Forums :: Resources :: Features :: Photo Gallery :: Vintage Radio Shows :: Archives :: Books
Support This Site: Contributors :: Advertise


It is currently Dec Mon 18, 2017 6:09 am


All times are UTC [ DST ]





Post New Topic Post Reply  [ 15 posts ] 
Author Message
 Post subject: Zenith 5S127 - wrong speaker?
PostPosted: Sep Wed 26, 2012 6:14 pm 
Member
User avatar

Joined: Feb Fri 25, 2011 8:00 pm
Posts: 876
Location: Dublin, Ohio
I bought a 5S127 awhile back and just started to get into it. FInished the recap and was ready to plug in the speaker. Then I noticed it was a 1000 ohm FC, not 2125 as shown on the schematic. Sure enough, I have a 49-165AB speaker, not the 49-143 speaker shown on the schematic. I searched this forum (using Google and not the built in search method..long story there) and found 2 articles that seemed to indicate there were some sets that were indeed built with 1000 ohm FC. See there threads:
http://www.antiqueradios.com/forums/vie ... p?t=126592
http://www.antiqueradios.com/forums/vie ... hp?t=38232

I also noticed that there are some changes in the radio different from the schematic. For example, R10 in the schematic calls for a 190K resistor, but my radio has a 240K resistor. That seems to make some sense since that resistor sets the bias for the 6F6 tube. WIth a lower value voice coil of 1000 ohms, R10 needs to be higher to get close to the -5V needed for the 6F6 grid bias.

So, can someone let me know that I did indeed not get hosed by a seller and have a valid variant of this chassis?
Thanks,
Lou

_________________
Website - http://www.louhaskell.com/radio.php


Top
 Profile  
 
 Post subject: Re: Zenith 5S127 - wrong speaker?
PostPosted: Sep Wed 26, 2012 7:03 pm 
Moderator
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 17494
Location: Berkeley, CA 94709
Hi Lou --

I haven't heard of any variation like this with the 5S127, though I admit that I don't know much about that particular model. I've never heard of them using two different speakers with very different FC resistances in any other radios, but I guess it could be possible. I can check the books I have when I get home, but unfortunately I won't be able to do it until tomorrow because I'll be away all this evening.

Good luck with all of this.

Bob

_________________
"The stupider it looks, the more important it probably is!" -- J.R. "Bob" Dobbs


Top
 Profile  
 
 Post subject: Re: Zenith 5S127 - wrong speaker?
PostPosted: Sep Wed 26, 2012 7:10 pm 
Member
User avatar

Joined: Feb Fri 25, 2011 8:00 pm
Posts: 876
Location: Dublin, Ohio
Bob, thanks for the reply. Any insight will be appreciated. Hopefully, there are others out there who have this variant and got them working. I am concerned that, if a FC of 1000 can work, it needs enough current to make the speaker work properly. If there is no way this will work, I will have to try to find the proper speaker and that may be a real tough problem. I have no idea how to search to see what radios this 1000 ohm speaker works with. Is there a Zenith parts to radio cross reference somewhere? That would be real handy.

_________________
Website - http://www.louhaskell.com/radio.php


Top
 Profile  
 
 Post subject: Re: Zenith 5S127 - wrong speaker?
PostPosted: Sep Wed 26, 2012 9:51 pm 
Member
User avatar

Joined: Mar Tue 30, 2010 2:24 am
Posts: 7748
Location: Lehighton, PA.
You're not alone:

http://www.antiqueradios.com/forums/viewtopic.php?t=134654

http://www.nostalgiaair.org/Forums/Messages/126/M0052126.htm

It would appear that there are two variations, as far as speakers, go for this model. It stretches reason to believe numerous people around the country would all replace the 2125 ohm with the same Zenith 1000 ohm speaker. Too many appearences of both the 2150 and 1000 ohm variations for it to be anything other than a factory change (i.e. early vs late) IMO (but I could be wrong).

Larry

_________________
kb3wbb


Top
 Profile  
 
 Post subject: Re: Zenith 5S127 - wrong speaker?
PostPosted: Sep Thu 27, 2012 3:09 am 
Member
User avatar

Joined: Feb Fri 25, 2011 8:00 pm
Posts: 876
Location: Dublin, Ohio
Rocketeer, it would appear that you are correct in observing the number of people who have found 1000 ohm speakers in their 5S127 radios. It would be good to hear from others who have seen this and comment on any changed values of components to account for this. For example, I mentioned the different value of R10 which sets the bias voltage for the 6F6 output tube. Also, I must assume that the 1000 ohm coil gets enough current compared to the 2125 ohm speaker to generate a sufficient field to drive the speaker magnet. Any observations for those with a 1000 ohm speaker are welcome!
Lou

