My antique radio restoration logs
The Philco 37-604 is a 5-tube AC/DC superhet circuit radio. It receives the standard broadcast band and one short wave band. The circuit is conventional, but the cabinet design is somewhat unique. The radio had seen some servicing in the past, but the radio had not been hacked excessively. This being the case, I decided to try and retain the original top and bottom chassis appearance if possible.
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 Philco part numbers referenced refer to the service information for this model in the Riders Manuals (the schematic for the 37-604 is not on-line on Nostalgia Air for some reason. There is a Philco model 604, which is similar, but uses standard base tubes vs. octal).
The filter capacitor block (Part 42) was missing and tubular filter caps were tacked in under the already crowded chassis.
The choke (Part 47) was not original and had been replaced in the past. The replacement unit was a small low-resistance choke. The original was supposed to be 2250 ohms, and likely a high inductance.
The AC switch was bad - open..
The AC line cord was crumbling.
Two tubes were not the correct type (they were GT rather than the correct G type used by Philco).
One tube was bad (25Z6GT).
The dial scale was cracked and the lettering was worn off or had been wiped off in a failed attempt to clean it.
One pilot lamp socket had broken off from its mounting bracket.
Both pilot lamp sockets needed to be rebuilt - deteriorated rubber, typical of Philco.
The rubber grommets supporting the tuning capacitor were stiff and had shrunk - there was too much play.
A capacitor (Part 49) had likely exploded - remnants were found. This capacitor was connected between the anodes and cathodes of the 26Z6G rectifier, and was likely only a 400 volt unit originally!
The radio used a bias cell, which was bad.
The 6Q7G grid lead was frayed. All other wiring was OK.
All coils, transformers, and the other filter choke were OK.
The speaker field and cone were OK.
Only a couple of resistors were out of tolerance (dogbone types).
The 6A8G cathode bias resistor was supposedly 200 ohms. The one in this radio measured 318 ohms (flexible wire wound type).
A fiber bushing for the bandswitch shaft was missing.
|The construction of this set makes servicing difficult due
to the presence of fragile coils on top of the RF unit. The set
cannot easily be turned upside down in order to gain access to the bottom
of the chassis without danger of damaging these coils. The RF unit
could of course be removed from the radio, but that would involve
disconnecting about 10 wires!
So I attached a piece of thin sheet metal to the outside of the RF unit, after removing the cover. This metal protects the coils from damage when the chassis is upside down (the tuning capacitor had already been removed).
|The original filter capacitor block had been removed. The replacement tubular filter caps tacked in under the chassis were removed, taking note of where they were connected. I built a replica container (not knowing what the original looked like) from an old metal box capacitor can cut down to fit under tuning capacitor bracket, and mounting tabs were soldered on. Holes were drilled to match the original mounting holes. The original capacitor was 16+16+8+10uf at 150+150+150+25 volts. I used 22uf 250 volt caps for all high voltage caps, and a 10uf/50 volt for the low voltage cap (25A6G cathode bypass).|
The filter choke (Part 47) was not original. The replacement was a low resistance choke. The original was likely 50-60hy, and 2250 ohms. The replacement was likely 3-5hy and 235 ohms. A question was posed on the Philco Phorum as well as on the Antique Radio Forums as to how to handle this. I got no answer. I did some research, and found in the Radiotron Designer's Handbook that the purpose of this choke and filter capacitor, separate from the main radio filter choke and filter caps, was to reduce flutter and motorboating due to B+ supply voltage changes (due to changes at audio rates as the radio plays). The 6A8G tube used in this set is very susceptible to flutter, motorboating, and modulation hum if the power supply is not rock stable. A suitable replacement choke (rated at 60hy, 2700 ohms, 8ma) was available from Hammond/AES, but it was physically the same size as the one installed. Since the replacement was good, I decided to simply increase the associated filter capacitor in value. The original was 8uf. I replaced it with a 22uf/250 volt unit. The radio worked OK, but it is not clear if this fix resulted in increased oscillator flutter (I did not notice any on short wave).
