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SC-502 Intensity Stuck at Maximum

Giovanni Carboni
 

Hello guys,

I am new here and this is my first question.

In my SC-502 scope the intensity is stuck to the maximum value and the knob cannot adjust it. I could reduce the intensity by reducing the beam current but I would like to fix the adjustable control. I measured voltages on the various transistors and checked waveforms but no results. Has anybody ecountered the same problem?

Giovanni

robeughaas@...
 

Giovanni: I had this problem and it was solved only after replacing all the high-voltage caps that go between the main board and the high-voltage auxiliary board. C881, C882, C883, C885 and C887. It's not practical to replace one at a time the way the caps span the two boards. You should also check diodes CR884 and CR885. These are relatively low-voltage diodes (225V PIV). Be careful with the large hybrid resistor network R893ABCD as it is fragile. If you have more questions, feel free to ask. I've restored several SC502's.

--
Bob Haas

Giovanni Carboni
 

Bob: thank you for answering. I'll take note of your remark, since luckily
I was able to fix it by replacing Q825 (in the process I also replaced Q815
and Q820 which were ok!)

73 Giovanni IZ5PQT