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Robyn T-123b

W

Wky Gatekeepr

Member
Feb 1, 2023
23
6
13
48
Western KY
Have the above radio humming bad when the power switch is turned in the off position, the hum stops when the power switch is on. But during transmit everyone says it has a bad hum, could this be the Electrolytic can capacitor next to the transformer causing this issue?

Thanks for all feedback
 

That doesn’t make a lot of sense. With the power switch in the off position and the standby switch open there should be no current/voltage in the power supply.
If you were to measure voltage on the caps with the above conditions met, I would say either the on/off switch or the standby switch are shorted or dirty or just plain conducting when they shouldn’t be.
A hum could be caused by an electrolytic capacitor or a shorted diode.
I would test the power and standby switches first. Solve that issue and track down the source of the hum.

73
David
 
When someone tells me his radio is 'humming' I'll resist asking "Did it forget the words but remember the melody?"

More useful question is to ask if the hum is 120 Hertz or 60 Hertz. If it's 120, this points to any of a handful of filter capacitors in the radio. The multi-section "can" capacitor is not the only one that can fail and do this. A radio that's 46 or more years old really should have all or nearly all the filter caps gone bad by now. The rectifier circuit that feeds into the filter capacitors is a "full-wave" circuit that charges the filter cap on both the positive and negative peaks of the AC coming out of the transformer. This effectively doubles the frequency of the 60 Hz AC when the filter caps go bad.

A 60-Hertz hum will be caused by some other fault than a failed electrolytic capacitor. A failed tube can do this but you won't see it very often.

If the standby switch is wired like the schematic shows, you should never hear anything when "standby" is selected.

And if somebody changed the wiring, who knows?

73
 

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Wky Gatekeepr said:
That is with it open
Click to expand...
I am new to tube CB radios and repair. I just got a Robyn T 123b, but cant find the photofacts to identify parts of the radio, by the pictures. First, can anybody share a full copy of the photofacts? Second, can I call on any of you for help as I go along?
 

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