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

[radio11]

479tx,did u solder the new caps to the lugs on the old multi sec caps?if so,that could still be the cause of your hum.

 

[falcon2448]

Could V13 also be the culprit?

Not likely. V13 operates in Class B Push-Pull and cathode to grid #1 leakage typically cancels out in Push-Pull amplifiers.

Looking at V13 you will notice there are 2 sections, pins 6, 7, and 8 which we will refer to as section "A" and pins 1, 2, and 3 which we will refer to as section "B". This type of amplifier circuit is referred to as "Class B Push-Pull".

In a Class B Push-Pull amplifier such as V13, when section "A" conducts during the first half of the audio sine wave, section "B' is not conducting. During the second half of the audio sine wave, section "B" conducts and section "A" does not conduct. This action of the Class B Push-Pull amplifier tends to cancel out cathode to grid #1 leakage hum. This does NOT mean the Class B Push-Pull amplifier will eliminate hum caused by cathode to grid #1 leakage in an earlier stage such as V12. Hum created by cathode to grid #1 leakage in an earlier stage such as V12, becomes part of the audio signal fed to the Class B Push-Pull amplifier and the output of V13, heard as hum in the speaker and in transmit, heard as hum in the transmit audio.

If the source of hum is caused by a bad electrolytic filter capacitor then the hum level would be much higher than it is AND the hum would not disappear when receive signals were present. In addition,the audio output heard in the speaker would be "distorted".

I agree with radio11, never use old can capacitor positive tabs as mounting points for new capacitors. The same AC voltage is being applied to the old AND new capacitor's positive terminal. If the old capacitor is on the verge of shorting internally, when it shorts the new capacitor also shorts.

Mike

 

[479tx]

I have the terminals of the original two cans completely disconnected electrically.

Falcon,

The hum doesn't necessarily go away when I turn up the volume, it just is easily drowned out. If you only turn the volume up so far (hearing static & other transmissions) you can still hear it behind the RX, just not overpowering.

Does it mean anything that the hum goes quiet when the squelch is turned up or the rf gain is turned down?

Like I said, I reinstalled the replacement capacitors 3 or so times to make sure I didn't have a cold solder joint, bad connection, etc. I had to extend a negative lead of one replacement capacitor & a positive lead coming from the radio on another replacement capacitor so I could place the capacitors neatly under the radio, otherwise, that's the only difference from stock.

I'm using mostly tested tubes. I think the only tube I'm using that I didn't have tested is V13. The tube in V13 was cracked when I got this radio so I put one that I had sitting around in there.

I found an electronics store today that's near my office that has a huge old drug store type tube tester in their lobby (owner says recently calibrated, they sell components and NOS tubes that apparently you can use for free.

I'm going to take all of my tubes for this radio up there tomorrow and test them out. It looked like The meter/controls of the thing only had two basic tests, shorts & strength. I know most of my tubes are probably not shorted because everything works, volume, squelch, tx/rx on frequency, etc.

Would a tester like I'm describing also rule out grid leakage? Or would it specifically have to have a function/knob for that test?

Thanks a ton for the replies. I'm going to figure this out!

 

[479tx]

I just spoke with the owner and he said he's got a Hickok tester and I can drop my tubes off with him in the morning and he'll test them thoroughly for 25 bucks and let me pick them up. I think that's worth it, I've got about 25 tubes for this radio.

1$ a tube. Much cheaper than finding a legit tube tester to purchase.

 

[Mudfoot]

I just spoke with the owner and he said he's got a Hickok tester and I can drop my tubes off with him in the morning and he'll test them thoroughly for 25 bucks and let me pick them up. I think that's worth it, I've got about 25 tubes for this radio.

1$ a tube. Much cheaper than finding a legit tube tester to purchase.

Gotta wonder how good these old testers are anymore. Do they make modern testers anymore?

 

[kc8adu]

first of all its heater-cathode leakage you want to check for not cathode-grid.
too bad you are not near dayton oh.
i would check em free on like a saturday when we usually are not busy.
i thought of another possibility.as much as folks mess with the insides of radio's you could have a lead dress issue.like a audio lead up against an ac line or a heater line.

 

[479tx]

Thanks for the replies, kc8adu.

I wish we were in the same city - that'd be handy. I'd definitely buy you a case of beer and or some lunch! Would be cool to learn a thing or two from someone about this stuff too.

The radio looked 100% stock when I got it. I don't think anyone has done any mods/touched it short of replacing a few tubes. And I've studied just about every photo online of an FS-23/3023/2340 and service/owners manuals.

Unless one of the + leads going to one of the terminals on the power supply canister/cap in the back of the radio is an audio line, and I've somehow relocated it to be touching an ac/heater line (had to do a little rearranging, nothing major) to get my replacement caps neatly soldered onto the bottom without being in obviously bad places.

The radio had this exact hum before I event went after the electrolytics. I could have a bad tube, but I guess I won't know for sure until I can test them all 100%. I can buy a bunch of new ones on e-bay and spend 100+$ replacing them all (with shipping) but I still won't know if the ones I got are any good.

I will ask the guy at the electronics shop tomorrow more specifics on the model tube tester than he has can test. Will specifically mention heater-cathode leakage and just look it up myself online if he gives me the model # tester.

 

[479tx]

Of the 18 or so tubes I had for this radio, I had them all tested.

The ones I got back as bad (he said leaking, shorted or one was so weak it was barely working)

2x6au6
2x6ea8
1x6bq5

I have enough "good tubes" to stock the radio back up, except I need to order 1x6ea8.

The two V12 (12AT7) tubes I had in the lot tested excellent, as did V13 (12BH7).

How likely is it that my hum was being caused by a 6au6 or 6ea8?

 

[479tx]

Well, fired the radio up last night with all known good tubes.

Hum persists. :blush::blink::bored:

Maybe I have some bad resisters.

I'm hesitant to think it's a wire dress issue (had to move two + leads to mount my replacement caps under the big power supply multisection canister) because it had the exact same hum before I did anything to the radio.

I guess I'm going to start testing resisters tonight.