Welcome Guest! Be sure you know and follow the Phorum Rules before posting. Thank you and Enjoy! (January 12) x

Thread Closed
Thread Rating:
  • 0 Vote(s) - 0 Average
  • 1
  • 2
  • 3
  • 4
  • 5

42-1010
#2

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.

John Honeycutt


Messages In This Thread
42-1010 - by Guest - 08-31-2009, 11:37 PM
Re: 42-1010 - by Raleigh - 09-01-2009, 01:10 PM
Re: 42-1010 - by Guest - 09-02-2009, 10:24 PM
Re: 42-1010 - by Guest - 09-08-2009, 08:41 PM
Re: 42-1010 - by Steve D - 09-08-2009, 10:55 PM



Users browsing this thread: 1 Guest(s)
[-]
Recent Posts
Philco Model 38-7: what caps & resistors do fail typically?
Since you stated earlier, you have a console 38-7XX.  They also had a 38-7T (table), and 38-7CS (chairside).  A very nic...GarySP — 01:18 PM
Part numbers to model cross
That I wouldn't know, but sometimes the radio document itself shows one, same as the capacitor cans' etc pinouts.morzh — 12:46 PM
Part numbers to model cross
Thank you morzh, that is exactly what I was looking for. Now , is there some where that shows pinouts for Philco power ...Jim Dutridge — 11:37 AM
Restoration of the Canadian General Electric A-87
Thanks for your help Michael. In fact, this video is only an intermediate result. Later I had to apply another tinting l...RadioSvit — 09:01 AM
Restoration of the Canadian General Electric A-87
Great job on the cabinet. PS. In the US notation, "kenotron" refers to specific type of tubes; we call re...morzh — 08:24 AM
Part numbers to model cross
This document has at least some tables of models and parts used. Example: Choke 32-7572, used in 604 radio. Search f...morzh — 08:19 AM
Restoration of the Canadian General Electric A-87
I also checked all the radio tubes on my Hickok 530 tube tester. The 5Z3 kenotron turned out to be faulty, all the other...RadioSvit — 08:02 AM
Restoration of the Canadian General Electric A-87
Well... While the varnish is drying up, I started repairing the chassis... Of course I started by replacing the pa...RadioSvit — 07:12 AM
Philco Model 38-7: what caps & resistors do fail typically?
Hello Martin, Welcome aboard our little community what great Model 38-7 Sincerely Richardradiorich — 12:30 AM
Philco Model 38-7: what caps & resistors do fail typically?
Welcome to the Phorum Martin. I count about 9 paper caps, the 3 electrolytic caps and 2-Y2 safety caps to replace th...RodB — 09:44 PM

[-]
Who's Online
There are currently 1328 online users. [Complete List]
» 1 Member(s) | 1327 Guest(s)
Avatar

>