24 Dec
2025
24 Dec
'25
7:14 p.m.
I found some time to look at this again today. A few answers below.
-----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.
Regards
Rob
Regarding some of the other points raised:
- 24.072 MHz is likely to be a magic number for video rates, with 24 MHz
YMMV
- the negative excursion may be clamped somewhere by diode(s) to gnd,
hence -1v2 .. - 1v4 excursion
- fast edges can be softened by a series resistor (22 .. 33 R say)
following the
driver, triksy to retrofit
- are the power rails clean, on a scope, only pertinent if the spikes are
asynchronous to the signal(s)
HtH; Martin
25 Dec
25 Dec
12:26 a.m.
New subject: Hot Video Shift Register on VT100
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.
12:47 a.m.
New subject: Hot Video Shift Register on VT100
-----Original Message-----
From: Jon Elson via cctalk <cctalk(a)classiccmp.org>
Sent: 24 December 2025 23:26
To: cctalk(a)classiccmp.org
Cc: Jon Elson <elson(a)pico-systems.com>
Subject: [cctalk] Re: Hot Video Shift Register on VT100
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.
I just double checked and on the other unit it is a 74S299N, identical to the one that
gets hot in the VT100. I suspect it is OK for it to get hot and maybe I should just stop
worrying about it. But it is puzzling as to why this one gets hotter. I will get my VT101
back soon and I can see how that compares.
Thanks
Rob
1:01 a.m.
New subject: Hot Video Shift Register on VT100
I just double checked and on the other unit it is a
74S299N, identical to the one that gets hot in the VT100. I suspect it is OK for it to get
hot and maybe I should just stop worrying about it. But it is puzzling as to why this one
gets hotter. I will get my VT101 back soon and I can see how that compares.
Should you be worried about why the other one is running "too cool" :-?
1:07 a.m.
New subject: Hot Video Shift Register on VT100
-----Original Message-----
From: Fred Cisin via cctalk <cctalk(a)classiccmp.org>
Sent: 25 December 2025 00:02
To: General Discussion: On-Topic and Off-Topic Posts
<cctalk(a)classiccmp.org>
Cc: Fred Cisin <cisin(a)xenosoft.com>
Subject: [cctalk] Re: Hot Video Shift Register on VT100
> I just double checked and on the other unit it is a 74S299N, identical
to the
one that gets hot in the VT100. I suspect it is OK for
it to get hot and
maybe I
should just stop worrying about it. But it is puzzling
as to why this one
gets
hotter. I will get my VT101 back soon and I can see
how that compares.
Should you be worried about why the other one is running "too cool" :-?
It's an interesting point, but the VT102 works just fine so I don't think it
is a problem. When I get my VT101 back I will have a third one to compare,
it might help to decide what the norm should be. Despite the fact that they
are three different models (VT100 hot, VT102 cool, VT101 still unknown) I
think the fundamentals are pretty much the same.
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Rob Jarratt