Car radio plays fine while the cover is off but not when on.

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Car radio plays fine while the cover is off but not when on.

Post by Fid on Sun Sep 04, 2011 3:12 pm

I have a GE car radio from 1948. It uses the 7C5, 7Y4, 7B8, 7A7 (2) and 7B6 Tubes. I've done the standard repair - replace all wax/paper and electrolytic caps and a couple of the postage stamp caps. If it's sitting on the bench with the top cover off, it plays great and pulls in all stations. If I put the top cover (made of aluminum) on it plays very weak and only pulls in one station. I've carefully inspected it and the cover does not touch any components when it's in place - in fact, all I have to do is hold if over the radio and I get this symptom. I've tweaked all the trimmers/padders to get maximum output from a weak station when the cover is off but as soon as I just place the cover over the top of the radio, I loose most reception. Anyone seen this or have any suggestions? It seems there may be some type of spurious interference when the cover is held over it (as I mentioned, it does not contact any components). It's a real puzzle. I've repaired many of this particular model with much success but I've not seen this symptom before. Oh, I have swapped out all tubes with another set, one at a time and all at once and the symptom does not change. Thanks!

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Re: Car radio plays fine while the cover is off but not when on.

Post by Bill Cahill on Sun Sep 04, 2011 8:55 pm

Some radios need to have the cover on because it affects the tunning. Also, make sure cover isn't shorting anything out.
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Re: Car radio plays fine while the cover is off but not when on.

Post by willy3486 on Sun Sep 04, 2011 10:36 pm

How does it play with just the cover on and no screws holding it? If it plays fine try putting in one screw at a time. I have had a similar issue and it wound up being a screw that was a little longer. I put a screw back in from another area and it did something similar.

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Re: Car radio plays fine while the cover is off but not when on.

Post by Fid on Sun Sep 04, 2011 10:41 pm

Thanks for the suggestions. As mentioned, if I just set it on there it causes the problem, no screws needed. And it is definitely not shorting against any components. Very strange - In fact, if I just hold the cover over it real close it causes the problem. Bill's comment may be the best approach - which means, if the cover is part of the tuning I need to install the cover, then align the IFs and tweak the padders/trimmers. I will give that a try as the alignment is actually pretty easy on it. Thanks again and keep the suggestions coming. I'd really like to figure this one out.

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Re: Car radio plays fine while the cover is off but not when on.

Post by Fid on Mon Sep 05, 2011 12:28 am

Problem solved! I looked at it again tonight. I noticed when I tried to adjust the main antenna trimmer, there was no change at all in output. The antenna connector goes straight into a choke coil, then to that trimmer. Turns out, that coil was open. Looks like the stray wires etc. acted like an antenna on the other side of the coil. When I put the cover on it, it was equivalent to driving under a bridge as it cut all of the stray signal out. Luckily, I had a parts radio I could take another coil from. I installed it and the dang thing plays great! Picks up stations I didn't even know existed here and it does so with the top cover on it! I appreciate everyone's input (no pun intended). This is a great forum.

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