Zenith C725 help!

[TUD1]

Well I finally got the tombstone fixed, now another radio took its turn to break! I got this radio at an estate sale a few weeks ago, and I have become very fond of it. It has (had) a very clear, excellent sound. But tonight, I was listening to it, and all at once, it went completely silent. I clicked it off, and I heard a ten second long buzz that slowly faded away. The first thing I thought was that the selenium rectifier had shorted. Any ideas? I really like this radio - I just got finished giving it the Novus treatment and the cabinet is in near mint condition.

[init4fun]

Oh no , The selenium didn't short , if it did , you'd know it .

It's highly likely that a wax paper capacitor did short , taking out one of the small resistors after the selenium , and making it go silent . I have the 8 tube model of that chassis with the "propeller dial" and on mine a bunch of the caps were disc and checked out fine so I left em but anything wax paper was either open (almost all of em) , shorted (a couple) and I replaced the electrolytics and the selenium with a silicon diode just for good measure as well (I left the disconnected selenium in place for future generations to marvel at) . Are you recapping these radios or just plugging them in as found ? As the very last 1960s tube gear rolls on up to 50 years old there are no wax paper caps in anything I get that I don't replace .

[TUD1]

I have many old 1930's radios with the original caps that I DO intend to replace. I know the problems they can cause, but I just don't have enough time to do the work. I only get to work on radios for three hours every Saturday.

[dieseljeep]

Originally Posted by TUD1

Well I finally got the tombstone fixed, now another radio took its turn to break! I got this radio at an estate sale a few weeks ago, and I have become very fond of it. It has (had) a very clear, excellent sound. But tonight, I was listening to it, and all at once, it went completely silent. I clicked it off, and I heard a ten second long buzz that slowly faded away. The first thing I thought was that the selenium rectifier had shorted. Any ideas? I really like this radio - I just got finished giving it the Novus treatment and the cabinet is in near mint condition.

It sounds like the electrolytic cap still holds a charge, for it to take that long to fade. First thing, I would do is to check the tubes for an H/K short. The 19T8 likes to give trouble, as well as the 35C5.
It's amazing, how many of those Zenith sets are still around, both the 7 & 8 tube jobs.
Owners were reluctant to throw them out, as they were rather pricey, $60 to $120 dollars, depending on the model and year.

[TUD1]

That's what I've heard - that the 19T8 and 35C5 are trouble prone. As soon as I get home, I'll try that. I hope it's something simple like that. I will be recapping this radio just as soon as I get an opportunity.

[TUD1]

I took the back off this thing and put my cheater cord on, and I discovered that none of the tubes are lit.

[Electronic M]

A tube heater is probably open.

[dieseljeep]

Originally Posted by TUD1

I took the back off this thing and put my cheater cord on, and I discovered that none of the tubes are lit.

As you probably well know, the tube heaters are in series. The 35C5 is first in the string and the 19T8 is the last. I always checked those first.

[TUD1]

Well I subbed in a known good 19T8 and 35C5 and nothing happened. Two down, five to go.

[wa2ise]

You could use an AC voltmeter to hunt down where you lose voltage along the heater string. This means you'd have to work on it hot. Start at the 19T8 and keep going until you find a tube with 0V on one heater pin and 120V on the other pin. That's the tube with the bad heater. It could be a dirty socket, try wiggling the tubes.

[jr_tech]

Or simply pull the tubes and ohm out the heater pins... look up the base diagram on line if you don't have a tube manual. Most 7 pin tubes use pins 3&4 for heaters and many 9 pin tubes use pins 4&5, but there are exceptions. Be sure to clean/deox the pins and sockets if you haven't already.

jr

[TUD1]

The radio's fixed! One of the 12BA6's was shot.

[dieseljeep]

Originally Posted by TUD1

The radio's fixed! One of the 12BA6's was shot.

An open heater, mid string is the exception, rather than the rule.
Old habits die hard, in practice I always check the 35C5 and the 19T8 first. Ohm meter continuity method. If I don't find those open, then I check the rest of them. If, after getting it working and it doesn't seem to work quite right, then I'll go through all the tubes with the tube checker.

[old_coot88]

With AM-only radios, open heaters happen most frequently to the rectifier (35W4) or output (50C5, 35C5). 95% of the time it's one or the 'tother. Always check those first for open heater.