Zenith 6P-457 no B+

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Zenith 6P-457 no B+

Post by Wildcat445 on Sun May 17, 2015 11:33 pm

This chassis is one of those bakelite, hung upside down chassis that Zenith produced in the early 1940's.  It has been laying on the end of my bench for several years now, and I am getting sick of looking at it.  I have recapped it and checked the preliminary components like field coil, output transformer, oscillator coil, things like that.  It has taken me several years to replace the four electrolytic caps in a can.  This is the most frustrating item I have ever worked on in my life.  The chassis is crammed and cramped and it is hard for me to run wires.  My eyesight is not what it used to be.  I finally got the last cap changed this afternoon and was ready for the first test drive.  Using my variac and dim bulb tester, I ramped the voltage up slowly to about 80 volts.  The tubes glowed brightly.  The pilot light is a 2.5 volt model that I do not have a spare of and it is burned out.  The original rectifier is supposed to be a 50Z7, which I understand, per my RCA tube manual is an obsolete type, to be replaced by a 50Y7, which I did.  The rectifier in the set when I got it was a 25Z6.  I have zero B+.  Nada.  Nothing at the cathode of the rectifier.  I have line voltage at the plate of the rectifier, not one volt on the cathode.  I have used two rectifiers, they test good, and get the same results with both.  How can that possibly be on an AC-DC set?  I have checked resistance from the cathode of the rectifier to B- (pin 7 of the ballast resistor, per the schematic) and can see the filters charging, so that seems okay.  I don't see anything open.  If I had the caps connected wrong or not at all, that may create a short or over-current situation, but it should not kill B+.  This chassis uses a voltage doubler setup on the rectifier.  With the exception of tube filaments, this set is dead as a doornail.  I'm determined to not let this thing beat me.  This project has gone on long enough and I need to get this thing off my bench.  I will be happy to scan and post the schematic if that would help.

WC

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Re: Zenith 6P-457 no B+

Post by CrazyCanuck on Mon May 18, 2015 7:33 am

There are two possible chassis model/radio model combinations for that set according to NostalgiaAir?  Did someone previously put the wrong rectifier tube in the set or rewire something according to the wrong schematic, and that is leading you to look in the wrong places for the problem?
just a thought ....

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Re: Zenith 6P-457 no B+

Post by Wildcat445 on Mon May 18, 2015 9:31 am

I thought of the chassis models after I posted this. It does appear that someone has been "working" on this thing previously, but what wiring I can decipher looks correct. There was a 25Z6 rectifier there when I got this set. According to the RCA tube book the 50Y7 has the same electrical characteristics as a 25Z6. Thank you, it looks like I need to take a closer look at that rectifier socket.

WC

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Re: Zenith 6P-457 no B+

Post by Bill Cahill on Mon May 18, 2015 10:46 am

I need to see the schematic. Could be you need to use the 25Z6.

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Re: Zenith 6P-457 no B+

Post by Wildcat445 on Tue May 19, 2015 1:39 am

Bill's comment got me to thinking.  I did not spend a great amount of time, and the wiring in this chassis is a huge mess, but it might be possible that someone rewired the rectifier socket.  Maybe to use a different rectifier than a 50Z7.  I removed the 25Z6 without really considering that someone had been there before me and changed the wiring.  There will be a 25 volt difference in heater voltage that will need to be considered.  Perhaps replace the 35L6 output tube with a 50L6.  I have a magnifying lamp I can use to take a closer look at the rectifier socket.  I may have messed up royally by tossing the 25Z6.  The chassis in this is the 5666, I believe.  There are two or three different versions of the bakelite chassis, as if one was not bad enough.  One version used a 35Z5 rectifier and the other two used 50Z7.  This whole thing is a strange setup.  It might actually work if I get this B+ situation straightened out.

