Should you be worried about why the other one is running "too cool" :-?

If similar ones don't get hot, I'd suggest pulling the part from the board. Use a 250 ohm resistor to ground on the outputs. If anything goes higher than 0.4V, there is a problem with the load. You want the part removed so you are not looking at the part pulling up. With the part still removed, it should also go over 1 Volts with a high impedance voltmeter or scope. Dwight ________________________________ From: Rob Jarratt via cctalk <cctalk(a)classiccmp.org> Sent: Wednesday, December 24, 2025 3:47 PM To: 'General Discussion: On-Topic and Off-Topic Posts' <cctalk(a)classiccmp.org> Cc: Rob Jarratt <robert.jarratt(a)ntlworld.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. Thanks Rob

(cctalk(a)classiccmp.org) also I have gone back again to look at this even more carefully. I am now fairly sure that there *is* a difference between the VT100 and the VT102. On the VT100 I get this https://rjarratt.wordpress.com/wp-content/uploads/2025/12/vt100-74s299-clk-s ignal-with-short-ground-lead.png. This is using a ground spring, with the 74S299 removed and the probe and ground spring directly connected to the socket. It shows a slight -20mV negative spike, but it varies between -20mV and -80m. On the VT102 I get this https://rjarratt.wordpress.com/wp-content/uploads/2025/12/vt102-74s299-clk-w ith-shorter-ground-lead.png. This used a longer ground lead and with the 74S299 in circuit, but even though the ground lead is longer the signal looks cleaner, there is no negative spike on the negative edge. In fact on the VT100 you can see that the signal then rises to about 0.4V, which is the minimum it goes to on the VT102. cause necessary or I think the difference must be that on my VT100 the DC011 output that goes to the 74S299 is directly connected. On later revisions of the VT100 there is an inductor and on the VT102 there is a resistor. These would presumably reduce the slew rate as you would suggest and indeed the slew rate seems lower on the VT102 trace. If this is the case then I guess the heat is by design, but the design was changed to stop this. I also wanted to respond to Dwight's suggestions about a couple of tests. I have done quite a few tests already actually: 1. With the load removed 2. Replaced the part. 3. Checked the apparent input spikes (see above) Dwight, I think this covers your first test with the 250 ohm resistor to ground, but I don't understand the last test you mention? Thanks Rob

Rob Series R on logic outputs are a classic "taming" mechanism, they also provide access to the net for more extreme measures (eg C to ground). Usu 22R or 33R close to the driver to bring the probable source impedance to 50Z - matching the nominal track impedance. The design may be evolved to include parallel (DC) termination or AC termination (series R & C) for multidrop loads. For 2025 circuits much of this is moot, the track lengths are so short, in the 1980's it was different ... IBIS models for devices of interest and the wisdom of HoJo's oeuvre are the classic references https://www.amazon.co.uk/High-Speed-Signal-Propagation-Advanced/dp/01308440… HtH Martin Ho Jo : Howard Johnston -----Original Message----- From: Brent Hilpert via cctalk <cctalk(a)classiccmp.org> Sent: 25 December 2025 19:12 To: rob(a)jarratt.me.uk; General Discussion: On-Topic and Off-Topic Posts <cctalk(a)classiccmp.org> Cc: Brent Hilpert <bhilpert(a)shaw.ca> Subject: [cctalk] Re: Hot Video Shift Register on VT100 The series R or L in the S299 clock line (that you mentioned are present in respective later models) are there to suppress parisitic oscillation / ringing. It may be that ringing - even if not going negative - is upsetting the internal transitions somewhat, for example resulting in some overlapping conduction in totem-pole drivers, and may well be why they added the series R/L in later models. If you wanted to experiment, you could try raising the IC a fraction of an inch on some extenders** - except for the clock pin where you shall insert a small-Ω R in series as the later model did (was it 36Ω?), you don’t necessarily have to find an L. Or if you’re careful you may even avoid extending the whole IC, by bending the clock pin on the IC out just far enough so it misses the socket pin while inserting the IC, and connecting the R in series (close in, not on long clip leads, could tack-solder one lead to the IC pin, then a fine wire on the other lead with a piece of slide-on insulation to get down between the legs to the socket pin). ** individual pins from machine-pin IC sockets can be nice for this, or an IC socket with wire stubs, or wire stubs soldered to IC pins seeing as how the removed IC is already solder-tainted, just use wire small enough not to damage the socket you're inserting into.

<cctalk(a)classiccmp.org> 50Z lengths are the suppression can be From the responses I have had it sounds like my suspicion that later designs added something to keep the part cool may well be correct. I will see if I can insert a resistor by bending the CLK pin out of the socket (I have spares if I break it) and see if I have a suitable resistor to hand. Thanks Rob

stock", a both to I tried both a 33 ohm and 100 ohm resistor. I wasn't able to put it on the DC011 output, instead I had to put the resistor on the 74S299 input as it is socketed and the chip is expendable. The video signal was still fine, although I can only use the External Video Output Connector to verify. However, the 74S299 still seems to get just as hot. Interestingly though the waveform did change. Using a 33 ohm resistor there was still a spike on the falling edge but it didn't go negative. Using a 100 ohm resistor the spike almost disappeared https://rjarratt.wordpress.com/wp-content/uploads/2025/12/vt100-74s299-clk-s ignal-with-100-ohm-resistor.png So now with the 100 ohm resistor it does look practically identical to the VT102 signal, but it still seems to get too hot. I think I will just put it back to what it was and live with it. I will try the 74F though when I get it. Thanks Rob