1935 GE Model A-53 AM/SW Tombstone Radio

[vortalexfan]

Well I figured out what my problem was, my one replacement filter cap that I installed to replace one of the old can caps on top of the chassis didn't have a good solid ground connection to the chassis, so I unsoldered the negative lead from where I had soldered it to the chassis, roughed up the metal chassis in the area where the negative capacitor lead was to go and then resoldered the negative lead to the chassis (that solved the incessant humming noise issue)

and the issue with the extremely distorted audio when being fed a signal from the RF Signal Generator, was because the grid cap for the 6J7 was grounding itself out (the grid cap wire lead had apparently lost enough of its rubber insulation inside the shielding sleeve that the shielding sleeve was causing the grid cap wire to ground out against the shielding sleeve, so I took and replaced the grid cap wire lead and BAM! the radio started working properly!

The only problem I have now is that the volume control seems to have some "dead spots" in it because it will go from completely silent to blow your ears out in a quarter turn and then I think the speaker might have some issues because the audio comes out completely distorted and barely audible at about quarter volume or a little less, and when you turn it up to about half way the sound is so distorted that you can barely hear what is being said on the radio.

Just thought I would give a little update on this radio.

[vortalexfan]

OK So another update:

I DID have this radio working fine last night, but then when I went to install a permanent power cord onto it tonight (instead of the temporary one I had on last night for testing purposes) I went to fire the radio up after installing the new permanent power cord and wouldn't you know it the radio went back to humming like a son of a gun again!

I checked all of the ground connections and they were fine, I checked all of the electrolytics and they were fine, and I even checked the 6J7 tube's Grid Cap wire shielding and that was fine so I don't know what to say about this thing!

Any ideas as to what to check next? Because I'm starting to run fresh out of ideas on this thing...

[dieseljeep]

Originally Posted by vortalexfan

Well I figured out what my problem was, my one replacement filter cap that I installed to replace one of the old can caps on top of the chassis didn't have a good solid ground connection to the chassis, so I unsoldered the negative lead from where I had soldered it to the chassis, roughed up the metal chassis in the area where the negative capacitor lead was to go and then resoldered the negative lead to the chassis (that solved the incessant humming noise issue)

and the issue with the extremely distorted audio when being fed a signal from the RF Signal Generator, was because the grid cap for the 6J7 was grounding itself out (the grid cap wire lead had apparently lost enough of its rubber insulation inside the shielding sleeve that the shielding sleeve was causing the grid cap wire to ground out against the shielding sleeve, so I took and replaced the grid cap wire lead and BAM! the radio started working properly!

The only problem I have now is that the volume control seems to have some "dead spots" in it because it will go from completely silent to blow your ears out in a quarter turn and then I think the speaker might have some issues because the audio comes out completely distorted and barely audible at about quarter volume or a little less, and when you turn it up to about half way the sound is so distorted that you can barely hear what is being said on the radio.

Just thought I would give a little update on this radio.

You probably have to remove the volume control and have a look inside!
IDK if the volume control is a log or a linear taper. Also, the schematic shows a 250ohm resistor in series, but the parts list shows that the resistor is part of the control. Some experimenting is needed!
Re: the next entry problem with the hum, maybe the 6J7 or the 6F6 has an H-K short.
Sure is one for the books.

[Jeffhs]

I cannot for the life of me imagine how just changing the power cord would cause a very loud 60-Hz hum as you are describing; after all, the power cord itself should not have any effect whatsoever on the audio output. If the radio had been working up to now, the only thing I can come up with is a heater-cathode short in one of the audio tubes. Given this radio is very old (pre-WWII vintage), that would not surprise me in the least. Vibration from moving the chassis could have caused such a short, especially if the tube were about to develop an H/K short eventually.

I would not operate the radio in its present condition if it does not have an AC line fuse (it is my understanding that most pre-war radios did not have such a fuse, which all too often led to very serious trouble if a tube were to develop an H/K short or a filter cap would short the same way). The house fuse might blow in case of a shorted tube or filter capacitor, but I wouldn't count on it.

[vortalexfan]

Originally Posted by dieseljeep

You probably have to remove the volume control and have a look inside!
IDK if the volume control is a log or a linear taper. Also, the schematic shows a 250ohm resistor in series, but the parts list shows that the resistor is part of the control. Some experimenting is needed!
Re: the next entry problem with the hum, maybe the 6J7 or the 6F6 has an H-K short.
Sure is one for the books.

