Beaten by VT100 PSU.

Peter Coghlan cctalk at beyondthepale.ie
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.

>
> 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.


More information about the cctech mailing list