Restoration Notebook: Converting the Oscillator Circuit in Models 19, 71, 89 and 91

The autodyne oscillator circuit as used in Models 19, 71, 89 and 91 is often very finicky and hard to restore to normal operation. With hard work and a little luck, 99.9% of these sets can be restored to proper operation without any modifications.

However, if you have one of these sets and are having a very difficult time getting the oscillator circuit to oscillate, here is an idea you may wish to try.

This modification involves removing the original five-pin tube socket used by the 36 detector-oscillator tube, and replacing it with a seven-pin socket to accommodate a 6A7 tube. It also involves completely rewinding the oscillator coil to different specifications, so that the 6A7 will work as a converter and oscillator.

First, here is the look of the front end of a stock 19 and 89:

And now, the front end of a 71 (the 91 is similar):

WARNING:

Do not attempt this modification unless you are knowledgeable at reading schematic diagrams!

Parts needed:

• Your old 71 oscillator coil
• Can of Deft clear lacquer
• Mylar tape
• Hot glue gun and glue sticks
• #31 or #32 AWG enameled magnet wire
• #38 AWG enameled magnet wire
• (1) 51K resistor, 1/2 watt
• (1) 32K or 33K resistor, 1/2 watt
• (1) 1K resistor, 1/2 watt
• (1) 300 ohm resistor, 1/2 watt
• (2) .05 uF, 600 volt capacitors (.047 uF will work just as well)
• (1) 110 pF mica or NP0 ceramic capacitor (100 or 120 pF will work)

The first thing to do is to take notes indicating which wire connects where on the 36 tube socket. Remove all of the wires connected to the 36 tube socket. Then, drill out the rivets holding the five-pin 36 tube socket in place, and remove the five-pin 36 socket.

Now, remove the oscillator coil from the set after unsoldering the wires from the coil.

The coil will look like this:

You need to remove every winding from the coil, until you are left with a bare form. The inner coil form will also need to be removed. The inner form will no longer be needed, and may be set aside in your junkbox for possible future use.

Now, here is a bare coil form with the inner form removed.

You now need to rewind the coil following the diagrams below:

We will not be using terminals 4 and 6, since we do not need a tapped secondary in this application.

Here is a rough drawing of the bottom view of the coil form.

Disregard terminals 4 and 6 because, again, we won't be using them. Besides, the 19, 89 and 71 coil forms do not have a terminal 6 anyway.

After you remove all of the old windings and the inner form, you should bake the form in an oven at 200 degrees for at least a half hour (an hour is even better), to dry out any residual moisture.

After baking, wipe the form down with a paper towel to remove any remaining wax. Careful - the form will still be hot when removed from the oven!

Let it cool, then spray a couple coats of clear Deft lacquer on the coil to help prevent the bakelite form from absorbing moisture in the future.

Once that dries, you are ready to rewind the coil.

This part is critical. Failure to follow these instructions exactly will result in a non-operational radio!

You will notice in the photo above that there are holes in the bakelite form. This will be of great assistance in winding the secondary of the new coil.

Yes, we wind the secondary first.

Taking the free end of your #31 or #32 AWG enameled magnet wire, poke it into a hole closest to terminal number 3 of the coil form. Feed the wire through the hole and pull it down from inside the form. Then wrap a few turns of the free end around terminal number 3. Now solder the wire to this terminal.

Next, carefully wind a single layer of 150 turns clockwise onto the form. Clockwise indicates the direction of winding as you look at the bottom of the form.

Do not leave any spacing between turns.

Once wound, apply a bit of tape to the end of the winding to hold the winding in place.

Now cut off plenty of extra wire; you need enough to reach back to the bottom of the form.

Again, find a hole in the form as close to the end of your winding as you can. Feed the wire through this hole and down through the inside of the form. Attach it to terminal number 5 and solder in place.

Now, with the secondary having been wound, you are ready to do the primary winding.

Apply a layer of mylar tape over the end of the secondary closest to the terminals.

Now, with your #38 AWG wire, feed out enough wire to stick out over the bottom end of the form, leavng plenty to wrap around terminal number 2. With the hot glue gun, apply a small dab of hot glue to the wire while holding it in place at the bottom of the mylar tape, close to terminal 1. This glue is necessary to hold the wire in place.

