Making parts for an antique radio

[Mike6158]

I made a post about this on another forum, it got hung up in moderation for three days, they released it, and then the site went down. Lets see how my luck holds up...

I volunteered to restore a coworkers 1941 Silvertone farm radio. It was his grandparents radio. They used it on their ranch in NE New Mexico. To this day it's a fairly remote area. It runs on a 90VDC and 2.5 VDC batteries. The 90V battery can be made from ten 9V batteries in series. That will last about 6 months. I've finished the electronic restoration and I'm not working on the only mechanical problem I've found. The dial pulleys are made of wood and one of them has failed. My initial plan was to make a new pulley from brass. Here's some photos to show what I need. I'm going to make both pulleys even though one is still good. More on that in a bit. No more skill than I have with my mini lathe I'll probably make a few that don't look like a pulley too. The "CAD" drawing was made with Mike CAD. The software is not for sale

The most pressing problem that I have is how to get the pulley off of the old (aluminum) shaft. I could chuck the hex end up and turn the ball down and then capture the new pulleys with a pin (1/16" hole to drill. That won't be fun). I could remake the shaft assembly out of brass, turn the axle to a larger diameter so it will be easier to drill, and then make the hole in the pulley larger. But I can't change the v groove OD, a little less than 1/4" so I can't increase the hole by much. Also, I don't know if brass on brass is a good idea. What's a good way to cut the v-groove. I thought about making a form tool but I really don't know how to do that. Thoughts? My guess is that this isn't a beginner project. That's one reason I bought 12" of stock

 

[RJSakowski]

Since you are making new pulleys, you could just split the old ones to remove the shaft. If you need the original for a pattetrn, you could glue the pieces back together again after removal. I don't expect that brass on bras would be a problem. The duty cycle and rotational speed are very low. A little light oil on the shaft will keep it operating for years.

Pulleys were used in two ways on these old radios. One was to convert rotational motion to linear motion and the other was to change direction of the linear motion. Pulley diameter when changing direction is not critical within reason. Tuning of the radios wa usually accomplished by rotating plates on a variable capacitor and the diameter of the pulley on the variable capacitor is critical to be in sync with the dial. Assuming that the tuining capacitor was not directly coupled to the tuning knob, the pulley on the tuning knob wouldn't be critical either.

If I would be making the pully, I would make a form tool with the angle matching th pulley groove. I would cut the grove muich like cutting a thread, with the compound angle set at 1/2 the included angle of the pulley groove. The actual angle won't be critical. The groove just has to capture the dial cord. I would put a radius on the tip of the form tool to match the dial cord diameter.

 

[matthewsx]

I suspect the originals were soaked then pressed onto the shaft where they shrunk to the point of being retained by the ball.

While it might not be as much fun as making them from brass I would think about using wood. It obviously worked good for a really long time and would keep the radio all original if you haven’t modified anything else.

I don’t know much about antique radios but can appreciate you doing the work for your friend

Welcome to the forum,

John

 

[Mike6158]

Thanks very much for your replies. I suppose the best way to learn how to make a form tool is to make a form tool. I agree that brass on brass will be fine. Wood on aluminum lasted 78 years so it's probably fine too.

In this case the tuning is different than what I've seen in other radios. Instead of dial cord moving a variable capacitor the dial cord moves a lever arm that raises and lowers ferrite cores in and out of coils (one for each band) and moves the dial pointer in the same cord. I should be able to match the inner OD of the pulley at least as close as wood is.

I thought about trying to make them out of wood but I feel like I'd be more likely to be successful with brass. I put a restoration post on the Antique Radio Forum and someone there also said that the pulleys were probably green (or wet) and were press on / shrink to fit. I don't throw any parts away. The original pulley assembly will stay with the radio.

This is the third radio that I volunteered to repair gratis. All three have been a challenge In the first one the insulation on every piece of wire was so brittle that touching it exposed a bare piece of wire. When there's 400 - 500 VDC in the radio bare wire is not an option. Wire routing is critical for noise suppression so I tack a new piece of wire to the old wire and fished it through. It sounds easier than it was but the radio knows how to sing now.

Here's a shot of the electrical restoration for this one. It wasn't very complicated. Just a recap. The black stripe on each capacitor (yellow thing) is to mark the foil side, ie the lead that should be grounded for noise immunity:

 

[RJSakowski]

I have worked on radios dating back to the 1920's and have never seen wooden pulleys. Brass was commonly used. Since you are using modern capacitors, the radio is not original any more nor is likely to ever be, given the scarcity of the old capacitors. If you did use wood, I would suggest stabilizing the wood. I would turn to a rough shape and then saturate the end grain with fresh cyanoacrylate (Super) glue. It will wick into the wood grain creating a strong plastic composite. After applying the glue allow several days for it to fully cure. The odor of glue should have completely dissipated. This allows for a better finish and really strengthens the thin sections. . I used this approach making some button head rosettes with great success. Super glue will also work well for fixing the pulleys to the shaft.

I have seen the ferrite core tuning before. In my case, they were used on a push button radio. Early automotive radios also used that method for tuning.

 

[Mike6158]

My oldest is mid 1930's. A lot of my projects are tube type Ham radios and tube test equipment. FM radios offer a challenge when it's time to align them but AM radios are pretty simple.

Even NOS caps from that era often won't pass a HV leakage test (and sometimes pass an ESR test). The electrolyte is usually long gone. Note that I didn't say all. I have run into so older caps that were still in good shape.

The Super Glue is idea is a great idea. If I could find some thin enough stock I could probably repair the existing pulley that way.

 

[RobertB]

If you boil them for 15 minutes or so you should be able to pop the ball back through them. You could make new ones out of something with a little give, like abs or nylon, that would push on over the ball end.

 

[Mike6158]

If you boil them for 15 minutes or so you should be able to pop the ball back through them. You could make new ones out of something with a little give, like abs or nylon, that would push on over the ball end.

I ordered some nylon and ABS. I might give that a try too.

 

[WarrenP]

I have worked on radios dating back to the 1920's and have never seen wooden pulleys. Brass was commonly used. Since you are using modern capacitors, the radio is not original any more nor is likely to ever be, given the scarcity of the old capacitors.

Actually, you cannot use old stock capacitors when rebuilding an old radio. Capacitors go bad just sitting in the shelf. Any refurbished radio has to have new caps.... they are actually safer too, it would be dangerous to use old capacitors.

 

[RJSakowski]

Actually, you cannot use old stock capacitors when rebuilding an old radio. Capacitors go bad just sitting in the shelf. Any refurbished radio has to have new caps.... they are actually safer too, it would be dangerous to use old capacitors.

Electrolytic capacitors go bad with time. Provided the foil and wax paper capacitors are properly stored, there is no reason why they should go bad.