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

And here are some pictures of its man and control PCBs: image 1 Image 3 image 2

I'd greatly appreciate your help on narrowing down the cause and find a solution to this issue.

  • 1
    \$\begingroup\$ 50 or 100Hz Humms typically come from the power supply, so a broken capacitor would be my first bet. Those big silver capacitors in the center look like they're leaking!? \$\endgroup\$
    – PMF
    Commented Jan 31, 2022 at 8:39
  • \$\begingroup\$ @PMF they are not leaking, it's glue. \$\endgroup\$
    – Justme
    Commented Jan 31, 2022 at 8:44
  • \$\begingroup\$ @Justme You are right, I thought those white stripes were a trace of a leaking something. But these ought to be there :-) \$\endgroup\$
    – PMF
    Commented Jan 31, 2022 at 8:51
  • \$\begingroup\$ @PMF, as Justme said its white silicone glue, visually those capacitors look fine. \$\endgroup\$
    – Esquirish
    Commented Jan 31, 2022 at 8:52
  • \$\begingroup\$ I’d be checking the pots first - they're looking a bit decroded. Maybe a spray of Caig deoxit might help? \$\endgroup\$
    – Kartman
    Commented Jan 31, 2022 at 11:48

1 Answer 1


It looks like hum is generated by balance-tone circuit. Volume control is previous to this module, also when direct is selected, hum is removed. I should check +15 and -16 V power supply, probably one these has excesive ripple. Try to measure ripple (if no oscilloscope available, some testers blocks DC when AC voltage is selected) in +15V (faulty components could be diodes D711&D712, IC701, capacitors C713, C715 and C313) and -16V (zener D716, D713&D714, C712, C714 and C314).

  • \$\begingroup\$ Thank you i'll follow your instructions and report back. Unfortunately i don't have an osciloscope, just a digital multimeter. \$\endgroup\$
    – Esquirish
    Commented Jan 31, 2022 at 8:56
  • \$\begingroup\$ @Esquirish You could try reading the voltage on the multimeter's AC voltage setting. \$\endgroup\$ Commented Jan 31, 2022 at 17:23
  • \$\begingroup\$ @AndrewMorton will do. Thanks for the heads up. \$\endgroup\$
    – Esquirish
    Commented Jan 31, 2022 at 20:07
  • \$\begingroup\$ @Bravale Thank you. You were absolutely right. Faulty component was capacitor C712. It looked fine at a glance but was actually bulging on the underside. Before taking it out i connected a new equivalent capacitor in series with the old one (with alligator clips) and the hum was gone. It was a rudimentary testing method but it worked. \$\endgroup\$
    – Esquirish
    Commented Feb 19, 2022 at 7:17

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