29 Nov
2025
29 Nov
'25
1:10 a.m.
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(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/vt102-74s299-clk-…
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@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 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.
>