[cctalk] Re: Hot Video Shift Register on VT100

Rob, I have a few silly thoughts, but maybe they can be useful. The negative going signal is something I would not be happy with, as it may damage the IC (over time). TTL does not like negative voltages on their pins. Is there between the DC011 (signal generating) and the LS299 (signal "user") wiring, or are they on the same PCB? In case of wiring, capacitive coupling may cause negative going voltages. Could a clamping diode with an as low as possible Vf (< 0.3 V) help to reduce the negative voltage? It is not a permanent solution, because the DC011 might be a bit unhappy with that diode, but for a test it might help to draw a conclusion. If all does not help, in getting the LS299 running cooler, consider some temperature conductive paste and a small "IC heatsink". Tie the heatsink onto the IC with a piece of waxed lacing. The IC can radiate more efficiently the generated heat, so that it can run cooler. This could drop temperature with some 10 degrees. Success hunting down this issue, Henk -----Oorspronkelijk bericht----- Van: Rob Jarratt via cctalk &lt;cctalk(a)classiccmp.org&gt; Verzonden: zaterdag 29 november 2025 08:11 Aan: 'Doug Jackson' &lt;doug(a)doughq.com&gt;om>; rob(a)jarratt.me.uk CC: 'General Discussion: On-Topic and Off-Topic Posts' &lt;cctalk(a)classiccmp.org&gt;rg>; Rob Jarratt &lt;robert.jarratt(a)ntlworld.com&gt; Onderwerp: [cctalk] Re: Hot Video Shift Register on VT100 Yes, it is a negative voltage. I measured this by attaching the ground lead to the ground connection on the 74S299. The DC011 that is producing the CLK signal is socketed, but the one on my VT102 isn’t. I would try swapping them to see what happens, but I don’t really want to touch a fully working board. Thanks Rob From: Doug Jackson &lt;doug(a)doughq.com&gt; Sent: 28 November 2025 22:59 To: rob(a)jarratt.me.uk Cc: General Discussion: On-Topic and Off-Topic Posts &lt;cctalk(a)classiccmp.org&gt; Subject: Re: [cctalk] Re: Hot Video Shift Register on VT100 Is that a negative excursion at the bottom of the waveform? now low - looks like -0.3v to me - With '"logic, that will likely turn on the protection diodes on the inputs, making them warm. Double check where the ground actually is on the CRO display. It's also interesting that the input doesn't even reach 4v, let alone 5v. Whats driving that? Kindest regards, Doug Jackson em: doug(a)doughq.com <mailto:doug@doughq.com> ph: 0414 986878 On Sat, 29 Nov 2025 at 08:52, Rob Jarratt &lt;robert.jarratt(a)ntlworld.com <mailto:robert.jarratt@ntlworld.com> > wrote: 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 &lt;doug(a)doughq.com <mailto:doug@doughq.com> > Sent: 25 November 2025 01:15 To: rob(a)jarratt.me.uk <mailto:rob@jarratt.me.uk> Cc: General Discussion: On-Topic and Off-Topic Posts &lt;cctalk(a)classiccmp.org <mailto:cctalk@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 &lt;robert.jarratt(a)ntlworld.com <mailto:robert.jarratt@ntlworld.com> > wrote: I think I can try this, the DC012 it drives is socketed. I guess CRO means Cathode Ray Oscilloscope? Not sure what you are suggesting I try here?