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Need help with Zenith 12U158

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Post by ricky_a5 on Sun Jan 28, 2018 11:32 pm

I am working on a 1937 Zenith 12U158 console radio. It has been recapped and plays, but every time you turn it on you must tap the detector coil metal housing to get it to work...then it will work for hours. But next day when you turn it on, same thing...nothing until you hit the metal housing a couple of times. Can't seem to find any short or obvious issue. I have not tried replacing the 6L7 tube in that circuit (I have one on order). Hoping some one can point me in the right direction. I now there are a lot of you much more experienced at radio trouble shooting than I am.

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Post by jukeboxman on Mon Jan 29, 2018 1:09 am

welcome to TRF-- lots of talented people here --you should get some info soon

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Post by ricky_a5 on Mon Jan 29, 2018 7:30 am

Thanks. Looking forward to hearing from someone.

Rick

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Post by willy3486 on Mon Jan 29, 2018 10:17 am

I am wondering if it is a cold solder joint inside or outside the case, or a wire corroded inside the housing. When you tap it that makes it connect. It also could even be a wire touching the case. Probably what happens tapping gets it to connect then the heat keeps it connected. After it cools down the next day its not connecting again.

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Post by John Bartley on Mon Jan 29, 2018 10:27 am

I second what Willy said. That is the first, most obvious place to look. Intermittent failures or failures caused by changes in temperature are the ones that I dislike the most. Sometimes they are just a pain to find.

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Post by ricky_a5 on Mon Jan 29, 2018 1:30 pm

Thanks for the ideas. What you said about heat keeping it connected makes sense now that I think about it, since I can turn the radio off and as long as I turn it back on within a few minutes, it works fine...but if I wait several hours before turning it back on, I have to tap on the detector coil housing once again. I will remove the metal housing and check all of the connections. Will let you know what I find. Thanks again!

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Post by willy3486 on Mon Jan 29, 2018 5:38 pm

ricky_a5 wrote:Thanks for the ideas.  What you said about heat keeping it connected makes sense now that I think about it, since I can turn the radio off and as long as I turn it back on within a few minutes, it works fine...but if I wait several hours before turning it back on, I have to tap on the detector coil housing once again.  I will remove the metal housing and check all of the connections.  Will let you know what I find.  Thanks again!


One thing you may want to do first now that I think of it would be to heat up the solder joints underneath the housing going to the coil and then turn it on. if its a cold solder join on those connections maybe heating it and putting a little solder will cause it to get a good connection without taking the case off.

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Post by ricky_a5 on Mon Jan 29, 2018 8:48 pm

Of course one thing leads to another, so as I was checking everything out, I must have bumped the HF detector coils (there's two of them on a wooden dowel-like shaft...they were actually loose), so I spent time reconnecting the hair-thin wire and glued the coils in place on the shaft. Thought this might have been some of my issue...but no luck still have a short or a cold solder joint somewhere. I did take the cover off and it doesn't appear to be touching any of the taps on the coil. In fact I tried it with the cover off...same issue as before. I will try heating up some of the connections underneath, like you suggested...but that will have to be tomorrow. Thanks again for your help. I really appreciate the talent that you all have.

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Post by ricky_a5 on Tue Jan 30, 2018 8:43 pm

Checked every solder joint on the base of the detector coil and neighboring tube sockets. Could not find anything loose. No change. I think I will wait until replacement tubes arrive. Radio plays well on Broadcast band and picks up a few stations on the three shortwave bands, but you still have to tap on detector can shield to get it to work initially. Will let you know what I find once I replace tubes.

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Post by ricky_a5 on Wed Jan 31, 2018 10:20 pm

Well, I replaced the 6L7 tube in that circuit, but it did not help. Actually what is happening now is that I can tap on the 6L7 tube or the Detector shield can and get it to work. So the issue must be just under the chassis in that general area. I guess I will start heating up every solder joint going to the tube socket and the detector. I have checked continuity on most all wiring in this area. What I haven't tried is to prop the radio up on its back so I can get to the underneath side of the chassis and very carefully probe with an insulated tool to see if I can find the bad connection this way. Of course I will keep my other hand well away from any part of the radio. I had wanted to avoid this plan, since I have to protect antenna connection, etc. when propping it up like this with it turned on. Is there any harm in leaving it on in this manner...since I really don't know what is being powered up for sure and what isn't getting voltage. That's another reason I haven't tried this yet.

