VM 1428 hum balance doesn't work

[AudioWizard]

Working on a 1428, the hum balance control doesn't have any affect. The pot, ohms out ok (200 ohms), the wiper has a good chassis ground. All filter caps and rectifiers have been upgraded. The power output are good on both channels. The 60hz hum is low, but the control has no affect at all, just like it isn't there. Any ideas?

 

[century tek]

The hum balance is usually for helping to cancel out the hum in push / pull output stages when one or both tubes age or are mismatched. I would think that if there is no change then the tubes are pretty well matched or balanced.

Sometimes you have to put your ear up real close to the speaker to even hear the slightest change.

 

[AudioWizard]

This hum balance is on the 6.3 AC filament transformer windings which goes to all the tubes except the phono preamp tubes. This hum setup is the same as Mac MC-30.

 

[gadget73]

Some hum balance pots don't do very much. Are you sure the residual hum is 60hz and not 120? It won't be able to do anything for 120 hum.

 

[AudioWizard]

I will measure the frequency to see, it sure sounds like 60hz hum, it is low level for sure. What brothers me is that it has no effect at all, this is looking at the output with a scope, the distortion analyzer output also shows no change with pot movement, just like its not connected to the circuit.

 

[knockbill]

Hum pots do go bad,,, is the wiper in good contact thru the whole turning radius...

 

[Dadbar]

Do you have the amp hooked up to inefficient speakers? You may need to hook the unit up to something more sensitive to hear the difference (like headphones)

 

[AudioWizard]

Do you have the amp hooked up to inefficient speakers? You may need to hook the unit up to something more sensitive to hear the difference (like headphones)

Sure see the hum on the scope and the control has no affect on the display. I think I will try to swap out the pot and see if that changes anything.