I'm restoring a 20s TRF console that has the three variable tuning cap pulleys ganged together with belts and springs. The pulleys at first glance look fine but upon closer inspection are swollen, riddled with cracks and warped. I can turn new duplicates easily enough on my lathe using a round of bar aluminum, but I have a question regarding their size. The center one measures an average of 2.135" in diameter where the others measure an average of 2.110". It's hard to get an exact reading on my caliper as, no matter where they're measured, the readings are all slightly different by a few thousandths.

Can someone tell me if these pulleys were typically exact in size or slightly different by design? Thanks.

Chris

IMHO: They would not shrink in size, so the smallest dimension is appropriate. The tuning condensers are all the same size and plate shape, so there cannot be any change in ratio, the pulleys would all be the same diameter. If you are making new belts, the diameter is not critical. If the belt tuned radio does not have tracking adjustments, probably does not, then a slight amount of "play" has to exist in the system. There should be slotted mounts for the two outside or "slave" tuners. The user is supposed to "rock" the main tuning condenser and that will "bump" the other two or three into tune. Some tracking can be accomplished by mechanical alignment at the center frequency if specified, if not then at 1mhz. If louder (more sensitive) high or low end of the band. Move the alignment frequency to favor the weaker end of the band by 50khz at a time. It will be a compromise unless major drastic changes are made like adding/subtracting turns to the RF coils or bending external plates of the condensers, do not bend cast potmetal plates, they will break! Do note any neutralizing adjustments if they exist. Neutralizing may involve using a disabled tube or an insulator on a filament pin. Adjust for NULL. Be sure all shields are in place and that any alignment of a radio with bottom chassis mounted coils be aligned on a wooden bench. Be sure to route any antenna lead well away from the detector tube.

If published alignment techniques exist, follow that instead of this advice.

YMMV

Chas

Chas --

Thanks for your reply of some length. It's very helpful. I suspect the intent was to cast all of these pulleys to an outside diameter of 2 1/8" since that's the closest fractional dimension to my measurements. I have the belts and springs still. This is for a 1927 McMillan early AC console. I have yet to find a schematic match for this set that uses Kellogg-type tubes -- let alone alignment instructions. Hopefully it won't be too much of an issue as TRF's aren't overly complicated.

Chris

Here are a few notes on swapping pullies between brands, and other pot-metal parts:

https://www.russoldradios.com/blog/rare-and-common

Russ