I have a United Motors (Delco) floor model radio
R-1254 that I assume has a shorted FM IF transformer (T-13 on the schematic). When I wiggle the transformer the reception comes and goes. I have looked it over pretty good and can't find any loose connections on the coil or leads going from it to other places. Most other radios that I have the IF transformer is phisically smaller and has only four lugs.What transformer can I use as a replacement.
Thanks Mike

The transformers should not be replaced. There are no shorted windings. Wiggling the transformers will not loosen a shorted winding. THe windings are covered in wax. Your problem is most likely bad mica condensers. Most likely there is a sheet of mica with silver sections deposited on each side of it. There will be two sections of silver on each side. The opposing sides of silver form the condenser for each winding of the transformer, totalling two condensers. The mica oxidizes and makes poor contact, or disappears all together. You must scrape the mica out so that there is none left to make contact/short. This is a very tedious and delicate operation. Then replace with mica condensers from 50 to 250 pF. Use one per coil. Select condensers that allow the IF to be brought to resonance. Mount the condensers under the can if you wish, so that you may change them until a correct value has been found. The correct value for the primary and secondary of one transformer might not be the same as that for another. The correct value for the primary of one transformer might not be the correct value of the secondary of that same transformer, either. The correct value for one winding will be correct for all others with the same DC resistance and same physical appearance. DO NOT TOUCH THE ADJUSTMENT SLUGS AT THIS TIME!!!! Select the condenser value that yields the strongest signal (use a signal generator to produce this signal). Then, once the closest value has been found (condensers don't usually come in infinite values...you only have so many to choose from, even from the best supply houses), touch up on the slugs and put your radio back together. Enjoy! Perhaps you might want to touch up on the other controls, too.

Thomas

Thanks Thomas I'll give it a try.

:The transformers should not be replaced. There are no shorted windings. Wiggling the transformers will not loosen a shorted winding. THe windings are covered in wax. Your problem is most likely bad mica condensers. Most likely there is a sheet of mica with silver sections deposited on each side of it. There will be two sections of silver on each side. The opposing sides of silver form the condenser for each winding of the transformer, totalling two condensers. The mica oxidizes and makes poor contact, or disappears all together. You must scrape the mica out so that there is none left to make contact/short. This is a very tedious and delicate operation. Then replace with mica condensers from 50 to 250 pF. Use one per coil. Select condensers that allow the IF to be brought to resonance. Mount the condensers under the can if you wish, so that you may change them until a correct value has been found. The correct value for the primary and secondary of one transformer might not be the same as that for another. The correct value for the primary of one transformer might not be the correct value of the secondary of that same transformer, either. The correct value for one winding will be correct for all others with the same DC resistance and same physical appearance. DO NOT TOUCH THE ADJUSTMENT SLUGS AT THIS TIME!!!! Select the condenser value that yields the strongest signal (use a signal generator to produce this signal). Then, once the closest value has been found (condensers don't usually come in infinite values...you only have so many to choose from, even from the best supply houses), touch up on the slugs and put your radio back together. Enjoy! Perhaps you might want to touch up on the other controls, too.
:
:Thomas

If the transformers have plastic bases, the capacitors could be molded inside of the plastic.

T.

Hi Mike and Thomas,
I think you are giving good information, Thomas. I have run into the same problem and dealt with it in pretty much the same way.
One thing that may give a hint as to the capacitor values is that with the transformer dissassembled I measure the capacitors. Often the capacitors take a short from primary to secondary, but since there was never much DC across them they may still give a good reading. Yours seems to be the intermittant type, you shoul get a good intermittant reading from it.
As an order of magnitude, FM IFs seem to have capacitors from 22pF to 50pF, AM has 80pF to 150pF.
Let me know if you have found different, Thomas.

Best Regards,

Bill
:If the transformers have plastic bases, the capacitors could be molded inside of the plastic.
:
:T.