posts'" > &lt;cctalk(a)classiccmp.org&gt; > Cc: 'Charles Morris&#39;&lt;charlesmorris800(a)protonmail.com&gt; > Message-ID:<014a01dc6215$b2b40610$181c1230$@ntlworld.com> > Content-Type: text/plain; charset="UTF-8" > > > >> -----Original Message----- >> From: Charles Morris via cctalk&lt;cctalk(a)classiccmp.org&gt; >> Sent: 30 November 2025 15:24 >> To:cctalk@classiccmp.org >> Cc: Charles Morris&lt;charlesmorris800(a)protonmail.com&gt; >> 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&lt;cctalk(a)classiccmp.org&gt; >>>> Sent: 30 November 2025 12:57 >>>> To: 'General Discussion: On-Topic and Off-Topic Posts' >>>> &lt;cctalk(a)classiccmp.org&gt; >>>> Cc: 'Hans-Ulrich Hölscher&#39;&lt;vaxorcist(a)googlemail.com&gt;il.com>; Rob Jarratt >>>> &lt;robert.jarratt(a)ntlworld.com&gt; >>>> Subject: [cctalk] Re: Hot Video Shift Register on VT100 >>>> >>>>> -----Original Message----- >>>>> From: Hans-Ulrich Hölscher via cctalk&lt;cctalk(a)classiccmp.org&gt; >>>>> Sent: 29 November 2025 09:17 >>>>> To: General Discussion: On-Topic and Off-Topic Posts >>>> &lt;cctalk(a)classiccmp.org&gt; >>>>> Cc: Hans-Ulrich Hölscher&lt;vaxorcist(a)googlemail.com&gt; >>>>> 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;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&lt;cctalk(a)classiccmp.org&gt; >>>>>> 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;gt;; Rob Jarratt &lt;robert.jarratt(a)ntlworld.com&gt; >>>>>> 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&lt;doug(a)doughq.com&gt; >>>>>> Sent: 28 November 2025 22:59 >>>>>> To:rob@jarratt.me.uk >>>>>> Cc: General Discussion: On-Topic and Off-Topic Posts < >>>>>> cctalk(a)classiccmp.org&gt; >>>>>> 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 >>>>>> &lt;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 &lt;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 >>>>>> &lt;robert.jarratt(a)ntlworld.com <mailto:robert.jarratt@ntlworld.com> > >>>> wrote: >>>>>> >>>>>>> -----Original Message----- >>>>>>> From: Doug Jackson via cctalk &lt;cctalk(a)classiccmp.org <mailto: >>>>>> cctalk(a)classiccmp.org&gt; > >>>>>>> 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 &lt;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. >>>>>>>> >>>>>>