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The Philco Model 38-9 from 1938 is a 6-tube AC Superhet circuit radio that receives the broadcast band and one short wave band. The radio had been serviced in the past but most of the original parts were still in place. I decided to try and reverse all prior servicing and restore the original chassis appearance if possible. The schematic can be found on-line on Nostalgia Air. Any part number references in the text below reference that schematic. |
My antique radio restoration logs
The radio was purchased on eBay and was said to power up and hum, but no reception (very likely the seller did not connect an antenna!). The radio appeared complete and in good condition. The radio is a 6-tube superhet that receives the broadcast band and one short wave band. Although it has 6 tubes, the circuit uses two tubes for the second detector/AVC and first audio amplifier, when both functions could have been performed by a single 6Q7G tube - tube count inflation! Features include a two-speed tuning control and three-position tone control combined with the on-off switch.
I always attempt to avoid purchasing radios that have been "restored" by collectors or flippers, and am looking for either all original examples or those which have been "lightly serviced" in the distant past by radio service shops, rather than peppered with new film capacitors. This radio had received some prior servicing, but had not been badly hacked or restored. Most of the original parts were still in place, including both filter capacitors..
The chassis was dusty, very dirty and had a few rusty areas. All tubes and shields were removed. The dust was vacuumed and blown off, top and bottom using an air compressor. After removal of filter capacitors 11/11A and 45 (for restuffing) and the tuning capacitor and dial assembly, the top of the chassis was cleaned using GoJo (white) hand cleaner and 00 steel wool.
The speaker cone, field coil, and output transformer were OK.
All coils and the IF transformers were OK.
The power switch (combined with the tone control switch) was bad. Likely the contacts had oxidized or were dirty. A shot of GC Big Bath spray followed by repeated operation of the switch cleared up the problem.
The tuner reduction drive was not operational (likely due to hardened grease).
The tuner anti-backlash mechanism springs were not in position which caused the tuner to jam at some point in rotation.
The power transformer was OK. The high voltage winding was balanced on each side of the center tap with 10 volts applied to the primary through a variac. It drew less than 10 watts at full line voltage (unloaded), and all filament voltages were correct. I always make this test prior to restoration, since some transformers may have shorted turns which may result in overheating with usage.
One section of the wire wound resistor 43 (Candohm type) measured 37% high and the resistance was unstable (it changed when the lugs were moved). The remaining two sections were OK.
Tubes: the replacement 6A8 (metal) was bad. The likely original 6K7G and 5Y4G were good. The likely original Sylvania 6F6G was very weak. The replacement 6K5GT was shorted, and the replacement 6J5GT was very weak. The 6A8, 6J5, and 6K5 should be G type tubes.
The shield bases for the 6J5G and 6K5G had been severely damaged in order to install GT type tubes.
The speaker cable was frayed and bare near where it enters the chassis.
The 6K7G grid cap lead was frayed where it entered the top of the IF transformer.
Five resistors were out of tolerance (more than 20%). One (part 12) was a 3-watt dogbone type. The remainder were carbon composition types.
The dial lamp was OK.
The line cord was rotted and unsafe. The original plug was long gone, and a length of heavy twisted cloth covered heater wire spliced onto the existing cord, terminated by a large appliance plug.
A phono jack had been connected between the second detector grid and ground using a short length of shielded wire.
The tuning capacitor rubber mounts were shot, permitting the tuner and dial to move around excessively. Fortunately reproductions of these complex parts is available from Renovated Radios.
Several of the rubber chassis corner supports were also shot, which meant that the control shafts were not properly centered in the cabinet. Again, reproductions of these parts are available from Renovated Radios.
I assume that all paper and electrolytic capacitors are leaky and thus should be replaced. I do not replace mica capacitors, but may test them in place if possible (usually this requires disconnecting one lead of the capacitor). When I replace a component, I always remove the original part and any wire fragments completely from a terminal. Other good components connected at the terminal are protected from heat using old medical clamps (hemostats). Excess solder is then removed using a solder sucker in order to expose terminal holes for reattachment of the rebuilt or replaced component.
Since almost all of the original parts were still in place I decided to try to maintain the original chassis appearance to the extent possible. Normally I rebuild all original wax-paper capacitors as well as the filter capacitors in their original cases (restuff them). Both filter capacitors were still intact and could be restuffed.
