Hi Rob Distrust of the instrumentation is always prudent, but an open mind is also always merited.
As your 74S299 is a standard footprint, you have the option of logic types other than F as alternates. The two things it may be worth minimising are slew rate, which will cause ringing, and switching current, which will cause ground bounce (due to inductance in the ground path). Subject to necessary drive capability my first thoughts for house trained 5v logic would be LS or HCT. There is a smorgasbord of logic types - TI have some usable summaries in pdf, including timelines. Good Luck Martin -----Original Message----- From: Rob Jarratt <[email protected]> Sent: 24 December 2025 18:15 To: Martin Bishop <[email protected]>; 'General Discussion: On-Topic and Off-Topic Posts' <[email protected]> Subject: RE: [cctalk] Re: Hot Video Shift Register on VT100 I found some time to look at this again today. A few answers below. > -----Original Message----- > From: Martin Bishop <[email protected]> > Sent: 30 November 2025 23:12 > To: [email protected]; General Discussion: On-Topic and Off-Topic > Posts <[email protected]> > 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 >
