The PHILCO Phorum

I pulled the chassis out of my 40-150 because the dial cord had slipped, and while I was realigning it I discovered the short wave band doesn't work.  I vaguely remember when I first worked on it in 2007 I could never get the SW band working.  It was one of the first radios I'd worked on.  I put it back together, thinking I'll never listen to short wave on this, anyway.

This time, I know more about radios, but I still can't get the short wave working.  Since the BC and Police bands work fine, I suspected the band switch.  I buzzed out all the band switch connections and it looks like it's wired correctly and everything makes good contact.

I checked the wiring of the coil, and that seems to be wired fine.  I have continuity everywhere on the coil.  Since it is just one coil with 4 taps, I don't think the radio would work at all if there were any opens in the coil.

I injected modulated RF into the converter tube with frequencies in the middle of each band.  I get a tone with each of the two lower bands, but not shortwave.  I also used a coil of wire near the loop antenna, connected to my signal generator.  Same result.

The oscillator does not seem to be oscillating on short wave, based on the "two radio" test.  The known good radio (a 42-350) squeals on the other two bands, but not short wave.

I changed the 7J7 converter tube for an NOS, thinking maybe it was having trouble oscillating at the higher frequencies.  No luck

The 7J7 tube socket wafer is badly cracked on this radio.  I'll have to replace it, but in the meantime I buzzed out the electrical connections between tube and socket, and I have good contact all around.  Anyway, the other bands work.

I don't guarantee that all my tests and checks were done correctly,  but I was pretty careful and repeated most of them more than once.

Scratchin' my head over this one.  If anyone can offer any more suggestions for troubleshooting the SW band, I'd love to hear them.

I would guess the 7J7 is one of those light tan sockets. Definitely replace it. You can use a brown Phenolic tube socket from a junker Philco and it will work fine.

Without looking at the schematic, I would also suggest carefully baking the SW oscillator coil...or hitting it with a hair dryer?

Yes, the bad 7J7 socket was one of the tan-colored ones. The rest of the sockets in that radio are brown-colored, and don't seem to be cracking at all.

I don't have a donor Philco, but I recently bought some loctal wafers from AES that should fit. Unfortunately, they are oriented with the key slot aligned with the rivet holes, unlike the Philco sockets which have the rivet holes between pins 3 and 4. I don't think that will be much of a problem.

The 40-190 I bought from Ron a year of so ago used an octal 6J8 converter instead of the 7J7. The octal socket wafer in that radio fell to pieces, too. What is it with the converter tubes in 1940 Philcos that they used such crummy sockets?

I'll try a hair dryer first, as you suggested, Ron. Not sure how hot to get the coil, though, or for how long. I assume I shouldn't be melting any of the wax it's covered with. Is the object of heating it to drive off moisture?

I have no way of proving this...but I have long suspected that the tan material may be a low-loss material intended for use with frequency converter tubes, although they also used this type of socket for the phono preamp tube in the 41-616.

Regardless of Philco's intentions, the material becomes just as fragile as the rubber insulation on the wires over the decades, and can be counted on to fail sooner rather than later now.

As for heating the coil - Do not worry about melting the wax. This is desirable as it has been found years after the fact, that wax is not as good as was first thought for moisture-proofing. In fact the wax will help a coil retain moisture. Just don't get it so hot that the coil, or coil form, gets damaged. You can safely bake these in an oven as long as the temperature does not exceed 200 degrees F. Philco model 89 oscillator coils usually require baking as well as rewinding, for this reason.

It's taken me a long time to finish this, but I finally replaced the converter tube socket and baked the oscillator coil in the oven for an hour with the oven set at 200 degrees F. The thermometer inside the oven tells me it only got to about 160, but I don't know if that's accurate or not.

Once I had the coil out and baked, but before I put it back in the radio, I measured the DC resistance between pins. I should have done that before baking, but it didn't occur to me. Between pins 2 and 3 I got 68 ohms, where the schematic tells me I should have 1.5 ohm. I scratched my head over that for a few minutes and then decided to reflow the solder on the pins to see what happens. After several tries I got the resistance down to about 6 ohms. It wouldn't go any lower. The resistance between the rest of the pins is about nominal, maybe just a little high, so I left them alone.

I put the coil back in, and now I get signals across all three bands. The SW signals are weak, but I just have the loop antenna propped up behind the chassis on my basement workbench, which is not the best place to listen to short wave. Still, now I know the oscillator is working on SW, which it wasn't before.

I'm not sure whether it works because I baked the coil or because I reflowed the solder, but at any rate it's working. Next I'll do a complete alignment and see how things sound after that. I haven't touched the SW alignment since I recapped the radio in 2007, so maybe aligning it will improve things.

Hi John (Raleigh)

Congratulations!

Since my last post in this thread, I have found that, according to the 1941 Philco parts catalog, the tan sockets have "special moisture-proof insulation". I also discovered that they seem to stand up to the heat of a soldering iron better if you leave the tube in the socket. This trick does not work every time, but it helps. Oh, and it seems to work better to use a small pencil iron on the contacts of these sockets instead of the big high-wattage guns.

Of course, all of that information does you no good when the socket is cracked. And I still say that those sockets were cheaply made.

Quote:I'm not sure whether it works because I baked the coil or because I reflowed the solder,

Perhaps a combination of both? In any event, yes, alignment should help.