Zenith am/fm table radio

[jln1966]

I have a Zenith am/fm radio I got on ebay last summer. I am now getting around to working on it. I am not sure of the model # but the chassis is #7c06. I beleive it is a 1959 model. It does not power on at all. It does have a selenium rectifier that will need replaced plus a recap. Also there is a 2 stage .0047 ceramic cap connected across the power line that is bad. The area around it is blackened and a lead that connects to the chassis is burned off. What would cause that and whet do I replace the rectifier with?
Thanks,John

[Tom Bavis]

Schematic is online: http://techpreservation.dyndns.org/s..._7C06_174.djvu

You'll need the DjVu plug-in from lizardtech.com to view it.

[Chad Hauris]

A 1N4007 1000 volt 1 amp diode should work well...if there is not a surge resistor ahead of the rectifier you can add a 100 ohm 10 watt resistor between the power line and the anode of the diode to help absorb the turn-on surge and slightly drop the voltage to approximate the selenium.
While you are in there replace the electrolytics and any paper caps too.
As for the cap on the powerline I would just remove it...have done this with Zeniths and had no problem. Also a fuse in the power line would be good, 1 amp should do.

[jln1966]

Would the rectifier being bad not allow the radio to power on? What would cause the cap on the power line to burn out? The components around it have some soot from it on them but look okay otherwise. Where do get the diode,Radio Shack or order one?
Thanks,John

[Nolan Woodbury]

John,

No question the best parts source for your Zenith is AES: http://www.tubesandmore.com/

It is my understanding that the rectifier changes alternating current (ac) into positive (+ only) current, so that's a pretty important step in basic radio function, I would think.

The fried cap on the power line could be the result of line surge or peaking, so heed Chad's tip and splice in an inline fuse. Cheap insurance to prevent it from happening again.

Power transformers are pretty sturdy and basic, but leaky or old caps can do them in. I'd suggest running power through an isolation transformer after the recap and bring the voltage up slowly. Hopefully, it'll all work out and you can get on with the business of enjoying your Zenith as intended.

[Randy Bassham]

Generally the cap across the power line develops leakage or shorts and it can just explode causing the black soot deposits, if there's a capacitor across the power cord now that's not exploded it's probably a replacement. The rectifier would not keep the tubes from lighting up. Your set uses a series string of tubes ie the filaments are in series, if one of them burns out none of the tubes will light up. If you don't have a tube tester you can check the filaments with an ohm meter, on the 7 pin miniatures that are in you set the filaments are connected to pins 3 and 4. Looking at the tube from the bottom count clockwise starting from the pin on the left side of the missing pin and go clockwise to number the pins.

[jln1966]

I went through the tubes this morning and one does have an open filament. It is a 12ba6. I have a utility tester that checks filaments I found at the dump several years ago. It still works. The light goes out it the filament is good. Thanks Randy for pointing out the tubes. Now I should be able to recap it replace the rectifier and the 12ba6 and it should come back to life.
John

[jln1966]

I do have one more question. There is a black striped paper cap connected to the power side of the rectifier on one end and the chassis at the other end. When I replace the rectifier with a diode is that cap necessary or do I put an orange drop in its place? I know I will need to mount a terminal strip where the rectifier is to connect them up. It has 6 paper and 2 electrolitic caps to replace.
John

[Randy Bassham]

Yes it needs to be replaced along with the others. Those are the Sprague black beauties some call bumblebee's because of the color stripes. You don't have to check it it's leaky and will probably be shorted soon.