Harman Kardon A300

@ccogopher Before you go to a tech you might compare the working channel against the bad one. I hope you have a schematic!!

Have you been able to sort out if the weak channel is in the power amp section?

Or does it occur further upstream?

Do the record outputs work OK?

Ice checked my cables and connectors and they all look ok. I’ve switched tubes and thoroughly checked solder joints. I do have a schematic and my hunch tells me that the problem is upstream of the power supply, but being a beginner, I’m starting to get into an area that requires more technical knowledge than I possess. I’m going to start going through it more methodically today but I’m doubtful that I’ll get it sorted out which is why I’m looking for a tech because I need a plan to fall back on if my efforts to solve this problem fail.

@ccogopher 'upstream from the power supply' means the AC power switch and power cord, so I'm thinking you meant something else.

The power supply is shared by both channels. So I think the power supply is OK.

Since you have a schematic, check the voltage on the plates of the 12AX7 which is the voltage amplifier to both power amplifier channels. The plate voltage should be about 130V on pins 1 and 6. For the 12AU7 you should see about 260V on pins 1 and 6.

You could also put a jumper between pins 2 and 7 of the 12AX7; this would force both channels to have the same input signal. If both channels then work, you'd know that the problem is upstream from the power amp section. You might want to read this article about restoring an A300.

Ok, thanks for the tip. I did find a tech but I’ll try any potential fix myself.. hopefully I can get this done and save myself some $