Back from a couple weeks of vacation and through my first week of classes, so I've had a little time to get back to the Electrola. When I left off, I had the power supply and amplifier working but no sound from the radio.
I cobbled together a little AM signal injector from my son's ancient Radio Shack "100-in-1 projects electronics lab," applied the output to the grid of the detector and, woohoo, heard a tone from the speaker. Moved back to the grid of the third RF amp and nothing. Stuck the scope on the grid of the detector and saw this eensy little AM signal, way less amplitude than I was pushing into the grid of the 3rd amplifier, so it was attenuating rather than amplifying. First and second stage RF amps were doing the same.
At this point, I had checked the plate and screen voltages and they were sort of reasonable, so it occurred to me that perhaps I should see where the cathodes were living. Turns out they were only about 15 volts below the plate and real close to the screen voltage. Oops.
So I took another look at the schematic. Here is a stripped down version of the first RF amp.
The volume control is a part of a voltage divider from B+ to ground that sets the cathode voltage for all the RF amp stages.
A bit of messing about with soldering iron and ohm meter revealed that the negative end of the 1K pot was open, leaving the cathode at nearly screen potential.
My first thought was to buy a dual gang 1K pot and a dual gang 50K pot and do some surgery to make a Frankenpot with 1K and 50K. But curiosity overcame me and I decided to drill out the rivets and see what was in the original and why it had failed.
The coppery thing on the left half is a spring that pushes the silver arc of metal against the carbon layer beneath it at one point. The problem is that on one end, the carbon layer does not come all the way to the rivet that holds the solder lug. It looks like a bit of the carbon was scraped or dissolved away. Not sure how, because the rivets were intact before I drilled them out.
Anyway, this got me thinking that if I could get some conductive paint, perhaps I could fix the thing. I found this web page:
http://www.instructables.com/id/Cond...nt-and-Conduc/
which I used as a starting point. Instead of Liquid Tape I used Barge Cement, a contact cement used in shoe repair. It is sort of like industrial strength rubber cement. I have used it in the past to lay HO rails onto ties without needing to spike them. Stands up well in my experience. I used acetone instead of toluene as a thinner because I had it on hand. My final mixture was 3 parts powdered graphite, 3 parts acetone, 2 parts barge cement. Let me remark in passing that this is an outdoor activity. Acetone should be used with good ventilation and powdered graphite is annoyingly difficult to clean up if you spill it.
Anyway, I mixed it up, painted on 3 layers, and let it dry. Here is before
and after
Next morning after drying overnight, the resistance had gone up about 100 ohms more. Here it is reassembled:
I just finished soldering it back in, powering it up, and I am now listening to KYW. I will be interested to see how well this holds up. I am optimistic.
Tomorrow it's time to put it back into the cabinet and make sure the record player portion still works.