Restoring MCS 3233: power supply hum won't go away

[gnarlyseven]

I got this MCS 3233 from my dad several years ago and in the process of upgrading it (complete recap, replaced output transistors) we've discovered a hum coming from the power supply that we can't get to go away. Thanks to this forum (so many helpful posts!!) we've been able to address most problems as they arise up 'til now. The filter capacitors are new and show no signs of damage, so we aren't sure what could be causing this hum. Changing the volume doesn't affect the hum, and its frequency is 121 Hz.

 

[painttoad]

I got this MCS 3233 from my dad several years ago and in the process of upgrading it (complete recap, replaced output transistors) we've discovered a hum coming from the power supply that we can't get to go away. Thanks to this forum (so many helpful posts!!) we've been able to address most problems as they arise up 'til now. The filter capacitors are new and show no signs of damage, so we aren't sure what could be causing this hum.

was the hum present prior to recapping the power supply board?

on the 3233 that i did, the screened polarity for 1 cap was backwards.

review your before pics then review your work.

 

[gnarlyseven]

was the hum present prior to recapping the power supply board?

on the 3233 that i did, the screened polarity for 1 cap was backwards.

We did find the reversed polarity on the PCB and made sure to install it properly. I can't remember if there was a hum before the recap- I remember the thump for sure, but I don't remember any hum because I'd leave the receiver on overnight and I'm a verrryy light sleeper.

 

[northpaw]

I'm not familiar with the receiver, but would suggest looking for a lost/compromised ground connection somewhere.

 

[redk9258]

Is the hum from the speakers or power transformer?