Beaten by VT100 PSU.

Peter Coghlan cctalk at
Wed Aug 17 04:14:16 CDT 2016

> 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.

> 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

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.

Peter Coghlan.

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