Help to repair oscilloscope Philips PM3207 (Algemeen)

door Onno-I , 02-11-2020, 08:27 (1274 dagen geleden) @ edison
Gewijzigd door Onno-I, 02-11-2020, 09:22

No, it's probably not the CRT.

T801 and V801 form a simple oscillator, the output of which is used to generate a negative high voltage.
It is a little bit confusing that the third winding of T801 is drawn separated from the other two.

How fast does your transistor burn?
Also check the rectifier circuit (C804 806 807 and V802 803). A short here will overload the oscillator/generator circuit. V801 might go but not in a second.

T801 should be a small transformer with 6 terminals.
It might have a short circuit in one of the windings (hard to check, could be done with a square wave generator and a scope, but your scope is broken...). shorted windings are probable with HV transformers, but it depends on how well built it is and how much power it must deliver. I don't have experience with this one. Does it look ok?

An interruption or short in C805 might cause trouble. You might replace it but you might burn another transistor if it's not the cause.

You say V800 was also burnt. This is odd because I think it would never get too much power on this position in the circuit. Because of V800, also check R804.

Maybe C802 is interrupted. Check its capacitance. It is needed to make V801 oscillate, without it, V801 would probably stay in an on-state and maybe burn.

You would also like to check the regulator circuit around D801. With V801 removed, solder a 82k5 (like R802) (82k is also ok, or even 47k) resistor temporarily between -12V and pin 3 of D801. The output on pin 5 should be the same voltage as pin 3 now when you switch the scope on. (with V801 removed, there is no HV, no smoke and no stress on T801). If it is not, D801 is defective or one of its resistors.. But I don't see if it could really fry your V801. It could give +10 or12V output if defective and make V801 work too hard, causing a too high HV (which might then destroy T801).
By the way, if C802 is interrupted, D801 would give a high output too, because there is no HV.
Do not forget to remove the temporary resistor after checking D801.

So the first two things are to check the two capacitors and the rectifier circuit.
Then the most likely cause is the transformer, which is a bit hard to verify, and probably hard to find a replacement for.
The regulation circuit might have a problem, check that too, just to be sure.

Onno