|The Philco Model 37-61 is a 5-tube AC superhet circuit
radio. It receives the standard broadcast band and one short wave
band. The circuit is conventional. This radio had seen some
servicing in the past but had not been hacked excessively. This
being the case, I decided to try and retain/restore the original top and bottom
chassis appearance if possible.
Previous servicing included several replacement tubes, a new line cord and plug, a hole-filler filter capacitor installed (but no tacked in filter cap to replace it), and a replacement on-off/tone switch (replaced by a standard potentiometer and switch). One very worrying item was noted: the metal cap that retains the type 5Y4G tube socket on top of the power transformer had been removed (tabs damaged) and was loose. All original wax-paper and bakelite block capacitors were intact.
The schematic for the Philco 37-61 can be found on Nostalgia Air. Any part numbers will refer to numbers on that schematic.
My antique radio restoration logs
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 and electrolytic capacitors are leaky and thus should be replaced (I always "restuff" the original containers if possible).
The on-off switch and tone control switch (Part 47) had been replaced by a potentiometer with switch.
It appeared that someone had removed the cap that holds the 5Y4G rectifier socket on top of the power transformer. The power transformer was immediately suspect and was carefully checked. First, I measured the resistance of all windings - all were reasonable. Then I connected the primary to AC through a fused variac and watt meter. Starting with 20 volts, winding voltages were measured. At full line voltage the wattage was minimal - about 5 watts. The two halves of the high voltage winding were within a few volts of equal. Filament voltages were also OK. The transformer did not heat up. But once the power supply section had been rebuilt, I once again applied power through a fused variac and watt meter and this time measured the B+. It was then I noticed that if the rectifier tube were moved slightly, the B+ went to zero and the watt meter shot up. So there was a short somewhere inside the power transformer. That's no doubt the reason that the cap had been removed.
There were also several wires that had been disconnected in the power supply area.
One filter capacitor (Part 44) had been removed and replaced by a hole-filler of similar size, but of the wrong type (lead wires, which were CUT, vs. a central terminal). The original insulator was not present (this capacitor is not grounded to the chassis). Wiring that should have gone to the ground lug of this capacitor was loose and not connected. No tacked-in tubular replacement was installed. Looks like an attempted restoration by someone who was confused and gave up!
The 5Y4G rectifier tube was missing. An 6V6GT was installed in place of a 6F6G.
The AC line cord was OK but not original - black vinyl (it should be brown rubber zip cord).
All coils and transformers were OK with the exception of the short-wave oscillator coil (Part 10).
The speaker field and cone were OK.
Five resistors were out of tolerance by 30% to 60%. All but one were dogbone types.
The wire wound bias resistor (Part 43) was fortunately good.
The wires to the (now loose) rectifier socket were unsoldered and the socket and insulator sleeve removed. The top cover of the power transformer was then removed. It was immediately obvious what the problem was: one of the rectifier filament leads had worn insulation and signs of burning. The inside of the transformer shell indicated arcing and burning. For a repair, the filament leads were covered with spaghetti tubing and the inside of the transformer shell was insulated with black electrician's tape - several layers on both sides (the high voltage side and the filament side). The top cover, socket, and insulator was then reinstalled. The socket cover was reattached to the transformer cover using epoxy and small self-tapping screws. Subsequent testing indicated no further problems with shorts. It is REALLY amazing that the power transformer survived this short. Perhaps it was saved by the 5Y4G rectifier tube, which suffered a direct short from B+ to ground.
All paper and bakelite block capacitors were rebuilt in their original cases using modern 630 volt film capacitors in order to maintain the original under-chassis appearance.
The two original can type electrolytic capacitors (Parts 20 and 42) were also rebuilt in their original containers. The cans were cut most of the way through on a Unimat lathe and the cut was completed using a hobby razor saw. The cuts were near the base in both cases so that the joint would be hidden by the clamps or insulating cover. The original contents were removed, the cases cleaned, new 450 volt electrolytics installed inside, and the two halves of the cans joined using 3/4" PVC plumbing couplings and epoxy. The non-original hole filler filter capacitor (Part 44) was removed and replaced by a Philco capacitor with insulator from a donor Philco Model 89 chassis. The replacement was rebuilt as discussed above.
