Zenith 5-S-319 - Humming noise...

Hi,

I'm working on a Zenith 5-S-319, all the paper and electrolitic caps have been replaced.

All the tubes test as good.

When turning the radio on and it warms up, it starts a loud humming. 

Any idea what I should do?

Thanks, Greg Rowland

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Check if the humming can be influenced by the volume control or other controls on the radio, this will tell you if the hum is entering the circuitry before or after the volume control.

Also, check if its 60 Hz or 120 Hz hum. As this radio has a full wave mains rectifier, any hum introduced by the B+ voltage will be 120 Hz (double mains frequency) if both parts of the rectifier valve are working. If its 60 Hz hum, its more likely that it is somehow directly induced from an AC voltage (i. e. the heater chain or capacitive coupling from the mains).

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I get a loud humming no matter if I change any of the controls, volume, etc.

Any other ideas?

Thank you,

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Torbjorn,

How do I check for 60 Hz or  120 Hz hum? 

I have a frequency counter, but not sure how to use it. 

The rectifier tube tests ok.

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Easiest is to listen to the hum to decide, or to look att the hum voltage with an oscilloscope.

Some tests you could make is grounding the control grids of 6Q7 and 6K6 via a capacitor, one in turn, and check if the humming disappears. Remove the 6Q7 and check if humming disappears.

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