I have traced the clock signal back. The MAS CLK input on the DC011 also has the negative spikes. This is coming from a 74S157. The input to that is an oscillator and that too has the negative spikes.
So, it seems the 74S157 is just passing through the inputs. There is a capacitor across pins 4 and 2 of the oscillator (Vcc and Gnd), would a failure there cause this kind of negative spike? It seems unlikely to me, but would be happy to be corrected. The oscillator is rated at 24.0734MHz. I can find lots rated at exactly 24MHz and I guess the difference in frequency is small enough not to matter, right? I will order the part and also a new capacitor for it in case that is where the fault lies and see. Thanks Rob > -----Original Message----- > From: Rob Jarratt via cctalk <[email protected]> > Sent: 30 November 2025 12:57 > To: 'General Discussion: On-Topic and Off-Topic Posts' > <[email protected]> > Cc: 'Hans-Ulrich Hölscher' <[email protected]>; Rob Jarratt > <[email protected]> > Subject: [cctalk] Re: Hot Video Shift Register on VT100 > > > -----Original Message----- > > From: Hans-Ulrich Hölscher via cctalk <[email protected]> > > Sent: 29 November 2025 09:17 > > To: General Discussion: On-Topic and Off-Topic Posts > <[email protected]> > > Cc: Hans-Ulrich Hölscher <[email protected]> > > Subject: [cctalk] Re: Hot Video Shift Register on VT100 > > > > 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! > > Yes, I have an IR Thermometer and the chip gets to about 40 celsius. I realise > that this is well below the maximum rating, but I can smell the heat and it > worries me a bit. If I knew this was expected then I would be OK with it, but it > feels anomalous and I have felt the chip to be hotter than this sometimes. > The observation about the negative voltages also concerns me a bit, although > I do have spares for the 74S299 should I need them. > > It looks now like the DC011 is just producing a DOT CLK output with negative > spikes. Maybe this is why later revisions introduced an inductor or maybe it is > just a bad DC011 or something is wrong with what is around the DC011. > > I am going to look at the DC011 inputs to see if anything looks amiss. The > other thing I would like to do is to see what happens if I can add an inductor > to the output of the DC011. But I don't know how to read the spec on the > printset to know what I might need. The printset says "BEAD, FERRITE > .1380DX.380LG", can anyone tell me what this might mean in terms of an > inductor that I could try? > > Thanks > > Rob > > > > > > 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 isnt. I would try swapping them to see what happens, but I > > > dont 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/vt102-74s2 > > > 99 > > > -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. > > > > > > > > > > >