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Post by Dan Walker on Thu Feb 01, 2018 11:16 am

Many times I have had to turn a radio upside down in order to
probe various points.
I find that using a bamboo chopstick works quite well if you have to poke around under the chassis while the radio is on.
I am working on a radio now that is installed upside down in the cabinet, so that will not be an issue.
Just prop it up making sure that the tubes don't get bent over and keep them away from anything that should not get hot.
Dan in Calgary

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Post by willy3486 on Thu Feb 01, 2018 12:34 pm

I have propped them up in all positions and never had issues. One tool I made years ago that was a big help was easy to make. Get a rubber bouncing ball that are sold in quarter vending machines you see all over the place. The same type candy and gum are sold from. The balls are a little smaller than the size of a quarter. I got one and drilled a hole about half way through just shy of a wooden dowel. I used a dowel about the size of a pencil. Then put epoxy in the hole you drilled and insert the dowel. I usually had a dowel about 6 to 8 inches long. After it dries insert the other end into a pencil sharpener and sharpen that end. You have a little "bad connection hammer" you can use. I would tap the rubber ball end to find the rough location then use the pointed end to find the exact location. The wood and ball is not conductive so nothing gets shorted or shocked. I had one for years and need to make another.

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Post by ricky_a5 on Thu Feb 01, 2018 8:00 pm

Thanks for your advice. I turned it up on its side and used a wooden dowel and managed to find one questionable solder joint. I fixed it and when I tried the radio again, it still did the same thing...but I accidently brushed by the 6C5 oscillator tube and the set jumped to life. I replaced it with one I just received and Eureka...it is working now. Have had the radio off and on several times waiting several minutes in between and it has come on every time. Thanks to everyone for pointing me in the right direction. Hopefully I will be able to return the favor sometime. Good luck on your projects and may God Bless.

Rick
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Post by willy3486 on Fri Feb 02, 2018 12:49 am

Glad to hear that its playing right again. I have a old Philco 37-630 I wanted to restore for over 30 years.  The original Chassis was too far gone, the speaker was shot,etc. I found a 38-5 chassis that almost fits. I don't usually piece two different models but I had looked for 30 years and found this one instead.

Anyway I did the usual recap,align,etc. I had a weird issue of only getting sound from 1/2 way over to the far right and only one station.. I checked all over it, removed the can covers to check the IF transformers. Then one night I was looking it over and saw on the diagram where two lines crossed. It looked a little like it had a circle drawn there like a connection but the more I looked I realize it wasn't. It went to one of the Philco bakelite caps so I removed all its connections and rewired the cap. I left that connection off and when I turned it on it played perfect.

So no matter how long you work on these you sometimes find one that stumps you and its a simple issue. Great job finding it.

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Post by ricky_a5 on Fri Feb 02, 2018 1:54 pm

I have an old Philco...I forget the model number...its a "Baby" cathedral from about 1933. I need to work on it when I finish this Zenith. What are you working on now?

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Post by willy3486 on Fri Feb 02, 2018 4:50 pm

Right now the Philco I mentioned. I also have a RCA from around the same time that is a tabletop I am almost done with. I probably will get these two and not start on any more. I usually am too busy in warm weather to work on them much.

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Post by easyrider8 on Wed Feb 07, 2018 5:51 pm

Did you check the voltages on the 6C5, If they are off the tube may not oscillate until it get a "bump" a fresh tube may operate properly with incorrect voltages.

Dave
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Post by ricky_a5 on Thu Feb 15, 2018 8:50 pm

Will check voltages on the 6C5 tube. Thanks for the tip. It seems to be working properly, but I will check voltages just to be sure.

Rick

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