first radio - suddenly quit working

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first radio - suddenly quit working

Post by 3 chord on Mon Aug 01, 2011 11:49 pm

Hello, new guy here.

So I picked up a real nice, original (I think) bakelite radio today at a used furniture store. $24 which I think is a good deal, no cracks, back panel is there, all seems ok for the price. It is a Coranado 6B11 tabletop, around a 1947 I think. Real good shape, nice marbled brown bakelight with matching knobs. Looked and sounded great in the store. Big sound.

So I get it home and it is dead, tubes and dial light don't light up. No sound, nothing. Not sure what happened but maybe I shouldn't have been playing it in the store (new guy mistake!). Did the car ride shake something loose or is it just what happens to old radios that have been sitting?

I pulled the chassy and the caps look old, paper, waxy looking things. Black waxy stuff in lots of places.

So what do I do next, test tubes, replace caps, both. Is there an output transformer on these things?

Any help would be great.

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Re: first radio - suddenly quit working

Post by Bill Cahill on Tue Aug 02, 2011 1:24 am

First, the radio should be re capped, and, have electrolytics replaced. If any of these parts suddenly shorts, it will dammage, er, burn out tubes.
Then, test tubes.
Bill Cahill
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Re: first radio - suddenly quit working

Post by willy3486 on Tue Aug 02, 2011 9:58 am

I do what Bill said. Replace the wax caps then test tubes. In about 95% of the time that solves the issue I have found out. Since it was working it sounds like there shouldn't be much wrong with it.

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Re: first radio - suddenly quit working

Post by 3 chord on Tue Aug 02, 2011 10:05 am

Thanks guys. I will start seeing what caps I need. Now when I think of caps I am thinking about the paper cylinderical ones, are there others too? I see Bill mentioned electrolytics. Not sure the difference.

Sorry for the dumb questions. Also should I replace the two prong power cord while I am at it?

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Re: first radio - suddenly quit working

Post by Bill Cahill on Tue Aug 02, 2011 11:14 am

Power cord replacement would depend on whether it is rotted, or, not. Condition is important to safety.
Electrolytics are the capacitors in the power supply that filter the dc voltages. They are crittical in the way you install them. The positive ends, must be connected where they belong. If you reverse these caps you will do great harm to the electrolytics, the radio, and, perhaps yourself.
I do mean the paper caps primarily on the capacitors to replace otherwise, as well.
Also, make sure through resistance checks that none of your resistors have gone substantially off value, usually no more than 20 percent off tolerance.
Caution, care, replace one part at a time, and, dressing are somewhat crittical. Also, if you have rubber wirring, and, it's rotted, that will need replacing, as well.
Bill Cahill

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Re: first radio - suddenly quit working

Post by 3 chord on Tue Aug 02, 2011 11:34 am

ok thanks, I think I can get started.

btw, although I know very little about tube radios (well i know very little period!), I have been using tube guitar amps for a long time so I will exercize the same caution when I am digging around in my radio.

I think this will be a fun project. There is just someting about glowing glass!

thanks again

Jarrett

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Re: first radio - suddenly quit working

Post by Bill Cahill on Tue Aug 02, 2011 11:44 am

Have you any electonics experience at all? If not, you need to learn things first. I wouldn't dive into it until I get some book, and, practical experience learning, plus, some basic needed safety, and, test equipment.
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Re: first radio - suddenly quit working

Post by 3 chord on Tue Aug 02, 2011 1:10 pm

Here are some pics of my radio. Looks like someone did a little mod. A pot of some sort was added to the power chord and to a pin on one of the larger tubes (power tube?). The non-original circuit also has a modern orange capacitor. Any idea what they were trying to achieve?

And I appologize if my photos are not great or the incorrect size but hopfully they demonstrate 1) the modification and 2) the radio I have.

thanks, Jarrett

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Re: first radio - suddenly quit working

Post by Bill Cahill on Tue Aug 02, 2011 6:02 pm

Without a schematic, it's hard to see what was done. The orange capacitor is fine. It's a replacement line capacitor. Other caps need replacing, including the big electrolytic cap with colored wires. The black wire is the negative. That is the cap most crittical to correct connections.
The red is one of the positive connections.
I see part of the symptoms. There is a burned out power resistor, and, looks like there was a small fire. A cap that looks like a mica cap has been singed, may be dammaged.
This radio definately needs proper rebuilding.
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Re: first radio - suddenly quit working

Post by 3 chord on Tue Aug 02, 2011 11:36 pm

Any idea what the previous modification was. The radio has a potentiometer mounted on the back panel (it can be seen laying one the bench in the second picture). It was spliced into the main power cord and the other lead goes to thru the replaced orange cap and then terminates at a tube pin.

I will probably remove this mess once I get a schematic but I do wonder what purpose (or detriment) this served.

And good luck today (apart from the helpful advice here on the forum), I found a local guy with a good supply of caps, resistors and other things that I will need.

As far as a fire goes, I think if there was it was a while ago, nothing smells burt inside the cabinet, just the old tube amp musty smell I would expect. Since the radio worked yesterday (I know, I should not have tried it Embarassed ), if the fire was recent I would think I would have smelled something (as I had the chassis out yesterday too).

