November 2025 - cctalk - classiccmp.org

by Murray McCullough

One of the greatest joys of classic computing was running what you wanted on your own computer. What has happened in the intervening years? Have ‘software walls’ created a computing environment that benefits software gate-keepers(owners of computing technology) by monopolizing creativity, freedom to program and establishing a defacto ‘true ownership’. Will the future be this or will it be more like the earliest years of microcomputing? Murray 🙂

6 days, 10 hours

26
51
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Classic computing - earliest years

by Murray McCullough

Hi everyone, According to historians, and I consider myself one, let us consider what classic/vintage computers were: The 1970s saw the three amigos: Apple II, TRS-80 and Commodore PET and the OS was DOS and its ilk + CP/M. The 1980’s saw the Dells, HPs and many others with MS-DOS & IBM PC-DOS from QDOS. We saw this and behold ’bring on the clones’(I just had to say this!) The era of old computers saw one generation building on the shoulders of giants who designed these wayback computers(with apologies to Wayback Machine). Today’s PCs and ARM machines are just the latest iteration of this theory(by the way not mine). Happy computing Murray 🙂

1 week, 3 days

36
159
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OCR and line printers

by Guy Fedorkow

Greetings Restorers, I think a number of us have wanted to restore software that's only available as a scanned listing from a line printer. The original printout probably wasn't the best typographic quality, and scanning doesn't improve it. As a first pass, OCR with tools like Adobe Acrobat can easily produce a rough draft of the content in text form, but it takes almost as much work to correct the many "typos" as it does to simply re-type the listing. It seems like, with all this high-tech AI processing around, it should be possible to take advantage of the limited character set, fixed fonts, and restricted grammar that one might find in a listing to resolve more of the ambiguities in character recognition. Does anyone have an approach that's more efficient than generic OCR and a long process of correcting typos on every line of code or comment? Thanks /guy

1 week, 3 days

9
17
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Friden Flexowriter models

by Peter Brown

I have recently acquired a Flexowriter. It looks very much like the one shown on this page - https://moca.ncl.ac.uk/iomedia/pt4.htm The identifying points are - blue color keys - tape punch and reader (both appear to be 8 bits) - white and red lamps on the front panel beneath the Friden logo The serial number plate shows F_V as the model and S P E C as the coding. Is anyone familiar with what this model is? What does the coding refer to? Peter

1 week, 4 days

6
9
0 0

Brainless Message Quoting

by Marvin Johnston

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$5a78ab40$(a)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>: >> >>> 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(a)doughq.com>;rob(a)jarratt.me.uk >>> CC: 'General Discussion: On-Topic and Off-Topic Posts' < >>> cctalk(a)classiccmp.org>; 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(a)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$(a)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>; 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>: >>> >>>> 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(a)doughq.com>;rob(a)jarratt.me.uk >>>> CC: 'General Discussion: On-Topic and Off-Topic Posts' < >>>> cctalk(a)classiccmp.org>; 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(a)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>; 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>: >>>> >>>>> 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(a)doughq.com>;rob(a)jarratt.me.uk >>>>> CC: 'General Discussion: On-Topic and Off-Topic Posts' < >>>>> cctalk(a)classiccmp.org>; 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$181c1230$(a)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>; 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>: >>>>> >>>>>> 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(a)doughq.com>;rob(a)jarratt.me.uk >>>>>> CC: 'General Discussion: On-Topic and Off-Topic Posts' < >>>>>> cctalk(a)classiccmp.org>; 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. >>>>>>>> >>>>>>

1 week, 4 days

4
3
0 0

VAX and OpenVMS

by Peter Ekstrom

Ok, started playing with the SIMH vax8600 simulator and OpenVMS 7.3 Hobbyist. I know they discontinued the hobbyist license program years ago, but does one actually need a license to run it? Or would an older version like 5.x or something be better? The OpenVMS community license is only for Alpha, Itanium and some x86_64 architectures so I'm sure that wouldn't work in a simulated VAX? - Peter

2 weeks

20
34
0 0

Hot Video Shift Register on VT100

by Rob Jarratt

(sending again from a different email address as I don't think my first email got through, apologies if this is a duplicate) I have a VT100 that I was working on a while back and then set aside for a bit. I have started to look at it again. I have noticed that the Video Shift Register, a 74S299, gets very hot, I can smell the heat and the chip gets almost too hot to touch, reaching almost 40 celsius. I have a working VT102 for comparison and the same chip there does not get so hot (it reaches about 30 celsius). I have already tried replacing the chip, but the new one gets equally hot. I looked at the signals the chip is receiving and the one that stands out as different is the CLK input (pin 12). It looks like this: https://rjarratt.wordpress.com/wp-content/uploads/2024/04/vt100-74s299-clk-s ignal.png. On the VT102 it looks like this: https://rjarratt.wordpress.com/wp-content/uploads/2025/11/vt102-74s299-clk-s ignal.png. It is much spikier on the VT100 and I was told at one point that this could be the cause of the hot running for the chip. Is that a reasonable assumption? Assuming the spikes are the cause of the hot running. I am trying to see why there is a difference. I have noticed that on the VT102 there is a 68R resistor between the DC011 which produces the signal and the 74S299. You can see this as R86 in the VT101 printset https://bitsavers.org/pdf/dec/terminal/vt101/MP-01066-00C_VT101_Family_Field _Maintenance_Print_Set_Apr82.pdf (p47 of the PDF). The VT100 printset dated Feb 82 https://bitsavers.org/pdf/dec/terminal/vt100/MP00633_VT100_Schematic_Feb82.p df shows an inductor L8 being used (PDF p17), but my VT100 does not have this and so must be described by the March 80 printset https://bitsavers.org/pdf/dec/terminal/vt100/MP00633_VT100_Mar80.pdf (PDF p17). Could the absence of L8 explain the spikier DOT CLK signal and the hotter 74S299? Incidentally, I suspect that the flyback transformer on my VT100 has failed. If anyone has a flyback transformer going spare, especially in the UK, then I would love to hear from you. Thanks Rob

2 weeks

12
22
0 0

Conductive material in rubber dome

by me＠scifidan.com

I just acquired a TRS-80 model 102, my first of that model. Everything is great other than three keys. The 2 keys works intermittently, q, and [ don't work. Having checked the schematic, the keys have no commonality on the circuit. Before I pull the cap off the switch, I know that the rubber dome inside has material on the inner part that meets with the silver contacts to complete the circuit. I'm told this material will wear out or lose conductivity. I reflowed the solder joints on the pcb and this didn't help. Other people have simply replaced the rubber dome from another dead 102's keyboard. But, I'd rather attempt applying new material inside that dome. I'm hoping there is an inexpensive and mainstream solution I can buy at home depot or something. Any tips? Daniel sysop | Air & Wave BBS finger | calcmandan(a)bbs.erb.pw

2 weeks, 1 day