Hello, I'm Jon. I've been recapping vacuum tube radios for about a year and a half. Nothing professional; I just don't like to see these classy looking, still usefully devices waist away(or blow up...). I have very little knowledge of electronics, but that doesn't stop me from trying it out.

At any rate, I've just recapped a Firestone 4-a-12(aka the Narrator). It was to be my main living room radio. I even spent the extra money to get the can instead of building my own multi section electrolytic capacitor. All the tubes were tested and a few were replaced. The thing looked like it had been worked on in the past. There were a few different generations of paper caps and some external electrolyics grafted to the can to get some old electrolytic back to measurement, I guess. I undid some of these uglier fixes to try to get it back to what it looked like in the schematics.

I am having an issue with FM. I'm getting nothing. AM is clear as a bell. To me, this would sound like an antenna issue, so I attempted to plug in an external antenna. Unfortunately, there is no guild on this one. Only four screws marked “D”, “J”, Blank, “A”. And I don't see any clues in the schematics. When I got the thing, there was a bar between “J” and Blank. And “A” had a piece of broken wire, like maybe someone had an old antenna and just cut it off.

Trying to get it to work, I've tried every combination of the four screws with a “T” type FM antenna. When I plug it into the “D”/”J” combination, I get some weak FM stations. They all seem blended and tuning the radio doesn't seem to actually tune anything. The tunning only makes the blended FM stations slightly louder of quieter.

And ideas?

First, "A" is for an external Broadcast Antenna. It just provides some coupling to assist the internal loop if so desired. If the AM broadcast band is OK as it is you can just ignore this one.

Originally there was a link on the blank terminal. If it is there you won't need to connect it to anything if the T type FM antenna is a folded Dipole with a twin-lead feed to the radio (this is pretty common). Basically connect one lead to D and the other one to J. This provides the input to the FM antenna coil.

If you don't have a dipole type antenna with two connections you basically Jumper the blank to the J (the bar you were talking about) then attach a single wire antenna to the D terminal.

The fact that things are not working well may be due to other problems including most likely an FM local oscillator that is not working. I would test or try another 12AT7. A tube that tests good may still not work in this stage, but a tube that tests bad probably will not work.

Curtis Eickerman

Thanks for the antenna rundown. Ill try to pick up a new 12at7 next time I am ordering parts.

If you still have the selenium rectifier, your DC voltages will be low, and that would affect the FM reception. Replace the rectifier with a 1N4007 diode and 100 ohm 5 watt resistor in series. The band on the 1N4007 rectifier should point to the filter capacitor.
Don