And thanks for your patience and help, it is most appreciated.

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Re: first radio - suddenly quit working

Post by Bill Cahill on Wed Aug 03, 2011 12:49 am

What is the tube number that control is connected to, and, which pin of the tube? Sounds not right.
There is a burnt looking resistor, and, that may be what failed.
Something did fail for you, and, we need to find out what, and, why.....
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Re: first radio - suddenly quit working

Post by Resistance is Futile on Wed Aug 03, 2011 5:24 pm

Looks like the volume control was bypassed with another external unit. since only the power switch is wired up on the external switch, it says to me that is what was overloaded. You definitely need to do some parts replacing.

If the blackened parts are close to the original volume control, Do not power up the radio. Do a wire check to make sure you don't have further shorts. You may have a wiring error.

I also see a bare end on one of the power leads. This may have not been soldered as it look exposed. Its the copper colored one. they may have connected it to an existing wire, or part and just did a finger wrap rather than solder. Check it out. If thats the case it may have come free and therefore no power. Looks like a da-fingerpoken mechanics vorken. Crying or Very sad
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Re: first radio - suddenly quit working

Post by 3 chord on Wed Aug 03, 2011 7:03 pm

I think when I change the caps and go thru this radio, I will just get rid of the modification circuit all together, all it consists of is one cap (the orange one) some wire that is spliced into the main power cord and the potentiometer.

When I did power the radio up, the main (original) on/off/volume pot worked so I am confused why someone would do this. But then again, anything could happed in +60years or so.

Too many folks think they can do better than just fixing what is wrong, sad, I see the same thing with botched mods on guitars and guitar amps Mad .

Oh, and to answer Bill's question, the power to the non-original circuit (pot) is to pin #2 on the 35Z5GT tube, the same pin the original power chord is wired to.

The more I read, the more I am starting to know what to look for (thanks to you guys around here!), the more I think I should have passed on this radio but I guess even if I over paid at $24, it will still be a good project. The case is in good shape and polishes up nice. I will stick with it and see how I do, hopefully for another $30 or so it will be up and running (I hope). Still cheap entertainment in today's world!

Any good leads on used 12SA7's, SK7's or SQ7's? Maybe the 35Z5 and the 35L6 too?? I will see what I need down the road.

Oh and more good news, found the schematic on-line yesterday, Free!! Looks like it is around a 1941 radio.

Jarrett


Last edited by 3 chord on Thu Aug 04, 2011 1:43 am; edited 1 time in total (Reason for editing : spelling)

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Re: first radio - suddenly quit working

Post by sprman on Sat Aug 06, 2011 1:39 pm

That looks like you got a mess on your hands.Take your time and be careful .Should not be to hard to fix it..Sprman : )

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Re: first radio - suddenly quit working

Post by Bill Cahill on Sat Aug 06, 2011 1:56 pm

3 chord wrote:I think when I change the caps and go thru this radio, I will just get rid of the modification circuit all together, all it consists of is one cap (the orange one) some wire that is spliced into the main power cord and the potentiometer.

When I did power the radio up, the main (original) on/off/volume pot worked so I am confused why someone would do this. But then again, anything could happed in +60years or so.

Too many folks think they can do better than just fixing what is wrong, sad, I see the same thing with botched mods on guitars and guitar amps Mad .

Oh, and to answer Bill's question, the power to the non-original circuit (pot) is to pin #2 on the 35Z5GT tube, the same pin the original power chord is wired to.

The more I read, the more I am starting to know what to look for (thanks to you guys around here!), the more I think I should have passed on this radio but I guess even if I over paid at $24, it will still be a good project. The case is in good shape and polishes up nice. I will stick with it and see how I do, hopefully for another $30 or so it will be up and running (I hope). Still cheap entertainment in today's world!

Any good leads on used 12SA7's, SK7's or SQ7's? Maybe the 35Z5 and the 35L6 too?? I will see what I need down the road.

Oh and more good news, found the schematic on-line yesterday, Free!! Looks like it is around a 1941 radio.

Jarrett

Sounds like it's wired wrong.That's the hot side of the line. End of control should be going to B-.
Bill Cahill

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Re: first radio - suddenly quit working

Post by 3 chord on Mon Aug 08, 2011 5:15 pm

Well now I feel silly. I had another look at the radio last night (I was having fun in Denver over the weekend so this was my first chance to get at it). Recall that I mentioned that there was an after market pot installed to the on/off circuit.

Well it is a push pull pot. I plugged in the radio, click the on/off switch on and pulled the pot switch to the out postion and guess what, the radio powered right up!

So this morning I took it apart, cleaned the radio case with dishsoap and water, blew out the chassis with compressd air (lots of dust bunnies flying out everwhere), ensured there were no faulty connections, inspected for evidence of a fire (there was none that I could tell), put it back together and it plays, works and sounds great! Very Happy

I kinda thought it would be something simple as the radio worked the day I brought it home.

I guess I will go ahead and change out the caps later on but it doesn't hum and plays strong so I will put this off until winter sets in here in Montana.

My next project, have fun with an external antenna.

Thanks for all the input trying to help me, but I guess there is no helping dummies like me. Embarassed

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