25 Dec
2025
25 Dec
'25
12:26 a.m.
On 12/24/25 12:14, Rob Jarratt via cctalk wrote:
I found some time to look at this again today. A few
answers below.
Looking at the 74S299 datasheet, they show current draw as
140 - 225 mA, which gives 700 mW to 1.125 W dissipation.
That SHOULD run hot. Your other unit that doesn't run hot
must not be a 74S299. If I read the datasheet right, they
say it can tolerate running in still air a 70C.
Jon
-----Original Message-----
From: Martin Bishop <mjd.bishop(a)emeritus-solutions.com>
Sent: 30 November 2025 23:12
To: rob(a)jarratt.me.uk; General Discussion: On-Topic and Off-Topic Posts
<cctalk(a)classiccmp.org>
Subject: RE: [cctalk] Re: Hot Video Shift Register on VT100
A short ground lead is 10 mm, from probe tip to ground spring see eg
https://www.aliexpress.com/item/1005007465068388.html
However, ground springs require ground planes surrounding test points, or
test / ground point PEC layouts - very unlikely on a volume PCB.
A 100 mm ground lead is neither short nor likely to be free of ringing.
To
eliminate CRO probe ringing seriously short
grounds are essential.
I pre-emptively bought an oscillator (24MHz and not 24.07342Mz) and tried it
on a breadboard. This gave me the same signal with a negative spike on the
falling edge. Although I don't have a ground spring I managed to shorten the
ground connection considerably and this did indeed clear up the signal and
it looked a lot squarer, without the negative spike. I lifted the original
oscillator, but the leads on it are too short to work on my breadboard so I
can't verify the same behaviour, but it seems highly likely that you are
right that the negative spike is not real.
That leaves me with the 74S299 still getting hot for reasons I don't
understand. Maybe it is just the design, but the same chip in a VT102
doesn't get nearly so hot. Someone else suggested replacing it with a
74F299, so maybe I will try this now that I have socketed it.