Heathkit H89 Video Question

From: Patrick Rigney <patrick_at_evocative.com>
Date: Sun Aug 31 00:41:00 2003

OK, I'm working in unfamiliar territory... and since there seem to be a lot
of video experts of late, this seems like the time and place to ask...

I'm working on an H-89 that seems to have a problem in the horizontal drive
on the video board. The unit powers up from cold to a good display,
properly sized, and stable. After the unit operates for 10-15 minutes, the
video breaks down, first by jittering slightly for 30-60 seconds, then it
goes to a single vertical line on screen that fades to black. At this time,
the filament on the CRT also stops glowing. If I leave the unit off for
20-30 minutes, it comes back with good video for a bit, then repeats the
above. If I turn the unit back on right away without a cool down, there's
no display/no filament glow.

After tracing the (good) horizontal output signal from the terminal logic
board into various part of the video board, and checking voltages where I
felt I could reach safely in a hot set, I discovered that if I shot
horizontal output transistor Q217 with cold spray I'd get video back right
away.

This unit is the one I posted about on cctalk a few days ago with the NOGDS
board... when I got it, it had a AA battery inside for a clock board that
leaked down onto the flyback transformer. The leak didn't appear excessive,
and most of the "material" on the flyback must have dried long ago and was
brushed away easily, leaving only a slight tarnish on the aluminum heatsink
it's mounted to.

My questions...

1) Although the flyback _appears_ OK, could the leak have caused some unseen
damage that is now causing excessive current through Q217 and shutting it
down? My limited understanding of flyback construction is that they are
sealed units. It takes several minutes (10-15) before the failure is
observed. Could a flyback failure allow operation for this long? How do I
test this thing?

2) If I proceed under the assumption that the flyback is OK and Q217 is
not... Q217 is a BU500: 1500V, 6A, 75W, SI-NPN. I normally buy from Mouser,
DigiKey, and Jameco, but none of them seems to have this part, and I wasn't
able to turn up anything useful Googling. A possible dealer or two in the
UK, but for a $2-3 part I was hoping for a more-readily-available source or
an equivalent. The closest I can find in the family is BU508A, which is
1500V, 7-8A, 120-125W (depending on whose specs you believe). Is that OK or
too far out of spec? Does anyone know of US source for the real McCoy?

Thanks for any help/advice! --Patrick
Received on Sun Aug 31 2003 - 00:41:00 BST