30 Nov
2025
30 Nov
'25
8:30 a.m.
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>om>; Rob Jarratt
<robert.jarratt(a)ntlworld.com>
Subject: [cctalk] Re: Hot Video Shift Register on VT100
realise
-----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 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
VT102?
Ulli
Am Sa., 29. Nov. 2025 um 09:44 Uhr schrieb Henk Gooijen via cctalk <
cctalk(a)classiccmp.org>gt;:
> Rob,
>
> I have a few silly thoughts, but maybe they can be useful.
> The negative going signal is something I would not be happy with, as
> it may damage the IC (over time).
> TTL does not like negative voltages on their pins.
> Is there between the DC011 (signal generating) and the LS299 (signal
> "user") wiring, or are they on the same PCB?
> In case of wiring, capacitive coupling may cause negative going
voltages.
> Could a clamping diode with an as low as
possible Vf (< 0.3 V) help
> to reduce the negative voltage? It is not a permanent solution,
> because the
> DC011 might be a bit unhappy with that diode, but for a test it
> might help to draw a conclusion.
>
> If all does not help, in getting the LS299 running cooler, consider
> some temperature conductive paste and a small "IC heatsink". Tie the
> heatsink onto the IC with a piece of waxed lacing. The IC can
> radiate more efficiently the generated heat, so that it can run
> cooler. This could drop temperature with some 10 degrees.
>
> Success hunting down this issue,
> Henk
>
> -----Oorspronkelijk bericht-----
> Van: Rob Jarratt via cctalk <cctalk(a)classiccmp.org>
> Verzonden: zaterdag 29 november 2025 08:11
> Aan: 'Doug Jackson' <doug(a)doughq.com>om>; rob(a)jarratt.me.uk
> CC: 'General Discussion: On-Topic and Off-Topic Posts' <
> cctalk(a)classiccmp.org>gt;; Rob Jarratt <robert.jarratt(a)ntlworld.com>
> Onderwerp: [cctalk] Re: Hot Video Shift Register on VT100
>
> Yes, it is a negative voltage. I measured this by attaching the
> ground lead to the ground connection on the 74S299.
>
>
>
> The DC011 that is producing the CLK signal is socketed, but the one
> on my
> VT102 isnt. I would try swapping them to see what happens, but I
> dont really want to touch a fully working board.
>
>
>
> Thanks
>
>
>
> Rob
>
>
>
> From: Doug Jackson <doug(a)doughq.com>
> Sent: 28 November 2025 22:59
> To: rob(a)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(a)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(a)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(a)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(a)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
> > > > > 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.
> > > >
> >
> >