28 Nov
2025
28 Nov
'25
3:52 p.m.
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>
Sent: 25 November 2025 01:15
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
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.
>