Help with 55 Century Sonomatic

[tdutch]

Hi, everyone! I'm new to the site and want to say hello, but also "help!"

I have a 55 century 2 door no post, and I've got everything working again EXCEPT for the sonomatic radio. I've narrowed it down to the antenna circuit. I replaced a dead tube, dead transistor, and dead capacitor, but I also have a dead compression mica trimmer capacitor (the antenna trimmer that is accessible from the outside). It looks like someone cranked it all the way clockwise (probably due to the other dead parts on that circuit) and it's pretty well cracked and ready for the junk pile. If I bypass it, I get sound, so I know that's the culprit.

 

The problem is that I have no idea what capacitance it has to replace it. I can't test it since it's fried, and the schematics I have don't say. The only writing on it is a funny A, then 7264297, and another funny A. I think it's a Delco 981652 model, which is just labelled as Sonomatic on the face.

 

Are any of your experts able to help a guy out? I'll post some pictures of the car in the near future.

[Lee H]

Does it look like this: https://www.ebay.com/itm/304387323952?chn=ps&norover=1&mkevt=1&mkrid=711-213727-13078-0&mkcid=2&itemid=304387323952&targetid=4580153136941822&device=c&mktype=&googleloc=&poi=&campaignid=437225723&mkgroupid=1224856224320864&rlsatarget=pla-4580153136941822&abcId=9300907&merchantid=51291&msclkid=222cb72abdfe12fcb504d2376434858f

I think those mica caps are all pretty standard.

[tdutch]

Very similar, Lee. Those capacitors actually all have different capacitance ranges, so while there may they overlap, they arent the same.

 

However, all the ones used specifically in GM radios might be the same for all I know. I'm hoping there's someone on here with conclusive information.

[tdutch]

Here's a photo of the one in my radio.

[Lee H]

If it were me, I’d find one that fits, and solder that in. Then check for output in the usual way. If you find that there is a sweet spot somewhere within the mechanical range of your replacement…you are golden. But if you find that it is still getting louder as you run up to max. capacitance, then solder in a standard cap (of roughly the same max capacitance as the trimmer) in PARALLEL with the trimmer. If the reverse is true, and it is still getting louder as you back off the trimmer screw, then solder in that cap in SERIES with the trimmer.

 

 

[Machine Gun]

I have the complete set of Rider radio service manuals that covers radios back to the 1920s. Most auto radios are included. Give me a day or two and I'll search your model number and see if I have the information you need. These manuals include parts lists for many models. That particular trimmer isn't something that would be expected to fail, hence no value might be provided on the schematic. A parts list may help, although some radio manufacturers provide only their replacement parts numbers and not their electrical values. If all else fails, I'd do what @Lee H suggests.

[Machine Gun]

@tdutch Never mind. I did a look just now and Buick/Delco/United Motors auto radios were no longer included in Rider manuals that late. Rider stopped publishing those manuals in 1954, with the final volume covering 1953 radios. When I have more time I will see if any of my other literature shows similar AM car radios so that you might have a starting point regarding what value capacitor to start with.

[Lee H]

The models I did find with an antenna trimmer didn’t identify the capacitance. Another way to attack this, might be to temporarily splice in a wide range variable cap, and find out what the radio is happiest with. From there, you should be able to find a compression cap with an appropriate range to fit the space available. There are many styles to choose from.

https://www.surplussales.com/Variables/Compression/CompCap1.html

[tdutch]

Thanks, Lee. That's what I'm finding--they omit the capacitance. Thanks for everything, everyone!