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-signal.png
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 <[email protected]> Sent: 25 November 2025 01:15 To: [email protected] Cc: General Discussion: On-Topic and Off-Topic Posts <[email protected]> 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: [email protected] <mailto:[email protected]> ph: 0414 986878 On Tue, 25 Nov 2025 at 10:48, Rob Jarratt <[email protected] <mailto:[email protected]> > wrote: > -----Original Message----- > From: Doug Jackson via cctalk <[email protected] > <mailto:[email protected]> > > Sent: 24 November 2025 23:04 > To: General Discussion: On-Topic and Off-Topic Posts <[email protected] > <mailto:[email protected]> > > Cc: Doug Jackson <[email protected] <mailto:[email protected]> > > Subject: [cctalk] Re: Hot Video Shift Register on VT100 > > Another test would be to disconnect the output and see if that runs the > device cooler. Maybe the load, while not a short, is simply too great for the > device. I think I can try this, the DC012 it drives is socketed. > > But I am leaning towards the inputs containing spikes that need to be > dissipated as heat in clamping diodes. A decent CRO on the device input pins > while it is out of circuit will show that as well. I guess CRO means Cathode Ray Oscilloscope? Not sure what you are suggesting I try here? > > Kindest regards, > > Doug Jackson > > em: [email protected] <mailto:[email protected]> > ph: 0414 986878 > > > > > On Tue, 25 Nov 2025 at 09:52, Peter Coghlan via cctalk < > [email protected] <mailto:[email protected]> > wrote: > > > > > > > Sorry if I wasn't clear in my earlier reply, but there were some > > > answers inline. The summary is that Vcc looks fine, there does not > > > seem to be any short on the outputs of the shift register and the > > > chip seems to be > > working > > > correctly. It just gets hot and I wonder if this could be due to the > > > CLK input being a bit spikier on the VT100 than on my VT102? Could > > > this be > > why > > > later revisions of the VT100 introduced an inductor on the DOT CLK > > > output from the DC011? > > > > > > > Perhaps one way to test the hypothesis would be to insert an inductor > > like those used on the later revisions in the DOT CLK output and check > > whether > > a) everything still works and b) the shift register runs cooler? > > > > Regards, > > Peter Coghlan. > >
