Some output transistor observations

Just had a pig of a job with the output transistors in my G05-801 monitor
(the four transistors, 2 x 2N3716's and 2 x 2N3792's, are mounted on a
separate heatsink above the deflection board).

I decided to swap around the two 3716's to see if that affected the 'curve'
to the bottom and left I was getting. Trouble is, once I'd done that, the
lower half of the display was squashed to a thin horizontal line in the
middle of the screen. So I swapped them back - same thing! Weird! (Damn
frustrating and annoying too!).

So I tested all sorts - continuity, any shorts between the transistor cases
and the heatsink, re-soldered the headers that these transistors plug into,
buzzed through all connections, tested the transistors with my diode
tester, etc, etc, etc. I even replaced the 3716's with brand new ones!
Didn't make a bit of difference. My frustration and annoyance levels were
increasing exponentially! What HAD I done?

So ......... I removed the rightmost 3716, and powered on again - the
*bottom* of the screen now started drawing okay, but this time the *top*
was squashed into a thin horizontal line in the centre of the screen. Hmmmm
.....

So, I put the 3716 back in and removed the 3792 above it (remember, I
hadn't touched these!). Switched back on and yes, the bottom half of the
display was squashed, as before, to a thin horizontal line. IE it must be
that transistor at fault, or it's connections to the deflection board.

So I replaced that 3792 with a brand new one, turned on and ..... bingo!
Full screen display! Yes! :-)))

So obviously that transistor was dead. Strange thing is though, it checks
out okay with my diode tester (although that won't check for 'leakage' I
believe) and the rest test the same.

So how did I cause this transistor to fail? I hadn't even touched it, just
swapped around two 3716's.

Oh, I also replaced the remaining 3792 - thought I may as well as at least
they're all new now - I'll keep the old ones for spares and the dead one as
a souvenir. :)

So to summarise, at least I now know that the two rightmost transistors
(3716 and 3792) control the Y axis deflection, but I must admit to being
slightly puzzled as to why removing the 3716 and leaving in the 'dead' 3792
(as noted above) caused the *bottom* of the screen to start drawing.
Doesn't make sense to me. Surely it shouldn't have drawn anything at all,
unless the 'dead' 3792 was doing *something*, it just wasn't *fully*
functional?

Discuss. :)

Cheers,
Phil (wondering what else he can break ........ ;-)

P.S. The curve is still there!

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Received on Mon Jan 14 02:47:03 2002