Zenith C730 making very loud buzz

[ldatlof]

Hi everyone. I am attemting to restore this nice Zenith radio. So far I have changed all the capaictors, and took out the selenium rectifier and replaced it with a Radio Shack 1N4005 diode (with 100 ohm 10 watt resistor). All the tubes are good. When I turn it on I get a really loud buzzing noise that gets louder.

I wonder if I used the right type of rectifier. The schematic has a weird symbol for the rectifier. Not like my other Zenith radio schematic. I have attached the schematic. Any thoughts on what the problem may be is greatly appreciated. Thanks.

[Nolan Woodbury]

Until someone better qualified forwards an answer, I'll give it a shot.

I checked my troubleshooting book for causes of loud buzzing and discovered a note that suggested many older tube-type radios and power amps use a "high voltage B+" power supply that consists of a power transformer, a rectifier and filter capacitors. I think this refers to the large, shielded "multi-section" capacitor (labled "C1" in your SAMS) thats rest on top of the chassis. Anyway, this converts the 110v from your outlet to the appropriate DC voltage for various circuits of the radio. Did you check / change this? Because of how it’s made and where it is, it is difficult to see if it’s leaking or leaving ‘white powder’. I know that it is nearly impossible to find NOS can-style stacks (you probably wouldn’t want them anyway) with most opting to fit a proper value electrolytic under the chassis. I did this (don't remember the value! Sorry) and left the can in place with leads clipped. If you want, I'll put it apart and tell you what it is. Let me know.

More good advice suggests mounting the new capacitor (the one I used was as big as a roll of dimes) as far away from the transformer as possible, as not to cook it. Zenith mounted it pretty close -the transformer is far right, the cap-stack almost dead center- but again, it is shielded for protection.

Hope this helps! I really love the C730; it’s my favorite Zenith radio.

[Nolan Woodbury]

Another option would be to re-install the original rectifier and see if that solves the problem? I didn't replace my rectifier, and my radio works great; plays every day.

Let us know what you come up with-

[holmesuser01]

Any chance the diode is installed backwards?

[Chad Hauris]

The schematic shows the multiple diode junctions of the selenium...a modern diode is just one junction with a single arrow. The "band" of the rectifier is where the positive DC comes out and this connects to the filter caps. The other end is the anode which connects to the AC input.
If all caps are new I would check and make sure they are connected right, especially the negative sides of the caps.
If the buzz is loud with the volume all the way down it is likely a power supply problem or less likely, heater to cathode leakage in a tube. If the buzz can be eliminated with turning down the volume the problem is elsewhere.
Caution as the power line connects directly to the chassis.

[ldatlof]

Originally Posted by holmesuser01

Any chance the diode is installed backwards?

That was one of my first thoughts. But according to Chad's post, it is in the correct way.

Originally Posted by Chad Hauris

The schematic shows the multiple diode junctions of the selenium...a modern diode is just one junction with a single arrow. The "band" of the rectifier is where the positive DC comes out and this connects to the filter caps. The other end is the anode which connects to the AC input.
If all caps are new I would check and make sure they are connected right, especially the negative sides of the caps.
If the buzz is loud with the volume all the way down it is likely a power supply problem or less likely, heater to cathode leakage in a tube. If the buzz can be eliminated with turning down the volume the problem is elsewhere.
Caution as the power line connects directly to the chassis.

The new electrolytic caps were oriented the right way they were just not properly grounded. Once I added a little solder to it (there was none before) 90% of the buzz was gone.

I think the next and hopefully last problem is that the G screw on the back cover is not connected to anything. If I put the phono/radio switch to the phono position the buzz is gone. Also there is zero FM reception. I am pretty sure I know where to make the connection from reading the schematic. I will let you know how it turns out. Thanks Nolan, holmesuser01 and Chad for your help. It's greatly appreciated.

[ldatlof]

The hum is mostly gone but there is no AM or FM singnal. I guess I am not sure where that G screw connects to.

[Nolan Woodbury]

Do you want me to pull one of my "C's" apart and takes pictures? I have a couple that I know have never been touched.

Let me know-

[Chad Hauris]

A quick test you can do for the AM converter stage at least is to put it near another AM radio...tune the good radio to a blank spot around the top part of the dial. Then turn the tuning dial on the Zenith on the AM setting..at some point you should hear a whistle on the test radio.
I would then check voltages but you will need an isolation transfomer to do it safely.
If there is no AM or FM it sounds like an audio stage problem.
If there is AM but not FM there is probably a bad convertor tube for FM or a bad IF can. I would check all the tubes if possible to rule out tube related problems.

[ldatlof]

Originally Posted by Nolan Woodbury

Do you want me to pull one of my "C's" apart and takes pictures? I have a couple that I know have never been touched.

Let me know-

Hi Nolan, that would be very helpful, thank you for the offer. I am looking forward to seeing the pics.

Originally Posted by Chad Hauris

A quick test you can do for the AM converter stage at least is to put it near another AM radio...tune the good radio to a blank spot around the top part of the dial. Then turn the tuning dial on the Zenith on the AM setting..at some point you should hear a whistle on the test radio.
I would then check voltages but you will need an isolation transfomer to do it safely.
If there is no AM or FM it sounds like an audio stage problem.
If there is AM but not FM there is probably a bad convertor tube for FM or a bad IF can. I would check all the tubes if possible to rule out tube related problems.

Hi Chad, I will do the AM radio test and see if I can get that whistling sound. I have an isolation transformer so I will do some voltage readings too. I will also check all the tubes. Prolly won't get to it until Wednesday when the kids are in school.

Thanks to you both. Loren

[Nolan Woodbury]

...but here is a partial view of the chassis, and the wiring in question:



...and a close up of how they attach to the terminal strip:



The green wire (looks blue in the photo, sorry) is for the ferrite antenna (which the power cord is also phased) the white the ground lead.

Here's a close up of the rectifier...not sure if it is original or not:



I wanted pull the chassis completely away from the radio and remove the bottom so you could see underneath, but this one has the speaker leads soldered and I didn't want to melt or cut them. If you need to see under there, let me know and I'll grab other radio.



Hope you get it working soon! The C730 is a wonderful radio; a real listening 'friend' if there ever was one.

[ldatlof]

Thanks Nolan, your pics are very helpful. Mine is hooked up wrong and the wires are not twisted together. I'll keep you guys posted on how its going. Loren

[Chad Hauris]

I have one of these that my friend found set near a dumpster! Replaced the selenium, all paper caps and electrolytics and it works great. I put 2 neon bulbs mounted on a terminal strip with series resistors behind the dial for a dial lamp. I would recommend removing that black cap across the rectifier...I had one of these type explode in a GE radio years ago so now I don't even try radios without replacing paper caps and electrolytics.

[Nolan Woodbury]

Old radios? Explosions? A chance for total and complete destruction? Cool! :-)

Seriously Chad, I only tried the radio once and it worked great, but I have better ones. I REALLY like the idea of illuminating the dial! Can I send you one and have you work your magic? I have a couple solid users that need the upgrade-