[GarySP]

Hello all.  Since I've got lots of time on my hands this winter I've made it my new years' resolution to get back to my long ignored radio restoration hobby. Since I'm back on a no bending,twisting, or lifting over 5 pounds for some time to come, I'm being a good boy and learning how to rewind RF coils.  I'm in the process of building myself a winding rig to mount the coils in.  Then I've got a model 20 parts chassis for three projects.  When looking at the parts list it shows the first RF transformer as part 3884-N, and the second and third RF transformers as part 3884-P.  I haven't pulled them out of the cans yet but I was curious as to what is different with the first RF as opposed to the second and third RF parts.  Probably a simple answer, but my expertise in restoration is a work in progress.  Thanks in advance.  Take care and BE HEALTHY!   Gary

[Ron Ramirez]

Gary

Since no one else has chimed in, I'll give it a go.

I am assuming this is the later version of model 20 with three trimmer condensers mounted to the tuning condenser.

Looking at the schematic, you will notice the first RF transformer's job is to couple the signals coming in on the longwire antenna to the first RF amplifier tube. The primary winding of this coil does not carry B+ voltage, only the signals out in the air.

The second and third RF transformers couple the amplified RF from the first stage to the second, and then from the second to third. Both of these have B+ on their primary windings. This, likely, is where they differ. In addition, the first RF transformer has one end of both the primary and secondary connected to ground, so it may only have two terminals with the grounded ends soldered to the mounting bracket of the coil (transformer).

I wouldn't be too concerned about the differences. The important thing is how many turns there are in the primaries of all three transformers. You already know the 2nd RF will be identical to the 3rd. Determine the number of turns and wire size of the primaries and you may proceed from there.

Oh, and just make sure you wind your new windings in the same direction as the originals.

[rfeenstra]

The primary of the antenna coil is actually a bobbin coil in the inside of the tube. It does not have the nitrocellulose problem that the other two might have. I rewound both of the primaries on coils 7 and 10. I have pics if you want.

[Ron Ramirez]

Thanks Rob. It's impossible for me to remember all this stuff. Plus, I do not own a 20 anymore.

[rfeenstra]

You astound me with what you do remember!  At age 70, I can't remember anything anymore. 

[GarySP]

Thanks guys. I will probably be calling on everyone else's expertise a lot until I return to somewhat normal. Take care and BE HEALTHY! Gary

[GarySP]

Thanks Rob and Ron. Rob, from what I read in your post the antenna coil shouldn't need rewinding? I'm still going to pull the coil and test it, just to see the difference between the other two. Ron. this is a later chassis with the three trimmer condensers. Thank you. Take care and BE HEALTHY! Gary

[rfeenstra]

Correct, no rewind on the antenna coil. The primary is a bobbin coil inside the top of the tube. No nitrocellulose. The secondary is like the others, much thicker wire and enameled. that is, of course, there wasn't a lightning discharge trough the primary!

[Ron Ramirez]

What Rob said. Don't automatically assume that bobbin winding in the first RF coil is good, or bad - test it before proceeding. It should measure somewhere in the neighborhood of 20 to 40 ohms...I think.

[GarySP]

Thanks guys! Quickly found what my limits besides no bending, twisting, or lifting over 5 pounds. New quirk is also no sitting at times, no standing at times, and no long walks just yet. (Don't ask me how I know.) Take care and BE HEALTHY! Gary