Well for sure I did swap out the 6F6 tube and the 6J7 tubes for known good tubes to check the H-K Short theory and of course it still has that nasty hum to it even after swapping in known good tubes.

I ohmed out the volume control pot just to make sure it was still good and it was only measuring 1 ohm all across the pot no matter where the volume control knob was set, which tells me that the volume control pot has failed and is perhaps what is causing my humming problem, because the volume control is grounded to chassis using the left side terminal and goes to the kathode of the 6A8 tube from the middle and right side tap of the volume control pot.

Is there any way to test this theory by somehow taking the volume control pot out of the circuit?

[vortalexfan]

Originally Posted by Jeffhs

I cannot for the life of me imagine how just changing the power cord would cause a very loud 60-Hz hum as you are describing; after all, the power cord itself should not have any effect whatsoever on the audio output. If the radio had been working up to now, the only thing I can come up with is a heater-cathode short in one of the audio tubes. Given this radio is very old (pre-WWII vintage), that would not surprise me in the least. Vibration from moving the chassis could have caused such a short, especially if the tube were about to develop an H/K short eventually.

I would not operate the radio in its present condition if it does not have an AC line fuse (it is my understanding that most pre-war radios did not have such a fuse, which all too often led to very serious trouble if a tube were to develop an H/K short or a filter cap would short the same way). The house fuse might blow in case of a shorted tube or filter capacitor, but I wouldn't count on it.

I think I figured out what happened, I think the volume control pot is what's causing the radio to hum, because I think the volume control pot (which was already sketchy when I had gotten the radio going) finally went to "pots" (pun intended) when I attempted to clean the volume control pot with contact cleaner to see if I could restore the volume control's functionality again (because as I had mentioned before the volume control pot when it was working before it apparently finally failed would cut in and out depending on where the volume contol was rotated.)

Does any one on here have any known working volume controls with a built in power switch that they could send me?

[dieseljeep]

Originally Posted by vortalexfan

I think I figured out what happened, I think the volume control pot is what's causing the radio to hum, because I think the volume control pot (which was already sketchy when I had gotten the radio going) finally went to "pots" (pun intended) when I attempted to clean the volume control pot with contact cleaner to see if I could restore the volume control's functionality again (because as I had mentioned before the volume control pot when it was working before it apparently finally failed would cut in and out depending on where the volume contol was rotated.)

Does any one on here have any known working volume controls with a built in power switch that they could send me?

I don't think there's too many volume controls of that value around!
Check it with all the leads disconnected and see if it reads the same.
Remove it from the chassis and look inside.
All kinds of strange goings on with that set.

[vortalexfan]

Originally Posted by dieseljeep

I don't think there's too many volume controls of that value around!
Check it with all the leads disconnected and see if it reads the same.
Remove it from the chassis and look inside.
All kinds of strange goings on with that set.

OK Will do.

[dieseljeep]

Originally Posted by vortalexfan

OK Will do.

I've seen non-AVC superhet radio circuits, but that one is a real gem.
I repaired the one I have or had, but I just replaced the bad caps and never realized that the set was that strange!

[dieseljeep]

Originally Posted by vortalexfan

OK Will do.

I located that chassis in my collection. I'm not sure if I have the cabinet, so I don't want to part it out.
It sure is a strange beast!

[vortalexfan]

Originally Posted by dieseljeep

I located that chassis in my collection. I'm not sure if I have the cabinet, so I don't want to part it out.
It sure is a strange beast!

how is yours wired up?

[dieseljeep]

Originally Posted by vortalexfan

how is yours wired up?

I think the same, but I'll look again to look at the connection.

[vortalexfan]

Originally Posted by dieseljeep

I think the same, but I'll look again to look at the connection.

Here's a picture of the underside of my radio, is there anything that looks out of place here compared to yours?

[DavGoodlin]

I had a look at the schematic and the 5000 ohm volume control seems to vary suppressor grid voltage on the 6K7 IF tube.

[dieseljeep]

Originally Posted by vortalexfan

Here's a picture of the underside of my radio, is there anything that looks out of place here compared to yours?

That volume control pot looks like a replacement and a wire-wound control.
The one in my chassis is smaller, possibly the original.