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Post by Wildcat445 on Sun Apr 21, 2013 9:48 pm

I am working on a Zenith console, model 6-P-457, chassis 5666. This is one of the infamous (famous?) Zeniths with the inverted bakelite chassis. This is the model with two bands and a 10"speaker. This matters, as the schematics are different. The schematic is at Riders 11-7.

Anyhoo.......I have replaced all the paper coupling capacitors, the power cord, the weird 2.9 volt dial lamp, a couple resistors that were out of range and am now ready to replace the filter capacitors. This thing has a four-section filter capacitor that I am going to have to re-stuff, as there appears to be nowhere else to stick the filter caps on this incredibly crowded chassis. There is a 20 mfd 350 volt cap, a 20 mfd 250 volt cap, a 20 mfd 200 volt cap and a 20 mfd 25 volt cap, the latter used as cathode bypass. I am going to use three 22mfd 450 volt caps and a 22fmfd 50 volt cap as replacements. I have a question. Actually two questions that hopefully some of you can answer for me. First, one of the 20mfd filter caps is wired, (if I am reading the schematic correctly) from a cathode on the 50Z7 recitifier to what appears to be the center tap of the rectifier? That is the first time I have seen such a circuit. Why is it wired like that? Is this one of Zenith's efforts to circumvent the RCA patents? Next question, this radio chassis has 6 tubes, one of which is a ballast tube. But if I add 50 plus 35 plus 12 plus12 plus 12, I get 121. Why the need for the ballast tube? This seems to have a fairly conventional series heater string. I am considering shunting the ballast tube and subbing the 35L6 audio output tube to a 50L6. Why would this not work? I am going to change the 50Z7 rectifier to a 50Y7, simply because I have one. Other than the rectifier being gone and the power cord being cut, I know nothing about the operating condition of this radio. I am flying in the blind here for sure.

I am going to wait for an answer on this before I continue with my repairs. I don't want to commit some type of faux pas that will cause the magic smoke to be released. This project has really been rather straightforward, and I have had little if any real problems. I have heard horror stories about this inverted bakelite chassis, so I want to err on the side of caution. All the coils check good to this point. That is fortunate, as they are oddly shaped and probably totally unobtanium. Who knows what will happen when I apply power, ever so slowly. Thanks for your help and attention.

Regards


WC





















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Post by Bill Cahill on Sun Apr 21, 2013 10:53 pm

That doesn't sound right. Got the schematic I can see?
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Post by Wildcat445 on Mon Apr 22, 2013 9:52 am

What is your concern, Bill? The way the filter caps are wired, or my proposed "in field modifications?" The schematic is in Riders, 11-7. Sorry I cannot post it here, since I do not know how. Perhaps that would be something I should learn.

Regards

WC

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Post by Bill Cahill on Mon Apr 22, 2013 9:59 am

I'm concerned how it's wired, and, I never heard of an electrolytic bing accross a rectifier tube. I think it's wrong.
It's easy to post photos.
What web host for photos do you have?
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Post by Wildcat445 on Mon Apr 22, 2013 10:18 am

I do not have a web host for photos. Never thought I needed one.

I have never seen an arrangement like that, either. I have traced the wiring and it is for sure wired like I explained. The rectifier, a 50Z7 is a voltage doubler. There are six wires coming out of the filter cap. One for the input filter cap, one for the output filter cap, one for the cathode bypass cap. These three caps have the negative going to common negative (B-) so that accounts for four wires. That weirdly wired cap, a 20 mfd, 200 volt electrolytic is wired with the positive end hooked to one cathode of the rectifier and the negative end to the center tap of the rectifier, where the dial lamp is connected. This radio has not been hacked, or even worked on, for that fact. All the tubes, including the ballast, are all Zenith branded with engraved base tubes. The rectifier was gone and the power cord was cut. I have had the thing for 30 years, and just decided to see if I could get it running as part of my scaling back my collection. It is probably worth more working than as a flower pot stand. Everything under (on top!) of the chassis appeared factory original. It has been sitting in my junk room for so long that the finish is falling off the cabinet. The grille cloth is the nicest part of the radio. I read a thread about inverted chassis Zeniths recently, and decided to get it out and tinker with it. The questions I asked are in response to some rather unusual features of this chassis and I appreciate your taking the time to help out.

Regards

WC

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Post by Bill Cahill on Mon Apr 22, 2013 3:51 pm

I reccommend Photo bucket.
Thanks. I try to help in any way I Can.
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Post by Wildcat445 on Mon Apr 22, 2013 7:01 pm

I am going to forge ahead with the repair of this radio. I will continue researching how and/or why this unusual circuit is wired like it is. I have figured out how the original wiring is configured. I will just fire it up with my dim bulb tester and variac and monitor everything carefully. It worked like this at one time, so it should work again.

Regards

WC

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Post by Dr. Radio on Mon Apr 22, 2013 9:16 pm

They are using the rectifier tube in a "voltage doubler" configuration. I'm having 'puter troubles so I can't post the picture of the schematic for you.

Here's an old discussion on your radio. Hope this helps.

http://antiqueradios.com/forums/viewtopic.php?t=165632
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Post by Dr. Radio on Mon Apr 22, 2013 9:24 pm

Finally Shocked Stupid computer "plug-ins". Mad

I have a question Zenith5666_zpsa9f2665e
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Post by Bill Cahill on Mon Apr 22, 2013 10:24 pm

Thanks for the great help, Dr. Radio.
Much appreciated, again.
Bill Cahill

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Post by Wildcat445 on Tue Apr 23, 2013 4:45 pm

Thanks Doc for the schematic. I am making progress with this thing. I am anxious to see how it turns out. It might work after all. Thank you all for your help with this

Regards

WC

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Post by Wildcat445 on Tue Apr 23, 2013 5:00 pm

Okay, now that we can see how the rectifier is wired, can anybody explain why it is like that? I would like to be able to better understand the circuit.

Regards

WC

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Post by Bill Cahill on Tue Apr 23, 2013 6:31 pm

Looks like a strange voltage doubler to me.
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