Audio muting circuit. Sony STR-2800L - UK Vintage Radio Repair and Restoration Discussion Forum

samjmann · 21st Feb 2023, 10:27 pm

The circuit attached was giving problems due to a substance that was spiled onto the the pcb. This is a re-draw of the original, there are two in the receiver.

In the fault condition there was no audio been passed through the FET. If I put a link between the pcb top surface and the cathode of the diode, it de-muted. The circuit is so sensitive that on the working channel if put a DMM meter probe on the gate of the FET, it modulated the audio with the 'test-current' of my meter.

I've since cleaned the board and things are 90% better. Both the source and drain read 0v. As far as I can see the gate is pulled negative to the -31v supply until the +15v is established.

Can anyone shed any light on the operation of the circuit?

Thanks, SJM.

21st Feb 2023, 10:27 pm	#1
samjmann Heptode Join Date: Feb 2014 Location: Nottingham, UK. Posts: 648	Audio muting circuit. Sony STR-2800L The circuit attached was giving problems due to a substance that was spiled onto the the pcb. This is a re-draw of the original, there are two in the receiver. In the fault condition there was no audio been passed through the FET. If I put a link between the pcb top surface and the cathode of the diode, it de-muted. The circuit is so sensitive that on the working channel if put a DMM meter probe on the gate of the FET, it modulated the audio with the 'test-current' of my meter. I've since cleaned the board and things are 90% better. Both the source and drain read 0v. As far as I can see the gate is pulled negative to the -31v supply until the +15v is established. Can anyone shed any light on the operation of the circuit? Thanks, SJM. Attached Thumbnails __________________ It's never been right since we've had it...