09-01-2009, 12:10 PM
Most of these old radios use speakers with field coils instead of permanent magnets. The field coil is part of the speaker, and is integral with the power supply, so simply hooking a modern permanent magnet speaker will not work. Unless the field coil is in the circuit, the power supply is disabled.
There is a workaround you can use to test it with a modern speaker, but I would suggest that you completely rewire and recap this set before you even plug it in. Philcos of this vintage used a lot of wire insulated with rubber that crumbles with age, leaving them vulnerable to shorts. Also, the filter caps may be dried out and shorted, which could smoke your power transformer. Any of the tubular paper capacitors could also be shorted, which could damage the power transformer and other parts. The paper capacitors are usually yellowish cylinders covered with wax.
The workaround involves wiring a power resistor of the same resistance value in place of the field coil to complete the power supply. In the schematic it is between the two filter capacitors. You might need two power resistors in series to get the same value as the field coil. You could connect it with jumper wires between the positive terminals of the two filter caps. The power resistor will get very hot. Be careful! Also, you are working with DC voltages of 300 volts or more, so be very, very careful.
You'll need to make sure you have a big enough power resistor. Ten watt resistors such as Radio Shack sells will probably not be big enough, but you could put several in series. You can calculate roughly the power resistor wattage you will need, based on the voltage drop across the field coil and the voltage dividing resistors on the schematic. If you know the resistance and the voltage drop, you can calculate the current and the power.
The formula is: (Voltage across the 1st filter cap - voltage across 2nd filter cap) divided by (Field coil resistance + the total of the voltage dividing resistors) = current in Amps. All of this information should be on the schematic. The current will probably calculate out around 100 mA or maybe a little more considering this is a 10 tube set. Current in amps squared times the resistance of the field coil = power. Current times the voltage drop is close enough approximation, and errs on the safe side.
Oops, I just pulled your schematic from Nostalgia Air, and I don't see the filter cap voltages anywhere, although the field coil and voltage divider resistances are there (parts 81 and 82.) You might be out of luck unless you just assume around 100 mA and use power resistors with plenty of margin. You might also buy the Philco service package from Chuck Schwark. Voltages might be elsewhere in the Philco service data. These packages are a very good value, and I plug them whenever possible.
There is a workaround you can use to test it with a modern speaker, but I would suggest that you completely rewire and recap this set before you even plug it in. Philcos of this vintage used a lot of wire insulated with rubber that crumbles with age, leaving them vulnerable to shorts. Also, the filter caps may be dried out and shorted, which could smoke your power transformer. Any of the tubular paper capacitors could also be shorted, which could damage the power transformer and other parts. The paper capacitors are usually yellowish cylinders covered with wax.
The workaround involves wiring a power resistor of the same resistance value in place of the field coil to complete the power supply. In the schematic it is between the two filter capacitors. You might need two power resistors in series to get the same value as the field coil. You could connect it with jumper wires between the positive terminals of the two filter caps. The power resistor will get very hot. Be careful! Also, you are working with DC voltages of 300 volts or more, so be very, very careful.
You'll need to make sure you have a big enough power resistor. Ten watt resistors such as Radio Shack sells will probably not be big enough, but you could put several in series. You can calculate roughly the power resistor wattage you will need, based on the voltage drop across the field coil and the voltage dividing resistors on the schematic. If you know the resistance and the voltage drop, you can calculate the current and the power.
The formula is: (Voltage across the 1st filter cap - voltage across 2nd filter cap) divided by (Field coil resistance + the total of the voltage dividing resistors) = current in Amps. All of this information should be on the schematic. The current will probably calculate out around 100 mA or maybe a little more considering this is a 10 tube set. Current in amps squared times the resistance of the field coil = power. Current times the voltage drop is close enough approximation, and errs on the safe side.
Oops, I just pulled your schematic from Nostalgia Air, and I don't see the filter cap voltages anywhere, although the field coil and voltage divider resistances are there (parts 81 and 82.) You might be out of luck unless you just assume around 100 mA and use power resistors with plenty of margin. You might also buy the Philco service package from Chuck Schwark. Voltages might be elsewhere in the Philco service data. These packages are a very good value, and I plug them whenever possible.
John Honeycutt
