Zenith g724

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Zenith g724

Post by Rickey on Wed Oct 17, 2012 5:17 pm

I was beginning work on this radio and noticed it has a 12at6, in place of the 12au6, shown on the schematic. The only sounds I get from this radio ,is a slight buzz, if I touch center tab of volume pot with a screwdriver, and scratch on it! Should I recap it if I can't get a little more out of it? I tried to get it to oscillate using another radio, no luck there. I tried all the tubes in a working set, except the 35b5, and they all worked.
Thanks, Rickey
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Re: Zenith g724

Post by Dr. Radio on Wed Oct 17, 2012 7:05 pm

I believe you'll want to get the proper 12AU6 tube as they Zeniths used tubes that did double duty for both AM and FM functions. Have you checked the B+ voltage?
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Re: Zenith g724

Post by Wildcat445 on Wed Oct 17, 2012 7:14 pm

Clean the band switch, if this is an AM-FM set. I don't remember ever getting a Zenith AM-FM that didn't have a dirty band switch fouling up the works. Get the correct tube for your radio, and make sure the other ones are correct, as well.

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Re: Zenith g724

Post by mr_ed01 on Wed Oct 17, 2012 10:44 pm

Per the schematic, Bietmans 1950, page 189, the 12AT6 definitely does not belong. If you don't have a 12AU6, a 12BA6 can be substituted for test purposes.

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Re: Zenith g724

Post by Rickey on Thu Oct 18, 2012 7:28 am

Thanks guys, there was also a 10 mfd cap in the set also.
I guess it was put in years ago, maybe to just boost the 80-40 multi capacitor? The set looks pretty much untouched otherwise.
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Re: Zenith g724

Post by Bill Cahill on Thu Oct 18, 2012 10:12 am

If it is wired accross one of the two other sections, yes, it was put there as a boost.
I'd replace all of them. If you find it was to boost the one, you won't need it again after replacing originals.
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Re: Zenith g724

Post by Rickey on Thu Oct 18, 2012 3:23 pm

Should I try to get it working, to some degree, before I recap it?
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Re: Zenith g724

Post by Bill Cahill on Thu Oct 18, 2012 3:52 pm

No. I'd start restoring it, myself.
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Re: Zenith g724

Post by Guest on Thu Oct 18, 2012 6:00 pm

My uncle had one of those in white. My cousin and I spent a few snowy mornings listening for the school closings on it.

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Re: Zenith g724

Post by Dr. Radio on Thu Oct 18, 2012 8:21 pm

Well, my opinion is....I would try to get at least a peep out of it before "shot gunning" replacement parts. The issue with many Zeniths is the IF cans suffer from the "dreaded silver mica disease". It's nice to at least see if you can receive audio on at least one band to "assess the health of the unit". I'd hate to see this scenario happen to you;

You replace all the electrolytics, the paper caps, out of tolerance resistors. You have installed correct tubes and cleaned the band switch and volumen control only to be greeted by no audio, low white noise or intermittent static. You're now in for the long haul (rebuilding the IF transformers) or stuck with a paper weight full of new parts.

This scenario does happen. Don't want to scare you away, just my 2 cents.
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Re: Zenith g724

Post by Rickey on Thu Oct 18, 2012 8:55 pm

Thanks guys, yes I would like to get something out of it in the way of a signal. I'm still not sure it's oscillating. I put the correct tube in the radio, 12au6. I tried using another radio, tuned to a weak signal ,and when the dead set was tuned to the same spot on the dial, the test radios reception greatly improved! Any thoughts?
thanks
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Re: Zenith g724

Post by Dr. Radio on Fri Oct 19, 2012 9:01 pm

Rickey,

Have you taken and compared any voltage measurements yet? I would check the B+ first and then assuming it's not way low, start checking your oscillator stage voltages.
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Re: Zenith g724

Post by Rickey on Sat Oct 20, 2012 8:27 pm

Does anybody have a better schematic they could send me for this radio? I got the Nostalgia Air ,and Radio Museum one, and I can't read the voltages!
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Re: Zenith g724

Post by Rickey on Tue Oct 23, 2012 7:58 am

Ok, I got 125 volts dc out of the selenium rectifier. I also have 125 volts on pin 5 of the 35b5. I get a hum out of touching the center tab on volume control, not loud. I still don't think it is oscillating.
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Re: Zenith g724

Post by Bill Cahill on Tue Oct 23, 2012 7:05 pm

Check your secondary voltages on both I F transformeres. If you get any positive voltage, you've found your problem. Bad I F transforeer........
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Re: Zenith g724

Post by Wildcat445 on Tue Oct 23, 2012 7:15 pm

I put my money on Silver Mica Disease. Zenith (and Admiral) are notorious for this condition on radios made in the early '50's. Positive grid voltages confirm the condition for certain. I have probably 30 Admiral and Zenith radios from this era and EVERY single one of them either had SMD when I got them, or developed the condition soon after they played for several hours. Anymore, when I get one of these sets, I fix the SMD first. Saves grief later. I wonder if Zenith and Admiral purchased their IF transformers from the same company? I like Admiral and Zenith sets from this era, but their IF cans are a PITA. I personally have never experienced SMD in any other radio other than these. And I have over 200 AA5-style radios.

Regards.

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Re: Zenith g724

Post by Rickey on Tue Oct 23, 2012 8:23 pm

Thanks Bill, I found an open wirewound resistor, 1000 ohms,
It's between the two electrolytics. Would this cause my problem ? Where do you check the secondary voltages?
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Re: Zenith g724

Post by mr_ed01 on Tue Oct 23, 2012 8:27 pm

That would cause the problem..........

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Re: Zenith g724

Post by Bill Cahill on Sat Oct 27, 2012 1:38 am

I agree. Replace that, and, hopefully, it shoud play.
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Re: Zenith g724

Post by Rickey on Sat Oct 27, 2012 6:25 am

I got it playing. Sounds pretty good. I got another problem, the pilot light stays on ,with the switch off. I can also faintly hear the radio, with the switch off?
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Re: Zenith g724

Post by Bill Cahill on Sat Oct 27, 2012 1:21 pm

First, see if the switch is working. Unplugged, put an ohm meter on switch. On you should have pretty much a short. Turned off, it should read open. Infinate.
If that isn't the problem, something may be amiss with the wiring.
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Re: Zenith g724

Post by Rickey on Sat Oct 27, 2012 2:06 pm

Thanks, Bill. I notice there is only wire wire going to the switch part?
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Re: Zenith g724

Post by Dr. Radio on Sat Oct 27, 2012 9:03 pm

Rickey,

I believe this radio has one side of the switch going directly to the chassis. The B- is tied directly to one side of the AC line when the switch is flipped. Any solder blobs or splashes linking the chassis metal to the switched circuit side could cause the "always on" condition. Check this if the switch checks out okay.
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Re: Zenith g724

Post by Leigh on Sun Oct 28, 2012 2:55 am

Dr. Radio wrote:I would try to get at least a peep out of it before "shot gunning" replacement parts.
That's a very dangerous practice. If a part fails during your peep testing it can cause other damage in the set.

Old paper capacitors and electrolytics should be replaced before you apply power to the set. It's good to check resistor values also.

Some people call this "shotgunning" when in fact it's just a responsible approach to radio restoration.
Components deteriorate with age. It's one of those "laws of physics" things, not a matter of opinion.

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Re: Zenith g724

Post by Rickey on Sun Oct 28, 2012 3:42 pm

Here's a photo of the switch. Note the arrow.
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Re: Zenith g724

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