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