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RF Choke on Philco 48-482 - Printable Version

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RF Choke on Philco 48-482 - jwidner - 01-21-2017

I am working on the FM portion of a Philco 48-482 radio. I have the AM/SW completed and working fine, but am having some issues with the FM. I was trying to measure the RF Choke Osc. Cathode on the FM Detector - L300 on the nostalgia air schematic. I also have the Philco Service bulletin for this radio, but cannot find the Henry value. My Sam's copy only lists the ohmage at 9 ohms.

The issue is that I can get no resistance reading through this choke at all, which leads me to wonder if it is working. I would like to possibly replace it but until I know the Henry value, I am not sure what to use. I could not find anything at Play Things of the Past, but do see a number of chokes at Antiques Electronics Supply, but need a Henry value or something close, perhaps.

The attached picture shows the choke plus I circled an odd piece of metal sticking out of the choke that does not seem to have an electrical connection to either lead.

Thanks,
Jim

   

RE: RF Choke on Philco 48-482 - Mondial - 01-22-2017

Jim, you might try repairing the choke. If the coil is open and shows infinite resistance, there is a possibility that the coil wire is broken where it terminates to the leads. One termination may be the metal piece sticking out of the coil form.

If you have a heat gun or similar heater you can melt off the wax and examine the coil more closely and see where the wire may be broken. Try resoldering the coil wire at the termination points.

If you have no success repairing it, I would consider replacement with a choke of around 100 microhenries. This is just a rough estimate based on its physical size and construction, and its use in the circuit.

RE: RF Choke on Philco 48-482 - jwidner - 01-25-2017

Thanks, Mondial. I did find the broken wire on it and was able to restore it. Now to get the FM properly aligned as I am getting some distorted FM signal, but without a scope or sweep generator I am finding it a little tough. Spent so much time on this radio over the past year, I plan on moving forward with it. I also have on order a possible replacement coil just in case that I might try putting in instead.

RE: RF Choke on Philco 48-482 - Mondial - 01-25-2017

Did you try the FM alignment as per the service manual procedure? A regular signal generator will do. It does not require a scope or sweeper, although they do come in handy for a final touch up adjustment.

http://www.nostalgiaair.org/pagesbymodel/637/M0013637.pdf

You should get pretty close by doing the zero beat adjustment of C300 and then finally optimizing the TC300 slug for best audio quality on a received signal.