Zenith C730

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Zenith C730

Post by mario tabernig on Sat Feb 04, 2017 4:05 am

After a recap and selenium rectifier replacement is working. However, on fm only works in a narrow part of the dial. I play with the oscillator coil in the tuner plungers but I only shift the working portion to other parts of the dial. I am at a lost here. Then  used other radio to tune a station properly on the dial but the high frequency stations of the fm band are not there and also the very low frequency stations of the fm band are not there eider.  I am only getting a partial FM band.
Any ideas?
Thanks

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Re: Zenith C730

Post by Bill Cahill on Sat Feb 04, 2017 3:36 pm

Unfortunately, you should not have moved that. Fortunately, you didn't go too far. Sounds like it could be leaky built in IF caps. Zenith started installing the caps in the base in 1949, and, yours may be a 1951 model, and, is definitely included in the troubled I F's. The best thing to do is to check with voltages. If you are getting a positive voltage on secondary, you have a problem.
Not sure what to tell you. Maybe someone knows a fast way to fix these.

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Re: Zenith C730

Post by mario tabernig on Sat Feb 04, 2017 4:44 pm

Thank you Bill. I herd of these problems but I did not know that this radios had the same problem. If I manage to open the can and disconnect the build in caps is there a way to introduce external caps and what value would they had to be?
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Mario

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Re: Zenith C730

Post by Dr. Radio on Sat Feb 04, 2017 5:05 pm

Don't tear into the IF (intermediate frequency) transformers. The issues you are describing is not consistent with IF transformer capacitor troubles. When silver migration occurs inside the base capacitors in the transformers you get intermittent reception or no reception with blooming thunderstorm like static. If the capacitor connections go open you get no or weak reception.

What you describe sounds like oscillator and alignment issues. You need FM alignment equipment and service data to get tracking correct again. You may also have a weak oscillator tube or out of tolerance components. Do you have equipment and service knowledge?

Not sure why this is in the phonograph section?

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Re: Zenith C730

Post by Bill Cahill on Sun Feb 05, 2017 6:19 pm

Yes, but, on advanced SM transformers, it goes to a low hum.
The Zenith ones have both capacitors squarely together, and, there's a way to take them apart. you'll have to thoroughly clean terminals to take it apart.

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Re: Zenith C730

Post by mario tabernig on Mon Feb 06, 2017 12:59 am

Well, I have some equipment but I am do not have a full understanding of FM all do I am familiar with AM and power amplifiers. there is the reason I am posting the question. I have a dual tracer scope 200MZ and an older Rf signal gen a  GW INSTEK GRG-450B. My service knowledge was acquired as a teenager and now I am 65 and newly retired. all do I worked wit electronics here an there over the years manly in logic circuits and amplification. I fully understand tubes and semiconductors.
Sorry if I got the question in the wrong place.

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Re: Zenith C730

Post by Bill Cahill on Mon Feb 06, 2017 1:17 am

Mr. Radio can help you on I F transformers. It's difficult, but, doable.
Maybe it would be a good time to bone up with some ez to rebuild five tube radios.

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Re: Zenith C730

Post by Wildcat445 on Mon Feb 13, 2017 2:08 am

I would also put my money on the local oscillator. If that set uses a 12DT8 for the FM oscillator, you might want to try subbing another tube known to be good and note what effect that has. I have had Zenith sets that were crabby about oscillator operation.

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