Beaten by VT100 PSU.

[Mattis Lind]

Normally I manage to repair the SMPSU that I dive into. But this time I
must admit that I am defeated.

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:

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.

I was thinking of breaking up the feedback loop and see what happens, using
an external +12V as an input to the regulator rather than the generated
+12V. But hasn't got there yet.

Anyone seen this type of behaviour? Tony, do you have some piece of good
advice?

/Mattis