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Ron's Super 71 Mod - Printable Version

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Ron's Super 71 Mod - Ron Ramirez - 03-24-2008

At the request of Steve in Missouri, here is how I modified the front end of my Philco 71.

As I mentioned in a previous post, the 71 chassis I used was a rust bucket. I stripped everything (including all of the tube sockets) from it; the only parts that remained on the chassis were the aluminum coil and IF shields which were spun on.

The chassis was then cleaned and painted flat gray.

I then reassembled the radio, using all but one of the original tube sockets.

But here is the interesting part.

The detector-oscillator tube socket was switched to a 7 pin standard base socket to accommodate a 6A7 tube.

I also rewound the oscillator coil to match the oscillator coil of a Philco Model 18. Like Model 71, Model 18 has an IF of 260 kc, making this an easy and ideal mod for this radio.

The detector-oscillator stage in my 71 mod is based on the detector-oscillator stage of a Philco Model 18; the only difference being the B+ feed to the oscillator "plate" coming through a single 32K resistor.

See the schematic below.

[Image: http://www.philcoradio.com/images/phorum/osc_010.jpg]

Winding the oscillator coil is not difficult; I will provide details in a later post.

You will have to change the RF coil secondary so that it connects to the AVC line instead of ground. This involves lifting the wire from the bracket which holds the coil in place (and grounds this wire), and making a new terminal of some sort to which you can connect the AVC line. A simple way would be to carefully fasten a long solder lug to one side of the RF coil, and attach the wire formerly connected to ground, to the new lug.

You will also need to remove the common connection between the cathodes of the RF and IF tubes, and provide each with separate resistor/capacitor networks, as shown. Note that the IF cathode resistor is 1000 ohms; this is critical. A lower resistance will allow the IF stage to oscillate. 1000 ohms reduced the gain of the IF stage in my 71 just enough to eliminate oscillation, while still allowing superior performance.

I stripped and painted a Model 89 chassis at the same time I did the 71; however, I have yet to build the 89 with the 6A7 tube. I did the 71 just to see if the old, inefficient autodyne mixer circuit could be improved upon. The superior performance of my 71 mod proves that it can be.

I intend to give complete details on all of the mods I made to my Super 71 (which includes an AVC mod, a volume control with a tap for bass compensation, and a tone control potentiometer) later on, on my website.

- Texasrocker - 03-24-2008

Ron, thats definately a "genious" retrofit!!! Another vintage Philco saved!! Congrats!! Icon_wink Randal

- Steve Davis - 03-25-2008

Thanks Ron, I think I will give it a try on my 91. I have extra coil forms and a model 18 that I can copy the oscillator coil. I will be watching for more information as you post.

Steve

- Ron Ramirez - 03-30-2008

Now comes the most difficult part of the conversion...winding a new oscillator coil.

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

Here is a schematic diagram of a Model 18 oscillator coil, the basis for our conversion.

[Image: http://www.philcoradio.com/images/phorum/osc_007.jpg]

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.

[Image: http://www.philcoradio.com/images/phorum/osc_008.jpg]

Terminal 6 is on top of the form, but again, we won't be using it.

You can reuse the existing Model 71 oscillator coil form, but you will have to remove the extra inner form and remove all of the wire from the main form. This form does not have a terminal on top, but as stated earlier, it isn't needed 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 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:

[Image: http://www.philcoradio.com/images/phorum/osc_006.jpg]

Sorry for the blurry picture. This digital camera and I don't always get along. I miss my old Pentax K1000, but I don't miss having to have the film developed!

But I digress...

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 in the partial schematic in my first post above, and enjoy your own Super 71. Icon_smile

- Texasrocker - 04-05-2008

Beautiful job Ron!!! Thanks for taking time to share your tips ( pics) here for all us to learn from!! I have re-wound several coils before, but they never looked that good!!, and my wrist was worn out from tension keeping count and applying the exact turns! My hats-off to you indeed!! RRE ROCKS!! Icon_wink TxRockr

- BDM - 07-22-2008

Just a bump to compliment the "Super 71" current post. Which continues here
http://philcoradio.com/phorum/showthread.php?tid=1244