The original AC switch on the volume control was bad, but responded to a good spray of Big Bath cleaner followed by rapid cycling of the control. This is fortunate, since the original control would be difficult to replace since it has a flexible shaft attached (as does the bandswitch). In this case, again fortunately, there were openings in the back of the switch cover where spray cleaner could be applied. This meant that the control did not have to be removed and disassembled for cleaning.
One pilot lamp socket had broken loose from its mounting bracket. It was repaired by inserting a nipple through the hole in the bottom of the socket and attaching the nipple and socket base to the original mounting using epoxy cement. Both pilot light sockets were then rebuilt, since the original rubber insulation had deteriorated. I use a 3/8" #6 fiber washer ground down to fit loosely inside the socket and use shrink tubing to insulate the wires. The original spring and contact were reused.
The dial scale was cracked and lettering was worn off or wiped off in the center in a failed attempt to clean it (NOT ME - I KNOW BETTER - ASK ME HOW I KNOW). A replacement was ordered from Mark Oppat at http://www.oldradioparts.net. Fortunately the markings on the dial needed for alignment were still present so I did not have to wait for the replacement dial scale to arrive. Once received, the new dial scale was installed using #4-40x1/8" RH brass screws and nuts (the original was attached with rivets).
The bias cell was rebuilt by removing the old content, cleaning out the housing, and inserting a LR44 button cell. When this is done, the negative terminal is now up and in contact with the flexible finger. This is the opposite of the original cell, where the positive terminal is up. In this case, the wiring to the cell was easily re-routed by swapping the position of a resistor and a wire to B-. In some radios this re-routing is difficult to do because the positive contact finger is connected to the chassis through the bias cell holder hardware (the chassis is not directly connected to B- in this set, since it is an AC/DC set - otherwise the chassis would be HOT!) The original cell was 1.0 volt. The LR44 is about 1.5 volts.
Only a couple of resistors were out of tolerance. The 6A8G cathode bias resistor was supposedly 200 ohms. Mine measured 318 ohms. I assumed it was bad. But when I removed the insulating spaghetti tubing, the color markings on the flexible resistor were Orange Body/Black end/Brown Dot: 300 ohms! So I left it in place. When I later measured the 6A8 cathode voltage, is was right on the money. So I suppose a change was made at some time. A 13K ohm resistor (Part 12) was about 100% high. I replaced it with a 12K old style dogbone which measured 11.26K ohms (-13.38%) but repainted the red end orange like the original.
All bakelite block and tubular capacitors were rebuilt in their original
cases using modern 630 volt film capacitors.
A capacitor (Part 49) had likely exploded - remnants were found. This capacitor is connected between the anodes and cathodes of the 26Z6G rectifier, and was likely only a 400 volt unit originally! I replaced with with a 630 volt capacitor.
Two tubes were not the correct types. GT types were installed. Philco used G type tubes at this point in time. The 25Z6GT was also bad - shorted. The correct 25A6G and 25Z6G were obtained and installed. The other tubes tested OK.
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 only issue was that at 118 volts AC input, the B+ at the rectifier cathode was only 105 volts - Riders says it should be 120 volts. I checked and substituted the 25Z6 rectifier, verified that 118 volts were getting to the rectifier plates, and even increased the input filter capacitor by 100% - very little effect. It is hard to believe that the rectifier could be putting out 120 volts DC with a speaker field load of 2442 ohms across the B+ (a load of 43 MA for the field alone). All other voltages were proportionally lower. I did verify that the other tubes were not drawing excessive current.
The set was next aligned. It was not possible to adjust the local oscillator on the short wave band at 18mhz. So the dial scale would not be correct on the high end. This is likely due to changes in the coils or capacitors in the RF section with age or moisture.
Chassis Before Restoration
Chassis After Restoration