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old tube radio, hum in tx/rx, already recapped

4

479tx

Member
Oct 12, 2011
36
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18
Hi all,

I'm working on an old Sonar FS-23. I have the TX and RX working very nice. There is a hum that comes from the speaker when the volume is turned down, it's pretty loud. Once you turn the volume up and hear static/signals it somewhat drowns the hum out.

The hum is very prominent in the TX audio as well. I have replaced all of the electrolytic capacitors in this radio already.

Someone suggested a loose power supply transformer, tried pushing down on the big black transformer in the back while the radio on and I was observing the rx hum, no change. All the transformers seem pretty well bolted down.

I'm also using tested/good tubes.

Any ideas?
 

The problem's in the power supply, and the first thing I'd try, assuming you don't have an oscilloscope handy, would be replacing the power supply filter capacitors. You may have gotten a bad one when you replaced them. While you're in the power supply area, take a look at the rectifier or rectifiers.
 
Thanks for the replies.

I guess its possible the ground connections to one or more of the replacement electrolytic capacitors I installed isn't so great. under the 4-part canister cap I installed 4 separate caps and had to get creative with getting all of their ground leads back to the original ground location.

I will give my recap work a double check. I had the exact same hum on rx/tx before I recapped the original electrolytics.
 
This is frustrating.

I've taken my replacement (brand new, from the package) caps out and recapped 3 times now and double checked my work each time. This radio only has 10 electrolytic capacitors. (4 in the biggest canister, 3 in the smaller canister and 3 other stand alone caps).

Hum persists in RX and TX.

Characteristics of RX hum:

- Hum starts a few seconds after the radio has been powered on. Around the same time as the meter slowly falls back down to its normal resting position to the full-on that happens upon power up.

- Hum is most prominent in RX when volume is turned most of the way down/all the way down. As the volume is turned up louder the hum gets drowned out by receive audio/static but it's definitely still there.

- Hum disappears as RF gain is turned most of the way/all the way down.

- Hum disappears as squelch is turned most of the way/all the way up.

Characteristics of TX hum:

- Always there, fairly loud background hum.

- TX audio otherwise sounds fantastic using a Turner +3 mic.

*** The RX/TX hum was present in this radio before I recapped it. It hasn't changed for better or worse, just the same. Radio otherwise works 100%.

I don't have a reliable way of testing the capacitors I purchased. They were all new in the package, not NOS. Tubes were all tested by a local CB shop (who unfortunately refuses to work on tube radio and amps anymore). Any tips on things I'm missing, or even better:

Anyone know anyone local to Houston that I could pay to check this thing out for me? I really have a thing for these radios and this one is in very nice condition, just want it working 100% so I can button it up and enjoy it.

Thanks.
 
check tubes on a good checker looking specificly for heater-cathode short/leakage.
tap on the tube as it warms up.or substitute others that are fine in another set.
fixed an old yellow machine for a neighbor that had been humming in rx and tx for many years.tech at the truckstop replaced every lytic in it.iirc it was a 12ax7.leakage was not enough to light the short light on a cheap tester but my jackson 648 found it.
 
479tx said:
Hi all,

I'm working on an old Sonar FS-23. I have the TX and RX working very nice. There is a hum that comes from the speaker when the volume is turned down, it's pretty loud. Once you turn the volume up and hear static/signals it somewhat drowns the hum out.

The hum is very prominent in the TX audio as well. I have replaced all of the electrolytic capacitors in this radio already.

Someone suggested a loose power supply transformer, tried pushing down on the big black transformer in the back while the radio on and I was observing the rx hum, no change. All the transformers seem pretty well bolted down.

I'm also using tested/good tubes.

Any ideas?
Click to expand...

Your problem sounds like filament to cathode leakage in one of the common receive and transmit audio tubes. The tube I suspect is V12, 12AT7.

V12A is used in the receiver as the first audio amplifier stage. In transmit V12A is used as the microphone amplifier. If hum is produced in V12 because of heater to cathode leakage then hum will be present in the receiver's audio and heard in the transmit audio. I recommend replacing V12 (12AT7).

Most common inexpensive tube testers cannot detect cathode to filament leakage that's causing hum. A tube that tests good in a tube tester may not always work, or it may work improperly, in the circuit it's used in. This is especially true with tube used in RF circuits.

73s
Mike
 
Thanks for the reply.

Pretty sure I've swapped V12 with another that I have on hand. Not certain the other tube is good. What are some other tubes that are used in tx/rx like V12?

I'm going to the radio shop this weekend to have all of my tubes tested again. I know last time I was there he was using a big tube tester that didn't look very cheap, and he was stating "no leakage" as he tested the various tubes. I'm not sure if there are multiple leakage tests.
 
479tx said:
Could V13 also be the culprit?
Click to expand...

yes but looking at the schematic from cbtricks v12 is more likely.
btw i had several of these rigs when i was 12 or so.seems at least one was given up for dead due to hum.got them dirt cheap and fixed them.i remember these had a much better reciever than any ss units of the time.
 

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