DIYbanter - View Single Post

In article ,
says...
If the tube filaments light, you have horizontal deflection.
The buzzing you hear is probably vertical deflection.

I don't see any activity in the tubes. Is it very subtle?

To see the tube filaments glow, you probably need to be looking
through the convergence board access hole in the front of the set.

snip
If that hasn't happened, a common electrical failure is for the high
voltage transformer T900 to break down, which causes the HVOT transistor
Q906 to overheat and short out, which blows open 0.33 Ohm fusible
resistor R934. R934 is hidden in front of the heat sink on the right
front of the board. It's not easy to get at, but easy to test once you
do. If open, it usually means T900 and Q906 are both bad. Couple hun
to fix, but usually worth it.

That's some good tips. I got my gloves on and started playing around
with that resistor. It was exactly where you said. My multimeter isn't
the best, so I don't know if it would detect that as a short. It
certainly didn't. I know I wouldn't get the specified resistance if I
tried plugging in straight, but I figured the equivalent power-off
resistance (assuming a day unplugged) showed 60k kOhms for that
resistor. What is most telling is it looks like a chunk on the right
side of it has burnt out. It's rough right there, so I suspect it has
blown.

Most multimeters will show a couple tenths of an Ohm with the
leads shorted. If the 0.33 Ohm resistor doesn't show in the same
neighborhood, it's bad. The actual value is not critical in this
application, it's there to be a fuse. And yours certainly sounds
like it's blown.

If that's a sign that it's blown, I'll clip it off at one end so I can
measure current through that path. Do you know how much current should
go through that when powered up?

I wouldn't recommend trying to measure the current. You may damage
your meter or other parts of the board, or more likely the power
supply will current-limit and you won't get useful data anyway.
Instead measure collector to emitter resistance of Q906 (use low
Ohms range). I bet it's shorted, and if so it's almost always
caused by T900 being bad.

I don't have a soldering iron on hand right now. But if it comes
down to that, I can get a Weller in a hurry.

Not the best tool for the job, but can be used in a pinch. If you
are not practiced in the art of desoldering, you might want to
experiment with some unimportant throwaway pc board until you can
remove large parts without peeling off traces. The flyback can
be especially frustrating, as it has a lot of large pins. You'll
need some solderwick or one of those solder-pullit things.

And let us know when you get to removing the HV wire from the old
flyback, it's a bit tricky too.