Soft hum at no volume in tube clock radio
 

I just finished recapping my Panasonic 720 tube clock radio (circa 1964). After doing some initial tests and cleaning the pot and switch with DeOxit D5 and F5, I noticed a soft hum with the volume turned down. Is this normal? I was just wondering if inexpensive radios were more prone to hum at low volumes than better-made radios. Also, could the clock motor introduce some hum to the radio circuit?

Pics

Could be the clock motor. Things to try: disconnect the clock motor. Try jumping a bit more electrolytic across each filter cap. If the first audio tube is not shielded, wrap a piece of foil around it and test clip it to circuit ground. The idea is to try to find out where the hum is entering the system. Sometimes hum is picked up by the first audio tube: try grounding its grid; what happens?

If it's a soft, non-objectionable hum, it may be perfectly normal. Many of the older tube sets, especially the AC-DC type tend to have a soft hum when the volume is turned all the way down but you put your ear up to the speaker.

Gilbert

Bad cap, incorrect value. Bad tube, gassy tube, short in tube. Not likely the motor, not likely normal. Since its am/fm, put the switch between bands to where you get nothing, like a tv tuner between 2 & 3, between the detents. Turn the volume through its range, if the hum is constant its a filter cap. If it changes look at the tubes. If its really hard to notice I hesitate to say its normal. I got a few tube clock radios, none have a low hum.

Nice looking radio....

The hum is a definite AC hum, the same frequency as the clock motor. Even though it's a Japanese radio, it has a GE Telechron clock which are known to get noisy over time. The caps were replaced with exact values save one: a 30mfd replaced with 33mfd. All tubes test good, two originals were replaced with my own stock. Hum is not band specific, heard in both am and fm. I could try shielding the 12BA6 or 12BE6, but I'm skeptical anything will change unless the radio is simply a bad design. I'm still doing tests so keep the suggestions coming.....

Just to add more suggestions: try disconnecting the clock motor and see if you still get the hum. Try flipping the ac plug at the socket. Is the on/off and volume the same switch? If so, deoxit again and check it. Check the leads and make sure there's no shorts or cat wiskers. If you have the Sams, check to see if it mentions any critical lead dressing. Check to see if the speaker housing is making contact with the chassis inside someway. Try blowing out everything with compressed air.

I've been there too. It really is a process of elimination and can be time consuming. Try taking out the filter caps and checking them on a meter, or swapping with others. I had a bad new cap once.

I couldn't get the switch to stay between bands. So I left it on AM between stations (This model radio is quiet between radio stations...no noise). I was able to increase the volume of the hum by turning up the volume knob. Is this a tube issue?

If the hum is varied with volume pot, try grounding the center terminal in the volume control (likely connected to grid of 1st audio tube).
Then try grounding the low side of the volume pot.
If hum stops, you either have a bad ground connection to the pot low side or a bad pot.

Try jumping a 47 mf [>16 volts] capacitor from the 35C5 cathode to ground (pin 1) read clockwise on bottom side, which makes almost any radio sound better but not related to hum.

The temporary shielding test would not be on the 12BE6 or 12BA6, but on the first audio, which would be a 12AT6 or 12AV6, usually.

I had a Zenith that needed larger value filter caps to get rid of same hum, NOW I can listen to my radio at whisper volume levels w/ NO hum.

Larger caps are expensive and minimum values were used due to cost where some hum was allowed. Upgrade caps

I shielded the 12AV6 and then tried replacing it....hum didn't change in either case. I should've mentioned in my original post that the hum is sort of like a 'buzzing' hum, in other words, it has harmonics. It reminds me of grounding issues or RF interference. I guess the next step is to disconnect the clock and see if anything changes. If it doesn't, I'll have to take the radio completely apart again and start checking the circuit for something that isn't properly grounded.

If there is a capacitor bypassing the AC line, make sure it's OK.

That "buzzing" is difficult to figure out sometimes, I've traced it to cheap switch mode power supplies in the area, but no saying it isn't internal

It could be the Telechron clock motor. I have a '68 Sony with the same soft hum, I traced it to the motor. In my radio the hum is only audible when the volume is turned down all the way, or the radio is off.