The PHILCO Phorum
RF Coil for Model 20 or 20A - Printable Version

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RF Coil for Model 20 or 20A - hrensin - 05-29-2018

Hi all;
I am restoring a Model 20A and need to replace an inter-stage transformer RF coil shown in the schematic as number 7.
This is one of the type mounted on a hollow tube like a toilet paper roll and inserted head into a metal can.
Any pointers to where I can find a replacement would be greatly appreciated. See attached schematic for reference.
Thanks.
Howard

RE: Inter-stage transformer for Model 20 or 20A - Radioroslyn - 05-29-2018

So what is the issue w/ the one in the set??

RE: Inter-stage transformer for Model 20 or 20A - hrensin - 05-29-2018

It appears to have a broken wire in the primary winding, but I have not pulled it out of the metal can yet as I want to line up a replacement in case its beyond me to fix.

RE: Inter-stage transformer for Model 20 or 20A - Radioroslyn - 05-29-2018

Very common issue on the early sets. Pretty simple to fix. Primary winding is on the outside so it's easy to remove and rewind w/new wire. Wire gauge isn't particularly crucial something like 32-40ga is fine. It's not a tuned circuit so the # of turns isn't critical abt 50 is good. The phasing (direction) dosen't matter.
I suppose you could cheat and use a resistor and cap to supply the hv to the 24A plate and a cap to couple the rf signal the secondary of the coil.

GL

RE: Inter-stage transformer for Model 20 or 20A - Arran - 05-30-2018

If you have to rewind an open primary on an RF coil, which is what I think you are describing, then it would be best to wind it in the same direction as the original winding.
Regards
Arran

RE: Inter-stage transformer for Model 20 or 20A - morzh - 05-30-2018

Isn't there an article here as to how to use just the secondary of the 20 interstage?

RE: Inter-stage transformer for Model 20 or 20A - Ron Ramirez - 05-30-2018

The original title of this thread was confusing.

Part (7) is the 2nd RF coil, not the set's interstage transformer. That is part (21).

I changed the title of this thread accordingly.

Icon_smile

[Image: http://www.philcoradio.com/images/phorum/schem/20b.jpg]

RE: RF Coil for Model 20 or 20A - morzh - 05-30-2018

A-ha! Well, as Terry said, those are not too difficult to rewind.

RE: RF Coil for Model 20 or 20A - KCMike - 05-30-2018

Here is some info on how to rewind the coil. Make note of where the wires on bottom of coil go to in the circuit before de-soldering to avoid a miswire.
https://web.archive.org/web/20160731140618/http://philcorepairbench.com/tips/svctip36.htm

RE: RF Coil for Model 20 or 20A - hrensin - 05-30-2018

Interesting ideas. Thanks for the suggestion, although I think the number of windings is important to keep the impedience and the resulting voltage on the secondary in the acceptable range.

RE: RF Coil for Model 20 or 20A - morzh - 05-30-2018

Terry is correct. Of course it is best to rewind to the specs, but in this particular case it is not overly important (within reason).

RE: RF Coil for Model 20 or 20A - hrensin - 05-30-2018

My apologies for misdescribing the RF coil. Being new to restoring old radios I have all I can do to keep my hands steady when working under the chassis.