> > 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. > > 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? 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? 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. 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. Regards, Peter Coghlan.
