1941 Monkey Ward mumbler

[electra225]

I have a nice old Airline set, unknown model, but it has a Wells-Gardner chassis, 6 tubes, untuned RF amp. It had shorted filter caps like they all do, so I shotgunned the caps. IT worked really well for about two hours, then started mumbling. I can touch one lead of a 250 pf cap with my signal tracer and stop the mumbling for about two minutes. As long as the signal tracer is connected to that cap, the radio does not mumble. I'm sure the alignment is off, since performance is rather weak. Selectivity seems to be good. The audio is clean at the second detector, but mumbles at the input of the volume control to all points to the speaker. It has/had a glass 12SK7 IF amp in it, that I replaced with metal. Pin 1 is grounded, but is unused with a glass tube. I've had the substitution of a metal tube with glass cause this in the past. First guess is that a cap is of the wrong value, is defective, or there is an open ground somewhere. Element voltages seem close enough. I have subbed all the tubes. This one will be interesting.

[hermitcrab]

The cap may be drifting when it heats up?...the signal tracer may be adding or subtracting capacitance...

[electra225]

Well, I have to admit that, since I "fixed" it this afternoon, it doesn't work at all. But, for what it's worth, it doesn't mumble.

I replaced the filter caps and the cathode bypass cap. Now I don't get anything thru the radio. If I use a signal generator, it seems like I can get a signal thru for a couple minutes, then it just goes away. This is one of those things that, when I find it, I won't want to admit what I found. There are some drifted resistors, but not enough to cause this. Next is to bridge all the mica caps to see if that helps. When I can get signal thru the set, if I touch a metal tube or the metal chassis, it quits. I have run the grounds to common negative and they all seem okay. Maybe check chassis grounds? I think this is the strangest thing I've ever come across. I've had this radio several years, and never have had it playing except for a couple hours. This is one time I'd been better off getting a baseline of performance before I tried to "fix" it.

[TC Chris]

Maybe check chassis grounds?

Especially ones that are lugs riveted to the chassis.

Chris Campbell

[electra225]

Volume control. I was right. I don't want to admit it......

I have never seen this condition. The volume control was scratchy, but no worse than I've seen a hundred other times. For some reason, yet unexplained, the volume control was losing ground. I jumpered across the hot terminal and the wiper on the volume control and got most of the volume back, but the audio was still garbled. I checked the ground connection from the control to common negative and it showed good. The control itself apparently is not grounding internally. I ran a redundant ground and got a wide-open radio that distorted due to too much volume, but it wasn't mumbly. Now I need a split-shaft volume control. Time to see what Mark Oppat has. The alignment is off because I have birdies between stations. We'll do the volume control first, then worry about alignment. I may replace a few resistors as well. The AVC resistor is drifted over 20%.

[Firedome]

So is mumbled synonymous with garbled then? I've heard the term used in context with indistinct or attenuated human speech but not with radio performance.

[electra225]

The word I wanted was "garbled". The word I thought of was "mumbled". I'm old. I had a senior moment.

"Mumbled", "garbled", "distorted", or simply "sounds like crap", all would have the same meaning to me, human voice or electronic.

[Firedome]

Righty-o, I was thinking it was some kind of a Mid-West expression, in this big country there's a lot of unique regional lingo, Baltimore had a lot of them.

Monkey-Wards for Montgomery-Ward seemed to be used just about everywhere though, we always called it that in MD, and our relatives in NY, MA, NC, and FL did too!

[hermitcrab]

old paper coupling caps in the audio will garble the sound ...

[electra225]

I recapped the radio when I got it several years ago. It quit working after about a couple hours, and I never did anything else with it until last weekend. I just felt in my gut that I was chasing a poor ground someplace. Everything else I checked, element voltages, resistors, wiring errors, all checked okay. I couldn't find a thing "wrong" with the radio, yet it wouldn't work. I did check grid voltages, both for a bad coupling cap and for grid emissions in a tube. B+ was a little low, 95 volts vs the 113 it called for, so I changed the filter caps just because. That did bring B+ up, but it did nothing for the distortion.

[Motorola minion]

This issue is a classic tube set problem and I learned because of it. Check any resistors from the grid to ground, also called "grid leak" resistors. Even the oldest radio has a capacitor feeding a grid, and this high-value resistor kept the grid from building up a charge. Grids must remain at or close to zero volts unless biased externally.

It "sounds" as if one of those grid leak resistors is open and this is why it does what it does. The garble/mumble is the grid becoming negative in respect to the cathode, gradually cutting off top portion of wave until nothing gets through. Turning up volume speeds up this process.

Especially in audio amp and output stages, the audio signal as an AC waveform is "rectified" by the conduction of positive portion of that wave going to a tube's cathode (to ground) and negative portion of that wave builds up a charge, cutting the tube off.

[electra225]

The problem, of at least the biggest part of the problem is the volume control. For some reason, it's not grounding. The ground connection on the volume control is floating. I don't know if the element inside is broken or corroded or just what. I can run a test lead from the body of the volume control to ground and get a somewhat better response. The volume control is also a high-resistance connection to the audio. I can jump across the input and the hot terminal and get full volume. I can run a redundant ground and get cleaner audio. Not perfect, but enough better that there is no doubt about the volume control being at fault. I have checked for a floating grid and positive voltage on the grid, as well as cathode voltages. I don't see an issue there. The first thing I need to do is replace the volume control. Then take it from that point. I know an alignment will be indicated, since my tinkering has that out of kilter, I'm sure.

[Motorola minion]

The volume control is part of the load for the detector, also AVC is developed across the volume control, all tied together