6 Nov
2014
6 Nov
'14
11:42 a.m.
It's not that there was a solid, bright vertical line - the image consisted
of the contents of the display (an ACSII block message box, plus a line or
two of text) squashed into the vertical. So there was a short, dashed
vertical line about 1.5" high, in the upper third of the screen. It was
dim, and became more dim over the course of a minute or two - eventually
disappearing totally, appearing to slowly slide upward past the upper limit
of the screen.
Ah... That sounds like the horizontal output stage stopped working completely (so no scan)
and the thing kept
working on the charge stored in the capacitors.
More on "disappearing" coming up.. I now
realize that it's important.
Could you put the old HOT back in and look at the base drive with a
'scope? What do you see on the base?
Having concluded that the original HOT was reading in-spec, I did
re-install it as part of my next-step troubleshooting. I then disconnected
the tertiary supplies (15V & 50V) by lifting leads of W3 & CR104,
respectively. Guess what? The 50V supply came up to 44V (the highest I've
seen it).. then the stupid stuff started..
I re-connected the 15V supply, and checked the 50V - still good. Then
re-connected the 50V supply - and it's still good.. AND the d*mn display is
working!! Now how obnoxious is that? Oh, and the HOT was running quite
cool, barely over 100F.
But not for long.. soon, the display faded out and the 50V supply crashed
to 20V. Ack, it's an intermittent..!
Replaced C114 & C401 (the stabilizng caps for the 50V & 15V supplies) just
for drill, to no effect. So this leaves a couple of possibilities.. There's
an intermittently shorting device in one of the tertiary supplies -or- the