[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

[Stormlord5500]

I have to get the 44 awg wire and I'm thinking of using kapton tape to wind on. I don't have the tape either. Money is super tight so this one might have to go on the back burner. Good news is the secondary winding is in excellent shape. The primary is dead. So I mapped it out and it's 21 turns of 44 awg magnet wire. Hopefully after I get the wire and the tape it will go well.

 I'm also going to have to retrofit a speaker temporarily because someone has re-coned it before and was sloppy with the glue and I removed the speaker and destroyed the cone. I have something that might work until I get a correct replacement (again $$$$). Good news is the OPT is still good so there's that I guess.

[morzh]

You do not have to use Kapton: buy a bottle of fabric glue and simply put a thick layer to fix the turns.

[MrFixr55]

Hi Stormlord and welcome to the Phorum.

I am scratching my head because, even if the primary of the antenna coil is bad, if there is nothing wrong with the secondary, hooking up a long wire to the convertor grid should definitely work.

You stated that you used the "2 Radio" routine to test for the local oscillator of the set in question and found it functional? The worst part of the autodyne circuit is getting ot to oscillate. The antenna coil has nothing to do with that; the oscillator coil does. If you can detect the local oscillator, the next step is to determine if it is 460 Hz above the station being tuned in.

Can you receive stations in the shop that you are working in? There is an amazing amount of RF interference to be found in houses nowadays what with cell phones, the Internet of Things, computers, video screens, CFLs, LEDs, etc. If the amount and nature of noise changes as you run up and down the dial, then you may e receiving too much interference.

Tryanother location. Good luck

[Stormlord5500]

It does oscillate. Here's the deal. If I touch a probe or my finger to anything in the secondary circuit of the antenna coil I get a response. I connected my TinySA on sig-gen mode to the terminal where the grid cap and band switch and tuning cap connect and actually pulled a station in. It wasn't super clear but I could hear it. Once I pull the primary off I will double check that the secondary is ok.

[Stormlord5500]

I do thank you all for helping me out on this.

[Stormlord5500]

The thing is when I touch or probe the antenna terminal, the volume control input, the wiper (output), and the primary terminal of the coil I get absolutely nothing. No buzz, no squealing, no response at all. So at the very least that primary is open or high resistance to where it won't work. Actually it is open now because I needed to get the wire size and where it has that one green dot just separated as soon as I put a tiny bit of pressure on it from a jeweler screw driver. The wire is definitely corroded. 

 I may try the fabric glue but, I still have to buy it. Right now it's not going to happen. Though I still want to use Kapton to replace the celluloid insulator. 

 I do however have some clear nail polish I was using for another project. Would that work?