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Zenith 7A04

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Post by bob1954 on Mon Dec 20, 2010 9:42 pm

Radio was working great so I was going to use my rf generator and the scope to check out the alignment. I had the set pluged in and droped the lead from the signal generator on the back of the radio and poped a breaker. (I think it shorted across the ant connection)Reset the breaker turned the radio on and buzzing noise, but if I turn the volume all the way up it clears up and you can hear the station. I repalced the electrolytics, the cap at the selenium rectiver, no help. Voltages seem to be about 10 volts low all through the set. All the tubes test good, no obvious burnt marks or exploded componets. I tried a different volume pot with no luck. Getting about 120/dc from the rectifier. Did I blow the chassis mounted transformer for the speaker?
Seems at times I break more than I fix.

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Post by Bill Cahill on Tue Dec 21, 2010 2:21 am

Your problem is you tried connecting the test equipment to a hot chassis radio.
You need isolation transformers on the radio, and, other sets to safely perform this.
You really don't need a scope in most cases.
Sounds as if you may have dammaged the antenna coil, and, or, volume control.
You put a strong ac charge through the radio.

That's why the pop, and, tripped circuit breaker.
Bill Cahill

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Post by bob1954 on Tue Dec 21, 2010 4:20 am

I know about the isolation transformers I have two of them, All I can say is DUH Embarassed I know it's not the volume control i subed it with a new on and it made no change. I know I don;t need a scope, was trying to lean something new. Just not what I learned!

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Post by GaryRabbitt on Tue Jan 25, 2011 3:33 am

Best place to learn a scope is on a fully working radio. You can see what the waveforms look like when all is well. Then when you are repairing a non working set, you will know what to look for.

With the voltage put through your set, I'd start Ohming your coils and see if you have an open somewhere. It may take time, but you may get lucky and find one(or more) right away. Use the ISO at all times Smile
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Post by bob1954 on Tue Jan 25, 2011 9:44 am

Thanks Gary, when I start ohming the coils is there any standard reading I shouold be looking for or is any reading like 3 ohms or above ok? I took them all out of circuit to test them and some where 3 ohms and others were higher. I treid my ring tester on them and that didn't work.

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