I would be happy to post the schematic if I could figure out how to do it. I'm not as smart as I thought I was in that regard apparently. Rolling Eyes

WC

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Re: Zenith 6P-457 no B+

Post by Dr. Radio on Tue May 19, 2015 9:17 pm

I believe this chassis uses the crazy hare-brained scheme of using the rectifier in a voltage-doubler configuration to raise the B+ to "hot rod" the output from a rather mundane chassis. I'll have to check...
Smile
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Re: Zenith 6P-457 no B+

Post by Wildcat445 on Wed May 20, 2015 10:02 am

Doc, my man, how's things, dude? Very Happy

Long time, no see. It is good to hear from you again. Very Happy

Hair-brained is mild to what I have called this contraption. I believe that Bill may have nailed this one. I have worked on this thing in spurts and have forgotten or ignored the small details. Absolutely not the correct procedure to troubleshoot a problem child. I need to get a few minutes to get back to it and see if somebody has rewired the rectifier for some reason. I appreciate all the help I can get on this thing. Embarassed

WC

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Re: Zenith 6P-457 no B+

Post by Bill Cahill on Wed May 20, 2015 11:43 am

Hey. A joke. What do you get when you cut a wire too short? Give up??
A. You get a short circuit!!!!!!!!!!!!!!! Laughing Laughing Laughing

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Re: Zenith 6P-457 no B+

Post by CrazyCanuck on Wed May 20, 2015 12:45 pm

Wildcat445 wrote:

Hair-brained is mild to what I have called this contraption.  I believe that Bill may have nailed this one.  .......... see if somebody has rewired the rectifier

WC

WC,

From what I see in the schematics that are available, the only way a 25--- rectifier tube would be used in there would be if someone did rewire it. If so, it certainly can be drawn "as-is", then a decision can be made to leave it as modified once repaired. However, if "restoration" is the goal, then reverting to the OEM wiring shouldn't be too difficult either.

CC

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Re: Zenith 6P-457 no B+

Post by Wildcat445 on Wed May 20, 2015 4:06 pm

My goal at this point is to just get it working. If I use a 25 volt rectifier, I will need to drop 25 volts somehow on the heater string. A 50L6 output tube would get me 15. Perhaps I could rewire it so a 45Z5 rectifier would work, whch would make the heater string voltage drop closer to correct.

WC

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Re: Zenith 6P-457 no B+

Post by CrazyCanuck on Wed May 20, 2015 5:26 pm

Wildcat445 wrote:My goal at this point is to just get it working.  If I use a 25 volt rectifier, I will need to drop 25 volts somehow on the heater string.  A 50L6 output tube would get me 15.  Perhaps I could rewire it so a 45Z5 rectifier would work, whch would make the heater string voltage drop closer to correct.

WC

Ok, having identified a goal for completion is good! Can you list the tubes that are in the set now (excluding the rectifier) ??

CC

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Re: Zenith 6P-457 no B+

Post by Wildcat445 on Wed May 20, 2015 7:17 pm



The Rider's schematic for this set is 11-7. This has the model 5666 chassis. Chassis 5659 and 5663 use a 35Z5 rectifier. None use a 25Z6. This set uses a 12A8 converter, 12K7 I.F. amplifier, 12Q7 detector, avc and first audio and a 35L6G audio output tube, and a 100-79 ballast tube. Plus the mystery rectifier. The models with the 35Z5 rectifier are table models with a 5" speaker. Mine is a console with a 10" speaker. My theory at this point is that the console version uses a voltage doubler rectifier to give more volume for the 10" speaker. It appears that someone rewired this chassis to possibly eliminate the oddball 50Z7 rectifier. All the tubes in this radio are engraved base Zenith, except, of course, for the rectifier. I tossed the rectifier that was in it when I got the set. I figured somebody had been "working" on this set, but never gave the possiblility that rectifier socket had been rewired a thought. My plan now is to compare tube basing on both the 25Z6 and a 35Z5 and see if and how they differ. Then I need to attempt to trace the wiring on the rectifier socket and see if I can tell what was done. It's not going to work with the correct rectifier. Thanks for your interst and help, CC. Very Happy

WC

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Re: Zenith 6P-457 no B+

Post by Bill Cahill on Wed May 20, 2015 8:03 pm

Not the same. 25Z6 is a dual section tube. 35Z5 is a single section tube. If that's what you put in, no wonder the radio won't work.

