I recently acquired a stereo amplifier, Pioneer brand, A-303 model. Upon testing it, i noticed it produces an audible speaker hum (at about 100hz) whenever the bass and balance knobs are turned.
I'm trying to narrow down the cause of this hum and hopefully, repair it. This amplifier was in pretty rough shape when i got it, i have since thoroughly visually checked it and cleaned it, its internal ground connections, solder points and capacitors appear to be fine (to my untrained eyes at least).
Here are my observations/notes on this issue:
- The noise/hum is produced via the speakers (not from the amplifier itself)
- The noise/hum is produced even when no input cables are connected
- The noise/hum is produced when when the ground connection is connected or disconnected
- The noise/hum is produced on both A and B speaker outputs
- When the balance knob is rotated, the noise is heard from the speaker being muted (for example if the balance is rotated towards the [R] position, the noise/hum is heard from the left speaker)
- When the bass and balance knobs are in their default positions, no hum is produced
- The treble adjustment does not produce noise/hum
- The noise/hum is not affected by the volume knob position (doesn't change when i rotate the volume knob)
- The noise/hum gets progressibly louder when the bass knob is rotated (both to the right [+] and left [-])
- If the noise/hum is heard because the bass knob is rotated, it disappears when the "direct" button is pressed.
- Other than the noise/hum being produced, bass/balance knobs are working as expected (they change the tone)
- The Loudness switch works as expected (does not produce noise/hum when activated)
The service manual with its schematic and PCB connections can be downloaded from the following page: https://elektrotanya.com/pioneer_a-303r_arp2854.pdf/download.html
I'd greatly appreciate your help on narrowing down the cause and find a solution to this issue.