Hi Rob, this is not intended to indicate the cause or solution, but merely to help analyze how critical your IC temperature is: Did you actually measure the temperature of the IC? I recently bought an inexpensive infrared thermometer (like:https://www.ebay.co.uk/itm/156810933790) that can measure the temperature of small areas relatively accurately. I made the experience that I cannot judge temperatures very well with my fingers.
According to Wikipedia, TTL ICs are specified for temperatures up to 70 °C. This temperature should obviously be avoided, but my fingers already perceive anything above about 50 °C as "hot". I hope this can help a litte! Ulli Am Sa., 29. Nov. 2025 um 09:44 Uhr schrieb Henk Gooijen via cctalk < [email protected]>: > 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 <[email protected]> > Verzonden: zaterdag 29 november 2025 08:11 > Aan: 'Doug Jackson' <[email protected]>; [email protected] > CC: 'General Discussion: On-Topic and Off-Topic Posts' < > [email protected]>; Rob Jarratt <[email protected]> > 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 <[email protected]> > Sent: 28 November 2025 22:59 > To: [email protected] > Cc: General Discussion: On-Topic and Off-Topic Posts < > [email protected]> > 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: [email protected] <mailto:[email protected]> > > ph: 0414 986878 > > > > > > > > > > On Sat, 29 Nov 2025 at 08:52, Rob Jarratt <[email protected] > <mailto:[email protected]> > 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-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] <mailto:[email protected]> > > Sent: 25 November 2025 01:15 > To: [email protected] <mailto:[email protected]> > Cc: General Discussion: On-Topic and Off-Topic Posts < > [email protected] <mailto:[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. > > > > >
