[SOLVED ] - k7500 B+ Too High then correct after shutting monitor off and right back on

johnczerwinski

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Working on a k7500 that would not power up. Got it working after replacing the (U101) UC3942. Replaced caps (few caps were bad) and Flyback. Replaced the B+ Pot

It powers up, but on the initial power the B+ goes to 165 VDC with no picture and no shutdown mode. I turn the monitor off. Turn it back on and the B+ is 117VDC (right on the money) and have picture, but the picture isn't the brightest and is has a slight jump to it. This pattern of B+ too high, turn off, turn on, and B+ at 117VDC is pretty consistent.

Wondering if there's a component(s) in the feedback circuit that's a bit out of spec causing it.
 
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Symptom
When powering up the monitor, had no picture and the B+ would be at 168 VDC. Turned off the monitor, then turned it back on, I'd get +117VDC and a picture (picture would wiggle bit). Turn it off and back on would result in a picture and 117 B+. Leaving the monitor powered off for several hours, would return the initial problem.

Fix
Replaced the U701 (LA7850). There was a LA7851 installed. Only had a LA7850 but both are compatible
Replaced the D118 (RU2 Diode)
cleaned the pots on the remote board (stopped the wiggling).

Hypothesis
Think U701 was bad and maybe the D118 diode wasn't performing under load (even though passed the typical multimeter test).
When you initially turned the monitor on, U701 would get enough constant DC to kick off the horizonal drive and flyback, thus getting 168VDC because nothing was really drawing current.
But C127 would get energized so when you turned it off and back on again, there was enough energy in the capacitor to kick off U701 and start the horizonal/flyback. After several hours idle, the capacitor would discharge then would be back at square one.
 
Thanks for this info. I've got 3 of these monitors that did this. The first 2 I replaced all the zener diodes, flyback and U101
which finally solved the problem. I have one more that still occasionally has this problem. Now I've got something
else to try.
 
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