Beaten by VT100 PSU.

[tony duell]

> 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

It's certainly an odd one...

There are several things that are curious. The first is the frequency
of the waveform. 329Hz or so according to your 'scope (assuming
I am reading it right). That is far too high to be mains ripple. It is too
low to be the oscillator in the SMPSU. So where is it coming from?

My first thought is to look for anything else that runs at that frequency.

The second oddity is that if it is 329Hz and you have a 700mV ripple then
(assuming the output capacitor is what it says it is), you have about 200-odd
mA of ripple current. That sounds a bit high to me. Maybe OK... 

Certainly the output capacitor is my first suspect. ESR meters do not tell
the full story always. I would try sticking something in parallel with it to
see if that has any effect.

Anyone else looking at it, be warned the printset is not easy to follow.
The one on bitsavers has a couple of pages swapped round, so the PSU
schematic is not on consecutive pages. And DEC have drawn the drive
transformers split between the 2 pages with the control circuitry
drivng the primary widings on one page and the secondaries and 
chopper transistors on the other. 

-tony