2016-08-17 11:14 GMT+02:00 Peter Coghlan <cct...@beyondthepale.ie>: > > > > It is a VT100 PSU (H7831). I tested it with dummy loads and it worked > fine. > > But when used in the terminal with the Basic Video board and monitor > board > > it gives a jumpy picture. Both horizontally and vertically. First I > thought > > that it was related to the monitor board but soon recognised that the > +12 V > > had a most peculiar waveform on it: > > > > I've seen a lot of VT100s with poor vertical linearity and of those, many > jumped vertically as well. I never got to investigate whether this was > poor adjustment and/or noisy controls or a more real fault. >
I could have filmed the behaviour but never did. But the size of the picture was jumping which goes well with the bad input voltage. The linearity was fine. > > > > http://i.imgur.com/d0z0NQS.jpg > > http://i.imgur.com/gQqmSN5.jpg > > http://i.imgur.com/P0dt5y1.jpg > > > > This waveform is only present on the +12V, not the +5V, not the -12V or > > -23V. > > > > So I connected just the Basic video board and a variable dummy load > instead > > of the monitor board. I used a Variac on the input. It turned out that > > there were no problems now with the +12V. Until I pulled out and > reinserted > > the keyboard. Then it was there. If I lowered the input voltage it was > > impossible to provoke this problem and also if I increased the +12V load. > > Further testing also gave that putting a few amps extra load on the +5V > > also made it resistant to this type of failure mode. > > > > The amplitude and frequency of this waveform is shifting by +12V loading > > and AC input voltage. > > > > The VT100 SMPSU is a primary switcher regulating the +5V. Then the +12V > is > > handled by a secondary switcher which is synchronised with the primary > > switcher. The other voltages have linear regulators. > > > > It looks to me that something in the regulation circuitry is not > behaving, > > thus oscillating. But what component has failed (or is out of spec)? I > > checked transistors. I checked the waveform from the 555 chip and ramp > > voltage input to the 555. But I cannot figure out what the problem is. > > > > I checked the 560uF output capacitor but my LCR meter said it was in good > > shape. Around 700 uF and very low ESR. > > > > If the waveform is present across the +12V output capacitor and +5V is ok > and > and the regulation only monitors the +5V, it sounds like the power supply > is > doing it's best and maybe the problem is in the +12V load? > Yes. The waveform is present on the +12 output capacitor. There are dual regulators. The main is only regulating the +5V and then there is a secondary to regulate the +12V. > > Alternatively, could there be an abnormally varying load on the +5V which > the > power supply regulation managing to smooth out but in doing so causing the > unmonitored +12V to vary? > The +12V is not unmonitored since there is a switching regulator there using a NE555 chip and a LM393 regulating the +12V as a secondary side SMPSU. The main regulator is a SG3524 chip. The waveforms from the SG3524 and the output on the +5V looks perfect. It is just the +12V that is bad which directs me to the NE555 and LM393 regulator circuitry. > Monitoring the current waveform in the +5V and the +12V might throw some > light > on this, maybe by putting the scope across a small series resistance in > each > line. > That is an interesting idea. I will bring in my current probe. > > I guess the 560uF capacitor referred to above is on the 12V line? The > corresponding capacitor on the +5V might also be worth checking in case > the regulation of the power supply is managing to compensate for it > being bad. > I will take a look at the +5V output cap as well just to be sure. If I put in the VT125 option board the terminal behaves just fine, so the loading seems to be of importance. > > Regards, > Peter Coghlan. > /Mattis