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Re: Zenith 6P-457 no B+

Post by Wildcat445 on Wed May 20, 2015 8:26 pm

Thanks, Bill. That was the next thing I needed to know. My guess is that if it would not work with a 50Y7 which is dual section, why would it work with a 25Z6 which is also dual section? Tracing the wiring is my next move, and should answer a couple key questions.

WC

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Re: Zenith 6P-457 no B+

Post by Dr. Radio on Wed May 20, 2015 11:08 pm

Yes, sorry I haven't been around much, lot of personal "stuff" has gotten in the way of any old radio "fun".

Hey, lookie what I found Smile

www.tuberadioforum.com/t2164-i-have-a-question

The link to the ARF discussion you started seems like it got a good response...were you going to try that route with the tube/dial light/resistor sub?
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Re: Zenith 6P-457 no B+

Post by Wildcat445 on Thu May 21, 2015 12:30 am

Since then, I have found that there has been someone "working" on this thing. All that would be appropriate if the thing was original. (I looked for that thread and couldn't find it). Now I am back to square one as far as the rectifier goes.

I have been trying to work on this in fits and starts. Not the right thing to do. I forget what I have done and chase my tail. I need to get some bench time on this in the next day or so. Maybe I can come up with something that will let me move forward.

It's good to hear from you again, Doc, and I hope all is well with you . And that you will not be a stranger. Very Happy

WC

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Re: Zenith 6P-457 no B+

Post by Wildcat445 on Thu May 21, 2015 3:28 pm

I have B+ now! cheers

Bill gets a feather in his cap for this one. He got me to looking down a path that did not include chasing my tail. I really should have known better. Embarassed

The 35Z5 rectifier was the correct one. I used my magnifying bench lamp and got a good look at the rat's nest under this chassis. It was obvious that the rectifier socket had been rewired. Quite a nice job from all appearances. The only give away was pin 6 which is unused in this setup and was a tie point in the factory setup. I have a little more hum than I think is normal, but it's gonna stay like it is. I have a few nasty wires on the antenna that I need to take care of. I'm going to make the antenna wires longer, since the chassis and antenna are installed as an assembly and is clumsy to handle. I almost have signal output from the speaker. I don't have any antenna connected, but I get oscillator squeal in another radio. Part of the hum may be due to no antenna. I'm not going to make any promises, but this thing may actually work when I accomplish a few small cleanup tasks. Very Happy

I'm not going to put this POS up for sale, though. I wouldn't wish one of these things on my worst enemy.

WC

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Re: Zenith 6P-457 no B+

Post by Wildcat445 on Thu May 21, 2015 5:25 pm

Slight progress, but progress none the less to report. This thing still kinda, sorta works. I found a cold solder joint on a capacitor on the 35L6 and I had the 12K7 where the 12A8 belonged. Wonder how that happened? Embarassed

Audio seems sorta okay. The problem is in the middle of the radio. Methinks that the IF is badly misaligned. Remembering that somebody has been there before me, it could be ANYTHING causing the problem. The next stab I take will involve a signal generator. B+ is sorta okay, certainly within 10% of normal. I believe that replacing the 35Z5 with a 45Z5 may be a benefit longterm. I may replace the 35L6 with a 50L6 as well. I need to account for a 15 volt drop. The 6.3 volt #47 dial lamp that I used seems to work just fine. Ridding the set of that 2.5 volt bulb was a blessing. I'm going to recheck my rechecking and make sure I have everything right up to this point. I have made more progress in the last two days than I have in several years, so I'm grateful for that.

WC

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