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.

[Jon Snell]

120HZ is main smoothing issues. Heaters will only cause hum if a valve has a cathode heater leak internally.
Only when powered on and running; drawing current.
Hums without an input, internal hum caused by poor smoothing.
Increases with master control; this is getting more tricky as it could be hum balance if DC heaters.
All controls including tone controls have an effect on the hum; bad ground or incorrect ground point on a pre amp valve.

Maybe.

[catalin gramada]

I think those amps are banned from 24th of February and no one may help with as it is a war crime...
For me it sound as an open signal ground return from you video.

[Zouto]

Since it‘s probably made in Saratov, Ukraine, it is humming strongly against warlord Putin to hum him out…..
I had similar problems with my 100H, exchanging all cheesy Input Jacks with solid reAns plus new and better Input wiring cured the hum. Checking and replacing Filter Caps is also a good idea.
Good luck
Zouto

[Chuck H]

...

[DrGonz78]

There are two 220k resistors in parallel of the two 220uf @ 300v caps, those caps sit in series for a 110uf 600v value. I first would make sure one of those 220k resistors wasn't way off the mark. Please post working voltages in the amp or at least the plate voltage on the power tubes and down the power rails. I would look at all the screen resistors on the power tubes and calculate the bias on each tube. Are the main filter caps new or very old? An esr meter reading is at low voltages and will not identify a faulty cap until it reaches high voltage. As you say the hum is increased by the master volume that seems to indicate that the hum is before that pot. Then it could be a hum from any location of the circuit from the input jack to the master volume control. Keep checking all ground connections through out the amp.