One must be VERY CAREFUL when doing repairs in the area of the band switch. The leads from the antenna and oscillator coils to the band switch are hair thin and very fragile! There are several capacitors in the area which have to be changed. Before starting repairs I took photos of the chassis, top and bottom, so that the original component placement and lead dress could be maintained.
The first IF amplifier grid cap lead was worn and could short to the transformer shield. This lead was replaced. To do so, the grid cap was removed and the transformer shield removed. The transformer itself and associated wiring was left in place. The grid lead was replaced using green stranded hookup wire. The new lead was left long so that it could be routed through the small hole in the top of the transformer shield as the shield was being replaced. The shield was reinstalled, the lead cut to the correct length, and the grid cap reinstalled.
The speaker cable (3 leads) was very stiff and there were breaks where the cable passed though a hole in the rear chassis. The clearance hole had no fiber grommet, unlike the hole for the power cord. Perhaps it had broken. A vinyl grommet was installed just in case. The speaker cable was removed from the radio and pulled out toward the front of the chassis. The knot was undone and the cable leads straightened. I used a heat gun on the cable to make it more flexible during the straightening process. The leads were separated and patched using shrink tubing. The twist and knot were restored and the cable reinstalled in the chassis, passing through the vinyl grommet for some protection.
The line cord was replaced by a new brown vinyl cord with molded plug. The original cord was a darker brown rubber.
Three original Philco paper-wax capacitors had been replaced (parts 24, 28, and 38 - all associated with the volume or tone control). I collect branded (Zenith, Philco, Sprague, RCA/GE etc.) dud capacitors just for cases where an original part has been replaced by a modern part. I was able to find the correct original Philco part for 24 and 28 in my dud stocks. I did not have the correct original Philco part for 38 (30-4467, .006mfd/600 volts), so I used Philco part 30-4591 (.006/400 volt) to replace it. All paper-wax capacitors were restuffed using 630 volt film capacitors. Capacitor 30 had been moved to the opposite side of the chassis. After restuffing it was returned to the location shown in the parts placement diagram.
My re-stuffing process is as follows:
Some original capacitor values are no longer available. In these cases I used the closest current value. I did not try to use multiple capacitors to get closer to the original values, since there was no room inside the original case. For C40, 0.008mfd I used 0.0068mfd. For C38, .006mfd I used 0.0056mfd. For C37, .03mfd I used .033mfd.
The original power supply filter capacitors 11/11A and 45 were removed and restuffed. Part 45 (30-2219A) was a 25mfd 475 volt wet electrolytic. It was restuffed using a 22mfd/450 volt axial tubular electrolytic. My restuffing process for this capacitor is as follows:
The process I used for restuffing the dry type capacitor 11/11A is similar to the process used for part 45. However all of the contents have to be removed, which is a messy and difficult process. I used a spade bit in my drill press to remove most of the material. This capacitor is a dual dry type capacitor with wire leads: 8mfd/475 volts and 4mfd/350 volts. It was restuffed using a 10mfd/450 volt and 4.7mfd/450 volt electrolytic. The original leads were cut, so they were replaced using the original wire colors. The replacement capacitors were secured in the case using hot glue. The two halves of the case were reattached with 3/4" PVC couplings as described above. In this case no masking tape was needed, since the capacitor diameter is smaller and the PVC coupling was a snug fit.
One section of the three section wire wound resistor 43 (Candohm type) measured 37% high and the resistance was unstable (it changed when the lugs were moved). The remaining two sections were OK. This resistor is used to provide bias voltages. I have found that these metal clad resistors often respond favorably to clamping pressure near their terminals when a section is unstable or has high resistance. I sometimes use a small C-clamp and firm pressure on the metal housing on both sides near each terminal. In this case, there was no way to use this method. I was forced to remove the resistor from the chassis and apply pressure on both sides of the terminals using my large bench vice. Each terminal is simply a band around the resistance element, which is wound on an insulator. Over time, the band expands or the insulator shrinks, resulting in an unstable, high resistance, or sometimes an open section. In this case, after squeezing, all sections were almost exactly correct and were stable (resistance did not change when the terminal lug was moved)! Compressing the metal case applies additional pressure from the band onto the resistance element. There is some risk to this procedure, since the fish paper insulation may be damaged in some cases, resulting in a short. For this reason, many collectors never re-use a Candohm resistor in a restoration, even when good! Instead, they remove the resistor and install terminal strips and individual wire wound resistors. This of course greatly alters the under-chassis appearance of the radio. I would also replace a Candohm resistor, and especially if part of it was bad, if being PAID for a restoration. But this radio will be a keeper (which will rarely if ever be played). And should the resistor eventually fail due to a short or open, no major components would be damaged - the radio will simply stop playing.
