Circuit diagram for this radio is at http://www.nostalgiaair.org/Resources/794/M0007794.htm. L3 of the antenna transformer (T1) is open. If I am reading the circuit diagram correctly I believe this coil is needed for reception of AM (referred to as the B band on the circuit diagram). Is this correct of can the radio receive AM with this coil open?

Thanks, Bob

If L3 was open circuited no aerial current would flow in T1 primary unless the contact S-1
to the left of the coil was closed, what ever band that happens in.

However, on a strong signal, some coupling might happen depending on how it is wound.

What is Band C ? Is that a police band on the dial?

Yes, L3 will be needed for the broadcast "B" band. L3 will be the coil with the most turns and of very fine wire. Maybe if you remove the coil assembly from the can and inspect it closely you will be able to see where the break is and repair it.

You may be able to pick up some of the stronger stations by connecting an antenna directly to the grid cap of the 6A8.

Band "C" is for 2.2 -7.0 mHz. I wonder what happened to band "A" on this radio?

Steve and Terry:

Thanks for the responses. I will take a close look at the coil to see if I can determine where the break is.

Bob

I finally got back to this radio. I wasn't able to find the break in L3 so am looking for a replacement antenna transformer and posted a want ad. In the meantime I checked the output voltages from the power transformer with all the tubes removed and am wondering if they are too high. Line voltage was 122. Voltage across the plate pins of the 80 tube was about 700v (314x2 on circuit diagram). Center tap voltages were about 350. Filament voltage for the 80 tube was measured at 6.2 (supposed to be 5.0) and filament voltages for the other tubes was measured at 7.8 (supposed to be 6.3). Are these too high?

Bob

Plug all the tubes back in and the speaker too and try again.
I have the same set and they do tend to run a bit hot.
But get back to us with new readings with all tubes loading things down a bit.
P.

In other words,
The output voltage of power and filament transformers are almost always 10-15 percent high when there is no load. The ratings are given for when there is a proper load on the various windings (tubes installed). So what you are seeing is quite normal.

Regarding the antenna transformer, the primary winding is not critical. I have replaced them in the past (when struck by lightning!) by taking a junker IF transformer winding (the larger types of the 1930's) and removing wire until it was about the same diameter as the original. You may have to fabricate a way to mount it. In my case, the primary was inside the coil and was sideways. In any case, it worked very well. Another possible source would be a wave trap coil. In fact, you could use the one in this set as a last resort (they are no longer needed).

I put good tubes in the set, changed the electrolytics, and attached an antenna to the grid cap of the 6A8. I did not change any of the paper caps or check for out of spec resistors. Will do that later if I can find a good antenna transformer or figure out how to repair the old one. I could not hear any stations, just got a low hum out of the speaker. Voltage across the plates of the 80 tube was about 570. Center tap voltages were about half that. Filament voltage for the 80 tube was 5.1. Filament voltages for the other tubes was 6.6. The radio was pulling about .8 amps according to the ammeter on my variac.

Dave Mc: I am not experienced enough to follow your suggestion on using an old IF winding. Can you provide some more detail? The wave trap coil in the radio is also open (this radio had a mice attack).

Bob

Can you describe the antenna coil and location of the primary winding that is open? Is it on the OUTSIDE or INSIDE the coil? If outside, is it bank wound (multiple layers wound in a criss-cross fashion)? If so, what are the approximate dimensions? If inside the form, what are the approximate dimensions? I have a stock of donor coils and in the past have been able to restore open or burned antenna coil primaries using parts from other coils and transformers. The secondaries, of course, are critical, and if anything went wrong there you would need an exact replacement - which will be difficult to find!

Dave Mc:

Here is a pic of the transformer. I believe the L3 coil is the top one. It is on the outside of the the coil form. There are no coils on the inside of the form. The form is 1 inch diameter. The coil has multiple layers (bank wound). The inside diameter of the coil is 1 inch. The outside diameter is about 1 3/16 inch. The height of the coil is about a little less than 3/16 inch.

Bob

If it were ME, I would first remove the coil from the radio, CAREFULLY documenting which wire goes to which lug - make good notes, and/or photos. Be careful not to break off any lugs - be gentle. Next, I would melt off the wax ONLY from the top coil using a heat gun (if you don't have one, get a cheap one from Harbor Freight). Then carefully inspect the coil for damage. The most likely failure point is where the end wires attach to the terminals. One trick I use to locate the break is to connect one lead of my DVM to the coil lug that goes to the INSIDE of the coil and then probe the outside of the coil with a new/sharp Exact knife with a #11 blade. If there is continuity, you may be able to disconnect the OUTSIDE lead from the lug and unwind turns, looking for a break. Loss of some turns will not significantly affect performance. If there is NO continuity, the connect the DVM to the OUTSIDE lug lead and probe the INSIDE of the coil for continuity. If the break is on the INSIDE of the coil, I have often been able to REMOVE the coil from the form (again, using the heat gun) and unwind turns from the INSIDE looking for a break.

I will look through my donor coil stock for a suitable coil with 1" or more inside diameter. If all else fails, and you are NOT able to find a replacement, you can always just scramble wind a coil of 100 or so turns on the form. It will very likely work fine.

get a couple .01 capacitors and jumper C12 and C17. disconnect C16. see if you get something better through the speaker with antenna connected to 6A8 grid cap

I removed the transformer from the radio yesterday and found the break in the L3 coil. I repaired it by splicing in a short segment of wire and now have continuity.

Further testing of the radio indicates very low B+ voltage. I am open to any suggestions on solving this problem.

Bob