The AM Forum

Some investigation of I.F. transfomers has reminded me of long-forgotten information about how they are made that is perhaps taken for granted these days, especially among those who use ready-made radio sets of any vintage, and especially low voltage circuit board type semiconductor gear where the impedances are lower and this kind of construction isn't seen.

Attached is a drawing of the 2nd IF transformer from a National NC-240-D. It's a very high quality transformer. Here are some comments and reasons why I think it's made this way.

1.) The assembly inside the can has a phenolic bottom plate with wire lugs, and a ceramic top. The tuning caps are permanently mounted to the top, or it is more correct to say the the ceramic plate is part of the capacitor pair.

2.) The single air core form is made of clear 'plastic' (printed '20') and the coils are separated by about an inch. The can slides down over all of this.

3.) The plate winding (input) is near the bottom of the can, and the grid winding (output) is near the top of the can where the shielding is better.

4.) The coils are wound in layers 'pie wound'? and varnished to hold shape. They appear to be identical or close to it.

5.) The B+ and GND leads go to the rotors of the capacitors, which ends are made as the tuning screws. The plate and grid connections go to the stators so that the tuning tool does not touch them. This minimizes de-tuning when adjustments are made. Swapping the B+ for Plate leads, or doing the same for the grid side, would probably increase this issue.

6.) There is a metal shield between the two capacitors, This is a heavy item permanently fixed to the top ceramic part and it is threaded for four sheet metal screws, which fasten the whole assembly very securely into the case as well as ground the shield.

7.) The stiff wire leads from the phenolic base go straight up past the coils to the ceramic plate, where they are soldered to the extensions of the capacitors that come down through the plate. The wires of the coils are connected straight to these stiff wire leads where those pass by. Between the stiff wires, the coil forms, and the can the whole thing is made very rigid, perfect for a tuned transformer.

Some other transformers have fixed capacitors and are are tuned by slugs, and some others that seem to be for higher frequencies have ceramic coil forms, but at 455KC ceramic is not necessary. Maybe I can post on these others later as I have to investigate them further to pick the most suitable ones.

closeup of coil. litz wire?

Patrick
Yes, Litz wire. National even listed those nice transformers for sale in their annual catalog.  Attachment from 1948 catalog.

Chuck K7MCG