HELP with Mitsubishi Pro-Jo fading picture!

Have this newer 65" Model WS65809 Mits in shop for service. After a few
minutes picture goes from perfect to a very "whiteish"
and washed out effect. You can still see your signal but very faint
due to the overwhelming brightness. Looks like a sudden loss of
croma or the brightness being turned up all the way (times 2). Have
checked everything on CRT pcb (no leak in transistors). Not seeing
anything in monitoring voltages. Problem is intermittent! No stray
caps at this point either. Can anyone assist us in this issue please?
T.I.A.
Jim

 

[JURB6006]

You must change all three AKB sensing transistors and the buffer that's on one
of the CRT socket boards. I think they're all PNP, what's more NO SUBS. ECG etc
sometimes do not work.

B & D or another of the more decent vendors of OEM type transistors can
probably supply them, but remember, there are certain characteristics to these
transistors that are critical. These have to do with B-E valence voltage and
the like. Like the reset transistors in the old NEC VCRs, they'd have you put
in a heat sink etc., poppycock, just use the right transistor, I found a bias
transistor for an audio amp worked forever because it could maintain it's gain
at low Vce. This case is slightly different, I believe it's the Vbe@ whatever
Ic and the resultant effective hfe. That's why I would urge you to use the
right transistors here. Mods to make something else work are simply not worth
the trouble.

You will find the same number of transistor on each CRT socket board, a PNP
(2SA or 2SB) and another on one that they all feed. Change all four, they are
cheap, and detecting which one has the minute almost immeasurable leakage is
impossible in circuit. What's more a static test is likely not to show the
problem.

Before you do ANY of this however, carefully inpect the area where the CRT
boards plug into the signal board, this problem could also be caused by a
coolant leak. Transistors could be OK, but the signal simply isn't getting
where it needs to go. If in doubt, put your ohmmeter on the 20 Mohm scale and
stick the probes on (almost into) the PCB about one mm apart, if you get
anyhting but open circuit reading, that's the problem.

JURB