Philco 37-624 Restoration

My antique radio restoration logs

The Philco 37-624 is a 6 tube superheterodyne circuit radio with three bands. It operates on 6 volts DC only, and was used in rural areas before electrification. It features a tuned RF amplifier stage and push-pull class B output. The build quality of this set is very high. The band switching is quite complex, with unused coils shorted out. This set uses the Philco RF subchassis construction, which is quite difficult if not impossible to rebuild in place.

The radio appeared to be completely original as found.  Since all the original components were still in place, I decided to try and retain the original top and bottom chassis appearance if possible. 

Schematic on Nostalgia Air


My usual restoration procedure is to first make a complete survey of the condition of all components.  The survey results guide my restoration strategy. If major and unique components are defective and cannot be restored, I may elect to sell the radio rather than restore it. I assume that all paper, bakelite block and electrolytic capacitors are leaky and thus should be replaced (I always "restuff" the original containers if possible).

RF Subchassis Servicing

Pilot Lamp and Filament Dropping Resistors

The pilot lamp was a 2.1 volt, 120ma bayonet type and is no longer available. The original was defective.  There is no current equivalent. It was suggested on the Philco Phorums to convert to a 6 volt (#47) bulb and rewire the lamp power supply to a 6 volt source. This was done. The power lead to the lamp socket originates in the RF subchassis, to one pin of the 1D5 used as a tie point. 6 volts is available from the adjacent pin, so the power lead was moved.  This change really does not change the under-chassis appearance. The problem is that the pilot lamp, paralleled by a 16.6 ohm resistor, completes the filament circuit for the 1C7 and 1J6 tubes. The combination needs to drop 2 volts at 240ma. So if the lamp is removed, then the resistor needs to be 8.33 ohms. The existing 16.6 ohm resistor (flexible type) was open. It lives inside red spaghetti tubing on the RF Subchassis base - 1D5 socket. The original tubing can just accommodate a 1/2 watt carbon resistor, but was about 2" long. The power dissipation needs to be 0.48 watts. So I decided to use three 2.7 ohm 1/2 watt resistors, hand picked from my parts bin to total 8.3 ohms. Since three resistors are used, each only needs to dissipate 0.16 watts. I tested the three resistors inside the tubing and applied 2 volts for one hour. They only got slightly warm, and the resistance did not change. So it looks like this is the way I will go. One alternative was that there is a guy in the UK who makes reproduction flexible resistors. But since the resistor would be hidden inside spaghetti tubing, I decided not to go that way.

Power and Tone Switch

The power switch was mounted on the back of the tone switch. The tone switch was OK, but the power switch was stuck in the ON position. No amount of contact cleaner sprayed inside would free it. I also tried soaking the switch in lacquer thinner for some time. That also did not work. I was afraid that the contacts were welded together. I did have a replacement power switch (IRC type 42) in stock, but the original was riveted onto studs of the tone switch, and reassembly would have required drilling and tapping the studs to accept small screws to hold the replacement switch. As a last resort, with a replacement switch in hand, I added some turbine oil! That freed up the switch - it then worked. I did soak it again in lacquer thinner to remove the oil. It continued to work afterward.

Bias Cells

The radio used 3 bias cells, totaling 3 volts, which provide bias voltage to the audio driver stage. Two of the cells were rebuilt using LR44 button cells, which equals 3 volts. The third original cell was left in place but bypassed under the cell holder. The cells were rebuilt by removing their contents, cleaning the case, and inserting a LR44 button cell, with the negative terminal up. The positive outside edge and back of the LR44 contacts the original case. The spring clip contacts only the negative center contact. This means that the polarity of the assembly must be reversed. This was accomplished by drilling out the rivet holding the original ground lug and first spring contact, removing the original ground lug (which contacts the metal spacer holding the assembly), and inserting a new ground lug on top of the assembly. This then becomes the new negative lead. The original ground lug was reused and wired to a rivet on the base of the second battery holder, thus providing a new positive lead, which connects to the chassis by way of the original metal mounting spacer.

More Flexible Resistor Trouble

The two flexible resistors (parts 43 and 44) which form a voltage divider across the filament of the 1F7 detector-first audio were intermittent and high in value. They were removed, and it was discovered that if the leads were recrimped they were OK. So they were re-installed. It is likely that corrosion had developed between the metal ends of the resistor and the lead wires.


The main power supply filter capacitor (part 64) is a metal cased unit mounted on the chassis. It was originally 4+8uf. It was restuffed using 4 and 10uf 450 volt capacitors.

All Philco bakelite block capacitors were removed, their contents removed, the cases cleaned with lacquer thinner, and new components installed inside.  All wax-paper tubular capacitors were removed and restuffed with modern capacitors (630 volt in all cases). The ends were then resealed using discarded RCA catacomb wax (a combination of rosin and beeswax, I think!)

Reassembly and Testing

The chassis was cleaned with GoJo and steel wool. All parts were then reinstalled. Any defective or frayed wiring was replaced. The tubes and tube shields were cleaned and reinstalled. An antenna were connected and the radio attached to my 6 volt power supply. The B+ voltage was monitored. The radio worked the first time! The set was then carefully aligned.

Below are photos of the chassis before and after restoration. Before:

After Restoration (below):

Chassis Top Photos - after restoration: