Sovtek MIG100H elusive hum

[luis.castillo]

Hi everyone,

I’ve been troubleshooting a hum noise in a Sovtek MIG100H amp. I tested all grounds it seems to be ok. I tried to measure in-circuit ESR and Vloss of the filter caps and they seemed to be bad (ESR 0.4 ohms and 25% Vloss), but then I desoldered one and the reading was good (ESR 0.1 ohms and 0.6% Vloss).
Maybe you can give a hint. These are the symptoms:
•   The hum frequency is 120Hz. (DC heater supply? How can I test that?)
•   It starts only when Standby is on.
•   Hums even without a guitar plugged
•   Increases with Master pot.
•   Volume, presence, bass, middle and treble pots affect hum somehow.

Here´s a video and maybe you can see something I don’t.
https://youtu.be/mOhLNX2V1r8

[Latole]

Schematic

[bmccowan]

If it were mine, I'd sell it and donate the proceeds to the Ukraine resistance.

[mresistor]

The heater V is 6VAC and if it was the noise it would be 60hz hum. Except V1 is dc. You could pull V1 and see if the humm goes away.  You say your noise is 120hz so that wouild be coming from the HV power supply most likely. How old is this amp? I notice it has the grey IC lytics in it that are known to fail quite often in Fender amps. Make sure all the grounds for the power supply are making good tight metal to metal contact.


Hum starting when taken out of standby indicates its in the HV power supply. If the amp is 10-15-20 years old it might be time to replace the filter caps.


Check all the plate resistors.   

[luis.castillo]

Schematic

I already have it but thanks anyway

[luis.castillo]

The heater V is 6VAC and if it was the noise it would be 60hz hum. Except V1 is dc. You could pull V1 and see if the humm goes away.  You say your noise is 120hz so that wouild be coming from the HV power supply most likely. How old is this amp? I notice it has the grey IC lytics in it that are known to fail quite often in Fender amps. Make sure all the grounds for the power supply are making good tight metal to metal contact.


Hum starting when taken out of standby indicates its in the HV power supply. If the amp is 10-15-20 years old it might be time to replace the filter caps.


Check all the plate resistors.

Thanks for your reply. I´ve been very busy but now i can continue with this amp. Yes, heater is VAC but i´ll try what you say, i'm using a frequency spectrum reader app in my cellphone (oscilloscope next time) so maybe is not good. The amp is from the 90´s but filter caps are "modern" (not just a brown cylinder like the other ones). Everything else looks original.
I didn't know that about those grey caps. Ill say they are at least 15 years old. That's a good point. Also Ill double check all grounds. I friend also suggested to check for a loose winding in the transformer but i guess that will be a 60Hz noise.
Thanks for your help!