I'm focusing all my attention on the plug in circuit board now. I downloaded
a data sheet for the chip that it contains. Apparently this is the PWM
horizontal deflection control circuit. So it is a VERY likely suspect.

Further testing has revealed that the pot setting (reference voltage) has
nothing to do with the problem. It controls the size of the raster.

I noted an intermittent mechanical issue though. Sometimes when I touch the
board with the back of a screwdriver, or even touch the pot in an attempt to
adjust it, the HV goes off. So I starting thinking "cold solder joints". The
board is mechanically unstable, being that it 'hangs' off the motherboard by
a tab. So I removed the motherboard and soldered all the connections from
this PCB to the main motherboard. Now I can tap, touch and adjust the pot on
it without the HV going off suddenly.

I'm testing further, but this ephiphany came about after I reassembled the
cabinet and the HV stopped working. The mechanical shifting of the chassis
could have disturbed the cold joint somewhere on the PCB connection.

This has been a deceptive, elusive problem, due to the intermittent nature
of the problem.


--
Best Regards,

Mark A. Weiss, P.E.
www.mwcomms.com
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