Philco Model 38-12 Field Coil

4/12/2009 4:33:45 PMGeorge F.

My Philco 38-12 was playing just fine when the audio stoped completely. Checked the tubes and everything seemed OK. Started checking resistance readings and found the field coil to be 300 Ohm to ground, should read 1500 Ohms across it, not to ground. Ok, so I need to get a replacement for this field coil. I have looked in the Radio Daze catalog without much luck. Anyone that provide some guidance here would be greatly appreciated

4/12/2009 5:05:16 PMWarren

One thing that is done, is to replace the field coil with a frame type filter choke. You do then need to replace the speaker with a PM type ..

4/12/2009 5:21:05 PMDoug Criner

You can replace the coil with a power resistor (and a PM speaker). If hum is objectionable, you can jack up the ratings of the filter caps a little.

If you use a filter choke, then you'll want the resistance to be as specified on the schematic - or else the B+ voltage will be out of whack. If it's too low, you can pad it with a series power resistor.
Doug

:One thing that is done, is to replace the field coil with a frame type filter choke. You do then need to replace the speaker with a PM type ..

4/12/2009 5:48:05 PMGeorge F.

:You can replace the coil with a power resistor (and a PM speaker). If hum is objectionable, you can jack up the ratings of the filter caps a little.
:
:If you use a filter choke, then you'll want the resistance to be as specified on the schematic - or else the B+ voltage will be out of whack. If it's too low, you can pad it with a series power resistor.
:Doug
:

Thanks Doug and Warren for your response, I like the idea of using a choke of the same resistance, any sources for this? Radio Daze, Digi-Key

:
::One thing that is done, is to replace the field coil with a frame type filter choke. You do then need to replace the speaker with a PM type ..

4/12/2009 11:12:58 PMMarv Nuce

Geo,
Make sure to disconnect other components from both ends of the field coil before measurements.

marv

:My Philco 38-12 was playing just fine when the audio stoped completely. Checked the tubes and everything seemed OK. Started checking resistance readings and found the field coil to be 300 Ohm to ground, should read 1500 Ohms across it, not to ground. Ok, so I need to get a replacement for this field coil. I have looked in the Radio Daze catalog without much luck. Anyone that provide some guidance here would be greatly appreciated

4/13/2009 2:19:40 PMEdd

Sir George:

Just now read this, but if you haven’t already progressed too far on this, could you try this?

Have the chassis pulled and resting on an insulated work surface, look at the Item 23. . . the speaker. . . and note the ground connection, and find that wire and open that circuit in its making connection to chassis ground.
( It MIGHT just be soldered to the speaker frame, such that it is dependent on that speakers mechanical connection to the chassis proper )

Pull the mounting of the speaker to the chassis such that the speaker proper is now mechanically and electrically floating from chassis ground.

Power up ye olde radio and see if it will /is / now work(ing) ?

(No touchee-touchee speaker frame !)

73's de Edd