EM84 magic eye does not close on FM on Philips B4X92A (Techniek Radio/TV)

door tdperez , Eindhoven, 23.10.2022, 15:34 (553 dagen geleden) @ ruudtx

Hi Ruud,

In order to check if there is a between R11-C30 and R18-R21 and EM84pin1, I've unsoldered the connection between R11-C30 and pin 5 of SK4. These are the outcomes:

1. No continuity between R11-C30 and (R18-R21 or EM84pin1)
2. When I turn the radio on with FM, I still have the same result (radio works, but EM84 does not move). Voltage at EM84 pin1 is still around 0.3VDC.

So that seems to rule out that the circuit at R11-C30 has anything to do with the problem.

These are the voltage measurements you suggested (with R11-C30 disconnected), with a strong FM station:

R18-R21: -0.3VDC
R11-C30: -0.3VDC
pin 1 EM: -0.3VDC
across C40: 0.010VDC
R18-C40: 0VDC
R21-C40: -0.004VDC

When I re-solder R11-C30 to SK4 pin 5, all the measurements are exactly the same with a single exception:
R11-C30: 0VDC (instead of -0.3VDC)

It seems the problem is somewhere in the FM detector/discriminator circuit. I have already replaced C41, C40 (twice), C38, EABC80 and aligned S21 and S22/S23. R37 and R16 measure correct. R18 and R21 measure about 10% their expected values, but that should be fine. Is there any chance that either a leaky C36 or C39 could be causing this problem?