Zenith g724

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Re: Zenith g724

Post by Dr. Radio on Sun Oct 28, 2012 10:45 pm

Hi Rickey,

It's kinda hard to tell, but does that volume control/switch have (3) places on the back side to connect power wires? If so, you may just have to move your connections to the other leg. Some switches have a point just to tie wires for convenience and aren't apart of the two sides of the actual switch. Wiring errors can happen sometimes with this arrangement.

I didn't have an exact schematic for your radio. I looked at a similar model G725, and if that is follows suit, your switch input should be one side of the ac power line, and the other side (the output of the switch) goes to the negative side of your electrolytic capacitors and the radio's B- buss. Please verify that according to your schematic.

Were you able to determine if the switch contacts are actually breaking power when you turn the radio off?

Also, I see you have an original wax capacitor still in place. Now that the radio is operating, for continued reliability and safety I would replace that wax cap.

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Re: Zenith g724

Post by Rickey on Sun Nov 04, 2012 7:05 am

Ok, I took the switch apart, and repaired it. I replaced the selenium rectifier, with a diode, and a 15 watt ,50 ohm resistor. The voltages are very close! I don't see any symptoms of silver mica disease. I learned a lot about troubleshooting, by getting it playing, without changing out,everything in it. I have since changed all the capacitors, and a few resistors.
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Re: Zenith g724

Post by Rickey on Sun Nov 04, 2012 7:11 am

Rickey wrote:Here's a photo of the switch. Note the arrow.
Some "tech", in the past ,had put the wire from the mains from the top of the switch,to the bottom. This left the radio on all the time, virtually. The "tech" did this to compensate for a bad switch. Learned a lot, doing this set. These are good sounding sets, and very good reception on FM, which is a little unusual, for this era.
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Re: Zenith g724

Post by Dr. Radio on Thu Nov 08, 2012 10:53 am

Rickey,

I'm glad you got it working. Hats off to you to go the extra mile and do "surgery" on the switch. The fruits of your labor paid off. Actually Zeniths of the late 1940s thru the late 1960s dominated in the area of excellent FM reception. Many have some not so pleasing designs (boxy, large cabinets), but the quality and reception is fantastic. cheers
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Re: Zenith g724

Post by Rickey on Thu Nov 08, 2012 3:57 pm

Dr. Radio wrote:Rickey,

I'm glad you got it working. Hats off to you to go the extra mile and do "surgery" on the switch. The fruits of your labor paid off. Actually Zeniths of the late 1940s thru the late 1960s dominated in the area of excellent FM reception. Many have some not so pleasing designs (boxy, large cabinets), but the quality and reception is fantastic. cheers
Thanks, Dr. Radio, I've learned a lot with this radio!
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Re: Zenith g724

Post by Rickey on Tue Nov 13, 2012 9:14 am

Just checking voltages ,and the voltages on pins 5 and 6 of the 12au6 read 45 volts ,not the required sixty. The voltage is close on the side of the resistor coming from the 1000 ohm resistor, but only the 45 volts at the 12au6. Interesting, my DVM reads the 60 volts for just a moment, and then drops to 45. The radio plays loud and clear on FM, and receives many stations. The radio plays loud and clear on AM, but only receives our local station. Earlier in my rebuild it was doing better on AM.
Thanks. The SAMs schematic shows 60 volts, the NA one 54.
http://www.nostalgiaair.org/PagesByModel/536/M0025536.pdf
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Re: Zenith g724

Post by Bill Cahill on Tue Nov 13, 2012 1:05 pm

Uh, oh... You got an easier radio to work on?
I know the problem, and, you ain't gonna like it..........
This radio is among the first of radios put out by them that has built in capacitors inside the bottom of the I F transformers. Apparently the silver mica caps in the bottom of the AM transformers have gotten leaky..
They are built into the the base that holds the terminals.
The only way they can be fixed is to remove the transformer, carefully take the base apart, cleanning the corosion off, and, re assembling it, then, putting it back together in the chassis.
It's called silver mica migration.
Sorry to give you the bad news.
Bill Cahill
They can be fixed, but, it's a royal pain..........

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Re: Zenith g724

Post by Rickey on Tue Nov 13, 2012 1:58 pm

Sort of what I figured too, Bill ! Is the voltage falling suddenly a sign?
Thanks!
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Re: Zenith g724

Post by Bill Cahill on Tue Nov 13, 2012 3:50 pm

It can be becuse positive voltage is leaking to other areas that shouldn't have it.
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