AK 20 (4640) restoration
 

Someone locally brought me a very nice looking AD model 20 (4640) for restoration. This is a 5 tube (all type 01A's) battery operated TRF set that requires 5V for the filament, 20V for the detector plate, and 90V for the other plates.

The problems that I found were the 2 megohm grid leak resistor was open, the first interstage (couples the detector tube to the first audio tube) audio transformer had an open primary, and the .3 uf plate bypass cap was leaky.

I installed a modern resistor, under the chassis, for the grid leak resistor and the old clip-in resistor was left in place for looks. The .3 uf cap was installed, out of sight, under the chassis. For the open transformer, I did a little circuit modification. I disconnected the old transformer and left it on the chassis for looks. I then connected a 150 Kohm between the plate of the detector tube and the 20V source (where the primary winding originally connected). I then connected a 1 megohm resistor between the grid of the first audio tube and the A- connection (where the original secondary winding connected). Lastly, I connected a .01 uf cap between the plate of the detector tube and the grid of the first audio tube.

After running a wire antenna out the window and connecting the battery eliminator, the radio came to life. I can receive all local stations and with moderate volume. Of course, I wouldn't expect room filling sound with an 01A output stage.

Now, I've got to see if the owner wants to buy an ARBE-III battery eliminator or see if he wants me to build a power supply. I recently looked at Ramsey Electronics' website and they have a battery eliminator kit; but, it cost more than the factory built ARBE-III models. The model I currently use is the kit that AES sells. And, I have two vintage 90V/1.5V models for the newer farm radios.

Anyway, I thought I'd share how I got around the bad interstage transformer, just in case someone else runs into that problem. I know I could have ordered an interstage transformer from AES; but, my modification seems to work OK and it will save the owner some money; as well as getting the radio back home quicker.