All original resistors more than 20% out of tolerance were replaced. I used dogbone type resistors as were used originally. One resistor in the RF subchassis (Part 13) was an old style carbon composition type - it was replaced by a very similar type that was within 20% tolerance. I picked out NOS and used dogbone resistors that had drifted to the correct needed resistance and then repainted them to match the original resistor's color codes. The replacements may continue to drift, as would most new carbon composition type resistors. But to me, maintaining the original look is more important than long term reliability of the radio.
|Here are the four replacement dog-bone resistors, ready for installation in the radio. The 9K 2 watt (Part 19) was marked 13K, but actually measured 8.8K! The 20K 1 watt (Part 18) was originally 15K. The 70K 1/2 watt (Part 34) was originally 50K, and the 51K 1 watt (Part 25) was originally 32K. All have been repainted using hobby paint.|
The original AC on-off switch and tone control switch (Part 47) had been replaced by a standard potentiometer and switch. The original was a three-position rotary switch. Position 1 was AC OFF. Position 2 was AC ON, and position 3 was AC ON with the tone switch contact closed (bass position). I placed a wanted Ad on the Philco Phorum Wanted List and also checked with Gary Schneider at Play Things Of Past - no luck. So I decided to install a 2-pole 3-position rotary switch until the original part could be found. In the process of searching through my stock of rotary switches, I found something very close to the original part! Other than the shaft length (it was too long), it had all the functionality of the original - how's that for luck!
|Here's the switch I found in my stock of rotary switches. It looks very similar to a photo of a an original switch, the repair of which was discussed on the Philco Phorum. I'm guessing that it is an original switch, but for a different Philco radio that required a longer shaft length. The switch was also GOOD. All I had to do was to clean it up, cut the shaft to the proper length, and file a notch for the knob.|
The RF subchassis was removed from the radio for rebuilding. The tuning capacitor was removed for cleaning and lubrication. The two-speed dial drive assembly was removed, disassembled, cleaned in lacquer thinner, greased, and reassembled. The tuning capacitor bearings were lubricated with lithium grease. The filter capacitor (Part 20) was removed for restuffing. The bandswitch and coil assembly was removed in order to gain access to the screw that holds the short wave oscillator coil (see next section). The bandswitch was cleaned using Big Bath spray cleaner. The rubber mounting grommets were replaced by new reproductions from Renovated Radios - part PHG-RF.
The feedback winding of the short-wave oscillator coil (part 10) was open. The coil was first removed from the RF subchassis. To do this, the bandswitch and coil assembly must first be removed from the RF subchassis to gain access to the screw that holds the coil. Once the coil was removed, it was obvious that the feedback winding was on the INSIDE of the form, on another smaller form. This form did not go all the way through the outside form. Also, one of the leads from the outside winding (heavy wire - about #20 or so) passed through both forms. This lead was pulled back through both forms and secured with tape to prevent the coil from unwinding.
A pencil soldering iron was then inserted into the center of the inner form and moved around until the wax started to melt on the outside form. At this point, I was able to insert a screwdriver into the bottom of the form and push out the inner form. The typical "green" Philco disease (corrosion due to moisture) was then obvious on the feedback coil. I began unwinding the coil hoping for a single break, but almost every turn was broken. So I removed the winding completely and replaced it with #38 DCC magnet wire. The wax impression of the original coil indicated correct number of turns, the spacing, and winding direction.
Once the new coil was rewound, it was sealed with rosin and then reinserted into the outer form. The trick here is to align the holes where the lead from the outside coil passes through both forms!
Once the radio was reassembled and tubes installed, power was brought up slowly using a variac. AC power was monitored using a watt meter, and a DVM monitored the B+. The radio came alive immediately and worked. Once at full power, voltage measurements were taken.
The set was aligned - no surprises.