Resurrecting a very old (2011) thread about PPS-4/1 - I'm looking for
data sheet on the mm76E - the pinout in particular - as I am trying to
sketch out the schematic of a home pinball game that used the MM78 CPU
and the MM76E for memory.
Bitsavers and everyone else has the sheet for the MM76EL which is the
40-pin version of the original spider chip and those pages talk about
how the EL does not require as fussy a voltage for Vss.
Anyway, if anyone has this data I'd appreciate it!
Thanks,
John :-#)#
On 2011-10-31 1:29 a.m., Eric Smith wrote:
> Benjamin Sølberg wrote:
> > So if you multiplex 8 leds you need 8 times the peak current to keep
> the
> > same luminance ?
>
> 8:1 multiplex requires 8 times the peak current to get the same
> average power. Years ago I saw some claims that with multiplexing you
> can use lower average power for the same perceptual brightness, due to
> retinal persistence. However, I've seen other people claim that this
> is not true. It would be entertaining to set up a double-blind test
> and find out.
>
> Too late now, but those keyboards were actually standard Keytronics
> ones, and not at all hard to repair.
>
> -tony
>
I'm fairly certain that the MG-1 I have is one of the Stern Hall machines
mentioned earlier.
But in re "standard Key Tronic"... The one here is certainly a
foam-and-foil capacitive device like a Key Tronic keyboard is, and surely
the mechanism is basically identical (as are its repair methods). It still
works --- for now. But the pads aren't little round tablets like the usual
KT fare but instead squares with one of the corners notched out (think: the
shape of Utah, but squarer). I do not look forward to those pads degrading
further and expect it may be necessary to get some kind of custom die punch
fabricated in order to produce replacements.
I can't easily retrieve the name of the keyboard manufacturer at the
moment, but I wonder if anyone knows more about this odd shape of
capacitive pad.
--T
>
Hi there!
Long shot here, I know, but does anyone on here have an IBM SP node, especially an earlier (non Power 3) Thin Node with MCA? But really any SP owners here at all?
Thanks!!
-Ben
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 🙂
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
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
I have a Connectix camera with a small 8-pin mini-DIN connector. The
pin placement looks like RS-232. All it says on the label is that the
FCC id is LKD1. The only thing I found about it online called its a
Macintosh camera. Logitech bought the Quick Cam from Connectix in 1998
but I can't find much more about it, or even if it's the one with FFF
id LKD1. Do you want it?
With all the talk going on, I ordered some ButtonWorx 4mm keypad
repair buttons to fix my old Technics stereo rubber dome remote and it
seems to work good as new. Now I just have to replace some capacitors
in the receiver itself.
-----------------------------------------From: "Daniel L via cctalk"
To: "General Discussion: On-Topic and Off-Topic Posts"
Cc: "Daniel L"
Sent: Sunday November 30 2025 5:40:55AM
Subject: [cctalk] Re: Conductive material in rubber dome
Well folks, it was a false alarm. My wife was looking at my open
switches and told me to zoom in with my camera. It turned out that it
wasn't the dome itself. The dome had left just enough residue on the
switch contacts to prevent consistent usage. So I sprayed excess
contact cleaner and a qtip and cleaned it vigorously. Voila, all three
switches work perfectly.
It took a better part of five hours to do all of them. I figure this
would likely happen again. And I was concerned the self adhesive
version would break off during use but, no. The material in the domes
is self adhesive. The dome material was also cast to be quite flat.
I didn't have to use any of the new bits since it's still in shipping
from amazon. But, I have them in stock because there are a number of
100s I will be refurbishing for resale at some point.
I'm glad the wife looked at it with her eagle eyes. I would've torn
off the original stuff and it wouldn't have worked. I would have
kicked myself.
I also replaced the leaky battery on the mobo so she's ready for
modifications! Looking forward for this project.
Daniel
sysop Air & Wave BBS
finger calcmandan(a)bbs.erb.pw
On November 26, 2025 5:04:11 AM PST, me--- via cctalk wrote:
>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
I am playing around with VMS 5.4 on a simulated microvax 3100 and want to
get decnet running.
The install media I have found is for decnet 4.0 end-node but it doesn't
install. Vmsinstal complains
about symbols not being defined and I am guessing I have the wrong version
for VMS 5.4.
Does anyone know of a decnet version that will work on VMS 5.4? I can't
seem to find one.
Thanks,
- Peter
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.
>>>>>>>>
>>>>>>
