Mystery Radio - Late 1930's?

Unknown Manufacture or Model. There are no markings of any kind on the radio. No letters, no numbers. The radio is smaller, 9.5w X 6"h x 5.5"d.  It's an early AA5 with a tube line up of 25L6 audio, 25Z6 rectifier, 6SA7 mixer, 6SK7 IF and 6SQ7 detector. And if your adding it up that comes to 68.9 volts for the heaters. 48.1 volts needs to go some where and there's not a ballast tube, so yes, it uses a resistive line cord. That could be trouble in it's restoration.

I searched the internet for pictures of radios to match this one to and here's what I came up with.

Mystery radio - Click for larger view

Sentinel - Click for larger view

Lafayette - Click for larger view

Looking at pictures in the archives at Radio Attic,  it almost looks like a combo of two radios. The top is this radio. The left is a Sentinel of unknown model that has a very similar dial and pointer, and the knobs look the same. Then the right Lafayette radio has a similar case with the exception of having only one knob below the dial. But the rest of the cabinet lines look the same. So who made this one? Was it a knock off for some department store?

On to the restoration...

    A pic of the radio as I received it. The cabinet is in great shape. Some one spent some time buffing a really nice shine on it. I won't have to touch the cabinet at all. Man I like that!
The dial cover was loose when I received it. I can see why. Some one used duck tape to hold it on! Oh well, a little glue will fix that.
    A quick look at the front chassis when it arrived. The speaker has a small tear that can't be seen in this pic. A little rubber cement should take care of that.

Pic's of the top and back of the chassis. Most parts appear to be there, and in good shape. That's good!

The underside looks standard to recap. Should be fairly easy to do. However the line cord seems to have several, make that 6, wires comming out of it. Humm, I'll have to investigate that!
Click for larger view

I gave the radio the "Dim-Bulb Test", and it failed misserably. There's probably a short somewhere and I think I'll start looking at the power cord. I think that's the most likely spot for trouble.

Ok, here's the deal. The resistor line cord (also known as a curtain burner) has several wires because it was a multi tap cord used on several radios. This radio used one tap for a pilot lamp, two for power and one was the resistor line. The rest wern't used, but other radios may have used them. I tested the cord and it was shorted. Opening up some of the cloth revealed that the wires inside had become brittle and cracked bad. This is typical on this type of cord as they get hot when used. A new cord won't drop the required 48.1 volts, so i need to do that another way. There are several options to do that, using a diode plus resistor or a large 8mfd cap, but I do need to account for the pilot lamp also. Well I'll think about it for awhile and decide on the best plan for this radio.

Click for larger view
Ok, here it is all capped. Notice the two electrolytic caps have their positive sides tied together. Humm, this radio design doesn't use the common practice of a filter choke between the two caps for hum suppression. Must have been built as a cheap radio. No wonder no one put their name on it.

Click for larger view

To solve the resistor line cord replacement problem, I decided to swap out the tubes with their higher voltage equivalents, which I had mostly except for the 35Z5. Basically you replace the 6SA7, 6SK7 and 6SQ7 tubes with 12SA7, 12SK7 and 12SQ7 tubes. The 25L6 is replaced with a 50L6 and the 25Z6 with a 35Z5. With these tubes the heater voltage adds up to 121 volts so there's no need to drop any voltage and no problems with diodes or caps shorting. All tubes are direct pin compatible except the 35Z5, but it only requires minor rearranging of some wires on the tube socket.

Some people don't like to use this method as it's seen as a major alteration of the radios design. However since this is an mystery radio, built with a cheap design, I don't think this method takes anything away from this particular radio. And this swap out was somewhat already part of this radios past. When I picked up this radio it had it's original tube compliment in it, but written in pencil on the chassis next to the 25L6 tube was 50L6 and next to the 6SA7 was 12SA7. So someone in the past new there was a problem with the cord and I believe was preparing to do a tube swap out. However for some reason they never finished, perhaps due to the cost of buying new tubes for a cheap radio. So in effect I finished the job some one was preparing to start. Here's some pic's of the top and front of the chassis after the recap and tube swap out. You can just make out the 12SA7 in pencil on the chassis at the top of the top view pic.

Ok, there it is. It's a nice looking radio, even if I don't know who made
it, and the cabinet is in fabulous shape.  Since some one took the effort
to polish it up, it's nice I could get the electronics working again. So now
this mystery AA5 joins my other radios, playing the sweet sound of music.
Just as this radio was originally made to do.
- Mike

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