Resistor 12, a 10K 3-watt dogbone type resistor was out of tolerance. I keep a stock of NOS and used "dogbone" resistors, and buy all I can on eBay and radio swap meets! Of course, most of these resistors, even NOS resistors, have also drifted in value and no longer have their marked values. My solution is to find a replacement resistor of the correct value and size as measured (ignoring the markings), and then repaint it to the needed value codes using enamel hobby paint! In this case, I used a 6.2K 3 watt dogbone resistor that measured 9.24K. It was repained using enamel hobby paint.
There were four carbon composition resistors also out of tolerance. They are specified as 1/2 watt and are quite unique in appearance. They are very close to the size of modern 1 watt carbon resistors. So I replaced them with modern 1 watt carbon resistors and repainted them to match the Philco color code scheme, which is body (1st digit), end (2nd digit), and a strip (multiplier). In some cases, the body and stripe are the same color and no stripe is used (490K). In other cases, the body and end colors are the same (99K). I have always thought that Philco used these weird values in order to avoid having to use a third color!
Here are the repainted reproduction resistors:
The shield bases for the 6J5G and 6K5G were severely mangled in order to install GT type tubes:
These were straightened as well as I could without removing the shield bases, which would have meant drilling out the rivets. Fortunately, the tube shields completely hide the damage. G type tubes were installed for the 6A8G, 6J5G, and 6K5G. The 6F6G was replaced (General Motors brand). The original 5Y4G and 6K7G were good and were reused.
The dial assembly and tuning capacitor were removed for cleaning, replacement of the rubber suspension parts, and repair of the reduction drive. The two-speed reduction drive mechanism was removed from the front of the capacitor. The trimmer capacitors and all associated hardware (including the mica sheets) were removed before cleaning so that all parts could be cleaned, and to avoid damage. The capacitor was then cleaned in my old Heathkit ultrasonic cleaner using dilute ammonia. This has to be done in several steps since the entire capacitor will not fit in the tank. Once cleaned, it was rinsed and then cleaned with soap, water, and old toothbrushes and dried using a heat gun.
As found, the anti-backlash gearing was damaged and was preventing rotation of the tuning capacitor rotor. Originally there were two driven gears. One was attached to the rotor shaft. The other was free to rotate on the shaft. There were two small springs positioned between the gears which put rotational pressure on the free gear so as to take up any slack between the driving and driven gears. This reduced backlash in the mechanism. Unfortunately, both springs were out of place. There was no way to re-position the springs. One of the two was jammed between the two gears. This spring was removed so that the tuner could rotate without jamming. Unfortunately, this removed the anti-backlash function.
The reduction drive was gummed up with hardened grease and difficult to turn. It was disassembled and all parts cleaned using lacquer thinner, Q-tips, and pipe cleaners. It was very difficult to remove the three ball bearings and spring from the assembly. Once all parts were removed and cleaned, the parts were lubricated using automotive distributor cam lubricant and reassembled. It then worked perfectly after its correct position on the tuning capacitor was determined - the small driving pinion must engage the driven gears on the tuning capacitor shaft correctly without excessive pressure.
The tuning capacitor rubber supports were replaced using part PHS-38-10 available from Renovated Radios (set of three). I also ordered the four chassis corner supports at the same time (PHS-COR).
Once the radio had been reassembled, the radio was powered up slowly using a fused Variac. This allows the filter capacitors to reform. A DVM monitored the B+ voltage. The radio came alive and actually worked on both bands. The set was then aligned. The IF transformers were way out of adjustment.
The radio worked well and sounded great with its large electrodynamic speaker, large cabinet, and loudness compensation type tone control. The set seemed to perform best with a 6' length of wire for an antenna! It did not seem happy using my 40' indoor antenna strung across my basement ceiling.
The cabinet was vacuumed then cleaned using GoJo (white) hand cleaner and 00 steel wool.
Chassis Before Restoration |
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Chassis After Restoration |
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