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

door tdperez , Eindhoven, 19.10.2022, 22:47 (557 dagen geleden)

I'm restoring a Philips B4X92A. After recapping and replacement of off-spec resistors, both FM and AM bands are working normally, with a loud and clear sound. The magic eye is working for AM reception, but does not move when the FM band is selected. The EM84 tube has already been replaced, as well as the 470k resistor R5 that connects pins 6 to 9/7. It lights up and appears fully open, not reacting to FM reception.

When FM is selected, I measure low (or zero) voltages in parts of the detector and AGC circuits (see diagram below). I have already measured all resistors (all within 10% in spec) and replaced a number of capacitors (the ones marked with green circles on the diagram).

There is a point where I measure 0 VDC in FM (see diagram), but I have non-zero voltages on AM (I did not write down the measurement, but can check again). It's a bit difficult for me to understand the switch (SK) diagrams, but based on continuity tests, it seems that the circuit indicated by the red line is the only one that is added when we press the FM bandswitch.

I have been in the hobby for a couple of years and so far could work my way out of most issues, but it's been several months that I'm stuck on this particular bug. Any advice is highly appreciated.

Thank you in advance!