[Stormlord5500]

Hello everyone! 

I've been working on a Philco 38 farm radio with the code 121 version chassis. As you all may know this set uses a rather rare tube, the type 15 pentode as an autodyne converter. I am not deeply versed in autodyne converter circuits. I'm still a few shades green at this kind of stuff.
 To preface my work on this radio, I have recapped it and changed all of the resistors because they were out of tolerance. I did not touch any of the micas. I checked all voltages on the tubes and they are within spec. All tubes are good. I checked all coils and audio xfrmrs for continuity. They all passed. The tuning capacitor is a little
dirty but no plates are rubbing. I used the two radio method and verified the local oscillator is running. The driver and push-pull audio is working. I checked the band switch and it's good. When I touch the detector grid cap and I get a buzz. So the detector is working. When I touch the grid cap of the IF amp and I get a response, like the buzz plus static. When I touch the converter's grid cap I get static. I also touched the tuning cap stator terminal on the antenna side. That gives me static. When I take my finger off of the grid cap or the tuning cap stator, I get nothing. The antenna terminal gives no response when I touch it. It has a 4 foot length of wire with a little kit AM transmitter pumping out signal and the radio acts like it's not even there. Even though I checked the antenna coil could there still be an issue with it? Could it be the volume control? It uses the volume control to throttle the incoming signal. It's directly connected to the antenna coil. What am I missing?

 Any help would be appreciated.
Thank you.

[Arran]

It is odd that they chose the autodyne mixer/oscillator in one version of this set, with the #15 tube, but chose to use a 1C6 in the Code 123 version. The #15 is also a lot more thirsty in that it apparently has a filament that draws 1/4 amp at 2 volts, the 1C6 draws maybe 1/10 of that. If there is something wrong with the tube I don't know what to suggest as a replacement, but it could be something simple like a bad mica cap stopping it from working properly. Another thing to check is the solder joints on the grid caps as well as the condition of the grid cap leads, it does not take much resistance to stop a tube from working as it is typically very low voltage on a grid. I think that a stage by stage signal trace might be in order, like feeding RF into the detector and working your way back to the IF amp, and so forth. A voltage check would be another way if you don't have a signal generator, something is clearly not there.

https://philcoradio.com/library/download...l.%201.pdf

https://philcoradio.com/library/download...l.%201.pdf

Regards
Arran

[Stormlord5500]

Thank you for the reply Arran.

I will do as you suggest and report back.

[Stormlord5500]

Well I found it and it's not good. The primary of the antenna coil is shot.The famous (or infamous) Philco coil rot got it. I tested the coil and even though I get continuity, the resistance is flaky.             

[morzh]

Well, you know what to do then.

[Stormlord5500]

Yes. Ball up in a corner and cry very deeply?  
I've never rewound a coil so first time for everything I guess. I have over the years read posts on forums and watched alot of vintage electronics guys on YouTube. This will be interesting. Thank God it's only 21 turns of #44 wire.

[morzh]

>> Yes. Ball up in a corner and cry very deeply?

It's a good start

Then try to get off the outer shell, take pen and paper, mark the view (top/bottom, to have a reference) and make a sketch, showing how the wire wraps around starting from the lug it is soldered to.
Count the turns.

Good news is, this is a small coil and I doubt there is more than 100 turns.
Get the wire gaged, then buy the appropriate magnet wired (unless you have it already) and simply wind the coil.

Then wrap it into ... Kapton would do.

[GarySP]

Not sure if it matters much, but measure where the winding sits on the tube and try to rewind as close as possible. Take care, - Gary

[Arran]

Ah yes, the Philco "Green Death" strikes again, someone suspected that cellulose nitrate based insulation might be to blame, which is what they used to make movie film out of, the explosively flammable kind. I have thought that a strip of acetate based photographic film would work, but there are so many types of sheet plastic around that you could just about use anything, PETE, Kapton tape, polypropylene.
Since it is a battery radio it is possible that the set wasn't stored in a climate controlled setting, like a barn, or attic, for many years, but that's conjecture on my part.
Regards
Arran