Hello,

My 1974 radio has a weird problem. It works reasonably well - for about 3 minutes. Then, it turns to static (at best) or just goes silent (at worst). I'm thinking it has something to do with heat buildup. I've worked on tube equipment quite a bit but know very little about solid-state stuff. Any ideas?

Thanks,
Jonathan

 

I highly recommend junking it and buying a modern reproduction radio that operates on 1/10th the amperage and puts out 3 times as much power. The average cost to repair these 40+ year old radios is around $250 and then you still have a 40+ year old radio.

I recently bought a reproduction radio from Antique Automobile Radios for $650 and its only 1/3 the depth of the OEM.

 

my retro sound replacement stero does that same thing although it comes back when I hit a bump so I believe there is a short somewhere....one of the PO's of my car butchered the stereo and wiring long ago..

BTW you could take the statement above about the stereo and apply it to the whole car and it still makes sense so while I see his point I can also see the point of keeping a classic ..... well a classic.

 

Just making a point but dont you see the irony here? when it comes to what we can and cant justify to ours selves its different for all of us and we all have different priorities..

 

I love the look (and depth) of the radio in the pic, but $650 is steep. Plus, I'm the type of guy who likes things to be as original as possible. I know that electrolytics do eventually dry out, so could it be those causing the intermittence I've described?

 

there are a lot of options for much less..

http://www.ebay.com/itm/Corvette-C3-68-76-RetroSound-Classic-Radio-Stereo-RC-900-Chrome-Retro-Sound-/251320761127?fits=Model%3ACorvette&hash=item3a83e27b27&vxp=mtr
http://www.ebay.com/itm/Corvette-OEM-AC-Delco-Mono-AM-FM-Radio-Receiver-1972-1974-/131581130761?hash=item1ea2d8b809&vxp=mtr
http://www.ebay.com/itm/Corvette-Original-AC-Delco-Stereo-AM-FM-Radio-Receiver-TESTED-41YFM2-1975-1976-/201406460156?fits=Model%3ACorvette&hash=item2ee4c2b0fc&vxp=mtr
http://www.ebay.com/itm/Corvette-Original-AC-Delco-Mono-AM-FM-Radio-Receiver-1972-1974-/200948677661?fits=Model%3ACorvette&hash=item2ec9797c1d&vxp=mtr

 

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Turn the radio bottom side up facing you.

Spray no more than one drop of DEOXIT onto the wire wound coil in the round hole on the right hand side near the front panel. You will see a screw driver slot in the hole.

Then rock the screw driver adjustment back and forth from end to end and leave it where you started.

The nichrome wire is corroded, heats up and the speaker bias fails leaving the amplifier out of adjustment.

Through the years many techs loved that problem! Quick fix, clean the visible face parts and send a big bill. BUT that's capitalism...

If it's an electrolytic, it's the 100MF across that control to ground.

Save the old radio! It will serve you much, much longer than an aftermarket. Plus, who will fix your aftermarket in 3 to 5 years? Not the manufacturer!

PS: it's easy to add an amp to that radio. Google really is your friend.

Good hunting!

 

I'll try that and let you know what happens. Thanks!

 

Turns out my radio doesn't have the slotted pot you describe. I removed the cover and cleaned all of the hex-head pots, and it didn't make any difference. Should the convector be getting hot enough to burn your skin? It's too hot to touch for more than a second. Normal?

 

see if you can send it to Jerry, he restored mine in the spring

 

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Then you have the later model build with the alternate "fixed" bias control components and yes, your convector can be running very hot. The 100MF cap can do that. I never trusted that fixed value Delco mod.

Still, the crackling could well be from the 100MF capacitor across that part of the bias circuit. That capacitor is notorious for breaking down all the way back to 63 models.

The variable bias control is handy if you want to add more speakers. Hooked two of Greg's aftermarket speakers up in series (4X6's and 4X10's) and wired the two strings in parallel for an install last week. Only had to touch up the bias control voltage. They sounded much better than I expected to hear.

You can get a can of freeze mist (or canned air) and spray the parts one at a time. That might help isolate the noise by extreme cooling. But it could be very difficult to find if it's not a cap. The 65 I did Monday had the frying sound and I finally tracked it all the way back from the amp to the 680 ohm collector transistor in the antenna pre-amp circuit. The resistor had gone wacko and was temperature sensitive.

Guess I can't help, sorry!

Comment to Gerry: many thanks for the thought! I can't help him as I only work on 72 and older. The 73-76's, especially the stereos, often have power switch problems and I will never make tiny parts to rebuild the cast and plastic original pieces again. L8R Gerry!

 

I totally understand. Thanks anyway:-)

 

I take it you have it set up for bench testing, a thought .....turn off all lights and watch it in the dark, examine the whole unit for arc-over. Sometimes a problem is obvious if we look in the right spot at the right time. I've been lucky a few times doing this. Good luck.