If you do this correctly, your wire will now be free of the glue about 1/16" from the edge of the mylar tape.

Now begin winding the primary winding onto the form. This is wound counterclockwise! You wind this winding on top of the mylar tape. Wind 17 turns, tightly spaced (no space between turns). Once you get 17 turns on the form, take a piece of tape and apply temporarily to hold the winding while you use some more hot glue to fasten the wire in place.

Allow the glue a minute or so to dry, then remove the small piece of tape you used to hold the wire in place.

Now apply a bead of hot glue from the end of the winding, across the winding, toward Terminal 1. As soon as you lay this bead of glue down, lay the wire across the bead quickly while the glue is still hot. This will hold the wire in position as well as help hold the windings in place.

Take this end, cut it leaving enough slack to wrap around terminal number 1. Wrap the end around terminal 1 and solder.

The coil should now look like this:

Sorry for the blurry picture.

The next thing you need to do is spray another coat or two of Deft over the entire coil for extra protection.

Once dry, install in the radio.

That's it. Connect the 6A7 circuit as shown below if you are working on a Model 19 or 89:

Change resistor (10) from 15K to 300 ohms. It now connects between the 6A7 cathode (pin 6) and ground.
Change capacitor (8) from 700 pF to .05 uF. It now connects between the 6A7 cathode (pin 6) and ground.
Add resistor (8A), 51K, from the cathode of the 6A7 (pin 6) to terminal 3 of the new oscillator coil (14).
Add capacitor (13A), 110 pF, between terminals 2 and 3 of the new oscillator coil (14).
Add resistor (11A), 32K, from terminal 2 of the new oscillator coil to the B+ line as shown above.
Remove the original wiring of trimmer (15) and connect this across the primary of the first IF transformer.
Remove the original wiring of trimmer (16) and connect this trimmer between terminal 3 of the new oscillator coil (14) and ground.
Connect terminal 5 of the new oscillator coil (14) to the "oscillator grid" (grid 1, pin 5) of the 6A7 tube. Also connect this same terminal to the tuning condenser section previously connected to the old oscillator coil (14).
Connect terminal 1 of the new oscillator coil (14) to the "oscillator plate" (grid 2, pin 4) of the 6A7 tube.

Now, use the circuit below if you are working on a 71:

Change resistor (18) from 15K to 300 ohms. It now connects between the 6A7 cathode (pin 6) and ground.
Change capacitor (19) from 700 pF to .05 uF. It now connects between the 6A7 cathode (pin 6) and ground.
Add resistor (17), 51K, from the cathode of the 6A7 (pin 6) to terminal 3 of the new oscillator coil (13).
Add capacitor (16), 110 pF, between terminals 2 and 3 of the new oscillator coil (13).
Add resistor (19A), 32K, from terminal 2 of the new oscillator coil to the B+ line as shown above.
Remove the original wiring of trimmer (12) and connect this across the primary of the first IF transformer.
Remove the original wiring of trimmer (15) and connect this trimmer between terminal 3 of the new oscillator coil (13) and ground.
Connect terminal 5 of the new oscillator coil (13) to the "oscillator grid" (grid 1, pin 5) of the 6A7 tube. Also connect this same terminal to the tuning condenser section previously connected to the old oscillator coil (13).
Connect terminal 1 of the new oscillator coil (13) to the "oscillator plate" (grid 2, pin 4) of the 6A7 tube.

Now, there is one more modification you will need to make. Because the 6A7 circuit is much more efficient than the 36 autodyne circuit, it will throw more signal into the IF amplifier circuit. This extra signal strength will cause the IF circuit to go into oscillation unless something is done about it.

Here's all you have to do:

Remove the connection between the RF amplifier tube cathode and the IF amplifier tube cathode, leaving the 300 ohm cathode resistor and .05 uF (.09 uF in some sets) bypass capacitor connected between the cathode of the RF amplifier tube and ground. Now add a 1000 ohm resistor, with a .05 uF capacitor in parallel with this 1000 ohm resistor, between the IF amplifier tube cathode and ground (as shown above). This will decrease the amplification of the IF amplifier stage to a level below the oscillation point.

Now turn your radio on and enjoy your own "Super" mod.