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?
Thanks Rob > -----Original Message----- > From: Rob Jarratt via cctalk <[email protected]> > Sent: 22 November 2025 17:53 > To: 'General Discussion: On-Topic and Off-Topic Posts' > <[email protected]> > Cc: Rob Jarratt <[email protected]> > Subject: [cctalk] Re: Hot Video Shift Register on VT100 > > Thanks for the reply. I had time today to go through some of these items. > > > -----Original Message----- > > From: dwight via cctalk <[email protected]> > > Sent: 17 November 2025 18:19 > > To: General Discussion: On-Topic and Off-Topic Posts > <[email protected]> > > Cc: dwight <[email protected]> > > Subject: [cctalk] Re: Hot Video Shift Register on VT100 > > > > If the part is getting hot, that can mean 1 of 2 different problems. > > > > 1. > > The part has failed internally. > > 2. > > An output has external short to one of the voltage rails > > 3. > > The VCC input has gone above 5V and this part has fail and is now > > loading > it > > down and hopefully protecting the other parts. > > > > For any of these, you will need to remove the part. The worst one is > > that > the > > regulator has failed as just removing the part and powering the system > > up > can > > cause more additional damage. > > > > First check the VCC voltage. If it is not 5V you may have a regulator > problem. If > > it is not 5V you'll need remove the part and use a variac ( or similar > > ) > to bring > > the power up slowly, watching the rail voltage. > > I don't think Vcc is the problem. With the part installed Vcc was 5.1V. With the > part removed it was still 5.1V. Pretty sure that is within the tolerance of the > PSU, and according to the datasheet I have this is within the limits for the > part. > > > > > Next since you've removed the part, you can now continue. With the > terminal > > powered up, we want to look for another possible short causing the > > part to run hot. Use a 50 ( about 47 is a 50 ) ohm resistor. Connect > > one end to > circuit > > ground. Attach it to each output pin and measure the voltage If > > another > input > > has shorted to 5V, you would see more than a 0.5 volts on the > > resistor it > is > > likely a shorted input. > > With the part removed and the terminal powered off, I used my DMM to > measure the resistance of the two 74S299 output pins (8 and 17) compared > to Vcc and to GND. Pin 17 was 1.2M to GND, pin 8 was open circuit to GND. > Pin 17 was open circuit to Vcc, pin 8 was open circuit to Vcc. > > I then did what you suggested and, with the part removed, connected the > output, pin 17, to GND with a 47 ohm resistor. I switched it on and the voltage > across the resistor was 0.03V. I also tried connecting the output pin to 5V and > the voltage across the resistor was 0V. So I don't think there are any shorts. > > I should explain that I think the 74S299 is working fine because, although I > have a problem with the monitor board/flyback, the external video output > works fine. > > Given that it works OK but is just hot, is it possible that this is just normal? Or > is it the spikiness of the clock input that is the problem? I checked the clock > input with the chip still removed and it is still spikier than on my VT102. > > > > > If all these don't seem to be the problem replace the part. Do it with > > a > socket. > > If you unsolder is make sure to note which pin locations top and > > bottom so that you can check them with a meter after installing the socket. > > I need to order some sockets. > > > > > Dwight > > ________________________________ > > From: Rob Jarratt via cctalk <[email protected]> > > Sent: Sunday, November 16, 2025 10:41 PM > > To: General Discussion: On-Topic and Off-Topic Posts > <[email protected]> > > Cc: Rob Jarratt <[email protected]> > > Subject: [cctalk] Hot Video Shift Register on VT100 > > > > (sending again from a different email address as I don't think my > > first > email > > got through, apologies if this is a duplicate) > > > > > > > > I have a VT100 that I was working on a while back and then set aside > > for a > bit. > > I have started to look at it again. I have noticed that the Video > > Shift > Register, a > > 74S299, gets very hot, I can smell the heat and the chip gets almost > > too > hot to > > touch, reaching almost 40 celsius. I have a working VT102 for > > comparison > and > > the same chip there does not get so hot (it reaches about > > 30 celsius). I have already tried replacing the chip, but the new one > > gets equally hot. > > > > > > > > I looked at the signals the chip is receiving and the one that stands > > out > as > > different is the CLK input (pin 12). It looks like this: > > https://rjarratt.wordpress.com/wp-content/uploads/2024/04/vt100-74s299 > > - > > clk-s > > ignal.png. On the VT102 it looks like this: > > https://rjarratt.wordpress.com/wp-content/uploads/2025/11/vt102-74s299 > > - > > clk-s > > ignal.png. It is much spikier on the VT100 and I was told at one point > that this > > could be the cause of the hot running for the chip. Is that a > > reasonable assumption? > > > > > > > > Assuming the spikes are the cause of the hot running. I am trying to > > see > why > > there is a difference. I have noticed that on the VT102 there is a 68R > resistor > > between the DC011 which produces the signal and the 74S299. You can > > see this as R86 in the VT101 printset > > https://bitsavers.org/pdf/dec/terminal/vt101/MP-01066- > > 00C_VT101_Family_Field > > _Maintenance_Print_Set_Apr82.pdf (p47 of the PDF). The VT100 printset > > dated Feb 82 > > https://bitsavers.org/pdf/dec/terminal/vt100/MP00633_VT100_Schematic_F > > e > > b82.p > > df shows an inductor L8 being used (PDF p17), but my VT100 does not > > have this and so must be described by the March 80 printset > > https://bitsavers.org/pdf/dec/terminal/vt100/MP00633_VT100_Mar80.pdf > > (PDF p17). > > > > > > > > Could the absence of L8 explain the spikier DOT CLK signal and the > > hotter 74S299? > > > > > > > > Incidentally, I suspect that the flyback transformer on my VT100 has > failed. > > If anyone has a flyback transformer going spare, especially in the UK, > then I > > would love to hear from you. > > > > > > > > Thanks > > > > > > > > Rob > >
