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This is by the way, but should anyone have an issue with one channel in their phono section not playing I have now come across my second repair (a 650) with the p-Channel jfet bad ( i.e. q11-2n5464/65). I also came across the same problem with a previous 500 I repaired. These p-channel Jfet are...

Change those 726 and 1313 transistors on the Power regulation Board(F2508). (Ksa992 and Ksc1845). I believe these are transistor 13 and 14

There are some 1313 and 726 transistors on the power supply board which must be changed too. Also on F2513. Use the 1845s and 992s. check your voltages ( especially if both channels are affected). Check your driver and pre driver transistors on the driver board. Check your resistors...

AU X701

hard to help to troubleshoot/help without a response from pppoffice as per Echo Wars query...not even sure there's still a problem.

right channel works so unlikely a case of missing supply rail ( they are supplied from the same place for both channels). unless, of course, there is a connection issue (loose connector etc).

did you swap the position of the driver boards to eliminate components in front and behind this board? As a first step you want to be absolutely sure its the driver board.

For completeness, the resolution was the change out of the two 1uf 50V capacitors (c11 and c12) on the filter switch board(F2611). I used film capacitors and the popping completely went. Note that these capacitors are switched into the signal path when the tone selector switch is flicked into...

Thanks for the responses. I recapped the power supply and amp boards but haven't touched the tone board. Will do more work on it.

Was there ever a resolution to this problem?. I am having the exact issue with my 7900. ie popping when tone filter switch is engaged or when the low filter switch is engaged.