More Star Wars HV problems, help please?

Six weeks ago I finished rebuilding my HV board which included
reflowing ALL the solder joints, recapping the board, replacing the W
jumpers, replacing all the main parts (VR1&2, BU406D, .8A 100v SCR,
MPSU07, 2N3904) and installed a brand new Wintron HVT. (I also
recapped the deflection board but that's not the problem now). It
has had daily home use since then and has worked perfectly.

Today we were playing it quite a bit and when we left the room for a
few minutes when we came back the screen was black. I turned the
power off and pulled the HV board out where I could look at it.
(BTW, this was also the previous problem).

When I turned the power back on, there was a loud sizzing rapid short
circuiting kind of noise which went on for at least 5 seconds. There
was no smoke or smell. When it stopped I looked over the board and
could see the overvoltage LED is on. I can hear deflection and game
sounds but there is no glow in the tube.

After making sure R17 (20k pot-HV overvoltage) was working, I tweaked
it with no change. I checked R12 (2.2Kohm resistor) and it is at
spec. I measured and am getting the correct +24/-24V and +30/-30V at
the test points shown in the guide. I am NOT getting 180 B+ voltage
- I am reading +26 at that point. I have measured that in the past
and had set it to exactly +180V. I haven't actually pulled the
BU406D yet, but I did make sure the collector is ohming out with the
heatsink and that the two legs have continuity down the traces.
Interestingly, (and maybe this is normal) I am reading +26 at all of
the Wintron hookup points.

After turning the machine off about an hour, when I turned it back on
the sizzling sound came back but this time only for about 3 seconds.
Later, it only sizzled for about half a second. Now, it no longer
makes any sizzling sound at all when I turn it on. I am 100% sure
that the sound was coming from the HV board itself because I had it
moved out of the machine onto a wood work bench, but I am not sure if
it was the HVT itself or something else making the noise.

Unfortunately I don't have an oscilloscope / HV probe / LOPT tester,
but I do have some spare parts like an MCI can and a 555 timer IC if
needed and I understand the basic layout of the board and components.

Any advice appreciated!

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Received on Sun May 11 13:18:13 2003