Buzz + ringing sound on left channel [solved]

[serpent68]

I was attempting to add bypass capacitors to the 1st and 2nd power supply cap, and also changed out the bypass cap to the main output caps. 

After installation, there is a loud buzzing sound with a high pitch ringing sound on the left channel.  The buzzing sound does not change with volume change.  There is no music coming from the left channel, but the right channel is perfectly fine.  The high pitch ringing sound seems to come and go.  When the amp is switched off, there is a very loud pop sound, and the buzzing sound remains in the left channel for about 1 second before dying off.

I have removed the bypass caps for the power supply caps but the problem remains.  Also re-soldered all the joints that I touched, but the problem is still there.

Any suggestion for quick troubleshooting?  Thanks in advance.

 

[Paul Birkeland]

I'd start by measuring the voltages and reporting any that aren't in spec.

 

[Doc B]

Bad signal ground connection

 

[serpent68]

Both the LEDs on the small PCB B are not lighted up.

Terminal 5 reads 0V instead of 90V
Terminal A1 reads 0V instead of 90V
Terminal B4 reads 0V instead of 90V
Terminal 9 reads 28V instead of 100V

What's wrong?

Thanks PB and Doc.

 

[serpent68]

I've checked all other voltages are within specs.  The LEDs on the big PCB and the small "A" PCB are all lighted up.  Seems to me like a one sided problem.  I've checked all ground connections resistance and they appear fine.

Appreciate any guidance on what to do next.  Should I re-solder everything on the small PCB?  Thanks.

 

[Paul Birkeland]

Resolder the center leg of the MJE350 on the small PC board by the volume pot.

 

[serpent68]

Problem is resolved.  Bad solder joint from terminal 4 to the small PCB.

Thanks PB for your advice.