Philco Model 20 (1934 vintage) is a TRF (not superhetrodyne) radio. It has all tuned circuits to receive and detect the audio from the air. Here are some pictures of the restoration.
The only restoration challenge is restuffling the large multi section capacitor. There in which the real challenge is minimizing the melted tar or pitch. I held the capacitor in front of a LP space heater and kept it moving. Softening of a layer that contacts the inside of the can is the goal.
I removed this capacitor from the chassis. I was later told this is unnecessary. You only have to unbolt the retaining screws and pull up on the can while gently restraining the bottom cover. It separates. The bottom is not affixed to the upper can. Then clip the wires, clean out the can and add new capacitors.
The smallest electrolytic that I had on hand was 10 uf. They work fine. 10 uf is well with in the capacitor specifications of the #80 rectifier tube (I always check). I used a Polypropylene 0.1 uf 630 volt as well for a direct value replacement of .13 uf across the choke. Riders Vol 2 page 2-3,4 Vol 1-6 from Nostalgiair.org
I found all but two resistors out of tolerance. Most beyond 100%. I replaced the resistors with two 1/2 watt in parallel to obtain 1 watt dissipation. Of course the individual resistors wattage are double the needed value. I have saved the original resistors in the event that someone would later intend to make reproduction "dog bone" resistors to maintain the vintage look.
And how about those Dog gone Dog Bone Resistors? Try Syl's web site.
After removal, clip the lead wires exiting the rivets, heat with a 40 watt focused spot light, and push the insides out with a small stick through a rivet. I have broken a case before, by using a screw driver.
Clean with contact cleaner and solder in new caps. Season to taste and Replace. Check your schematic or see, http://www.philcorepairbench.com/partinfo.htm and select the Bakelite Blocks for what is inside. Thanks to the Philco Repair Bench for maintaining such a valuable Resource!
Alignment is simply putting the RF generator on 1400 khz, setting the dial on 140 (1400 khz), placing a loop of wire near the antenna (a length of wire) and adjusting the three compensating capacitors mounted on the variable air capacitor. Some models only have one or two.
If tracking is off, bending of the outer most disc of the air capacitor is needed. This radio tracks accurately. No adjusting is needed. However, several, not all, bent plate sections were apparent on the center section rotor.
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