17 Aug
2016
17 Aug
'16
10:11 a.m.
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...
When I used a variable load on the 12V both the amplitude and the period
varied. More load less amplitude.
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.
Tested by putting a 1000uF in parallell. No difference. Interesting note is
that regardless of capacitor, at the very moment of turning the power off
the picture becomes absolutely stable.
Added the AVO option. Then I have perfect picture. So the extra loading
makes a difference.
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
/Mattis