_________________
Website - http://www.louhaskell.com/radio.php


Top
 Profile  
 
 Post subject: Re: Zenith 5S127 - wrong speaker?
PostPosted: Sep Fri 28, 2012 6:35 pm 
Member
User avatar

Joined: Feb Fri 25, 2011 8:00 pm
Posts: 876
Location: Dublin, Ohio
OK, got the recap complete and fired it up with the 1000 ohm speaker. I adjusted the input to 112VAC as indicated on the schematic for voltages. The B+ ran hot at 280 rather than 240. I measured the 6F6 bias at -15V. The spec says -5 volts. A tad off I thought. But then measured the total drop across the voice coil at 50V. So 50V across 1000 ohms FC gives 50ma total b+ drain. That is quite typical I thought. But if the FC were 2125, the drop would be over 100v, b+ down to what it should be, but the grid bias would be -30V approx. That does not jive.
It would seem that a 1000 ohm field coil is what was intended for this radio. Also, the tube manual recommends a bias more like -15V, not the -5v.
So it would be interesting to hear if anyone can comment on the B+ and grid bias voltages for a radio with either a 1000 or 2125 ohm field coil.

_________________
Website - http://www.louhaskell.com/radio.php


Top
 Profile  
 
 Post subject: Re: Zenith 5S127 - wrong speaker?
PostPosted: Sep Fri 28, 2012 8:46 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 4504
Location: St. Louis, MO, USA
Hi Lou,
Here is a Zenith parts identification list, by radio model, that some kind soul posted a while back.
http://antiqueradios.com/gallery/d/1530 ... 1496689a7b
It shows the number 49-143 speaker as the correct one for this model.
There is no 49-165AB speaker listed for any model.
Must be a genuine "undocumented change."

You calculations and figurings all seem accurate to me but there may be one thing you may have overlooked.
The voltages were taken with a 1,000 ohms-per-volt meter. When measuring low-current high impedance circuits like grid voltages, it usually "loads down" the circuit and will give a much lower reading than what is actually present without the meter.

In this case, when measuring the 6F6 grid voltage, it would be like putting a 10k or 25k resistor in parallel with R10. Your modern meter will not do this, so you will get a much higher (and more acurate) reading.

I'm sure the actual intended voltage at the grid is 15v or more. Probably more. I've a hunch they intended the 6F6 to operate at a lower current than it is capable of handling. As you point out, 100v drop across the field coil nets a 30v grid voltage. Looking at the curves for the 6F6 that would seem to give a plate current of less than 10ma. But with a lower plate current, the total current through the field coil would be lower and there would not be 100v drop across it, and the grid voltage would be less than 30, which would increase the plate curent..... It probably "balances out" somewhere in the middle.

Your elevated B+ of 280v seems understandable since you now only have 50v drop across the field coil. If that causes you worry, you could always add a 1,000 ohm resistor in series with the field coil and revert to the original R8 and R10.

But I think you're going to be okay as long as you keep the grid voltage up so you don't tax the output transformer or the field coil's capabilities.

_________________
Terry Davidson


Top
 Profile  
 
 Post subject: Re: Zenith 5S127 - wrong speaker?
PostPosted: Sep Fri 28, 2012 9:54 pm 
Member
User avatar

Joined: Feb Fri 25, 2011 8:00 pm
Posts: 876
Location: Dublin, Ohio
Terry:
Ah, yes, I forgot the effect of the 1000ohms/volt meter! And yes, good analysis of the equilibrum of the grid voltage versus plate voltage. I do plan to put a 1000 ohm resistor in series with the field to see where the voltages settle out.
Thanks,
Lou

_________________
Website - http://www.louhaskell.com/radio.php


Top
 Profile  
 
 Post subject: Re: Zenith 5S127 - wrong speaker?
PostPosted: Sep Sat 29, 2012 2:00 am 
Moderator
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 17494
Location: Berkeley, CA 94709
Hi Lou --

I checked my Zenith references and they all show the 5S127 as having the 2125 ohm 49-143 speaker. But...the 5A127 export version uses the 1000 ohm 49-165 speaker. Is yours the export version with the huge power tranny with taps visible on the back of the chassis for different voltages?

Here is the schematic:

http://www.nostalgiaair.org/Resources/907/M0024907.htm

Bob

_________________
"The stupider it looks, the more important it probably is!" -- J.R. "Bob" Dobbs


Top
 Profile  
 
 Post subject: Re: Zenith 5S127 - wrong speaker?
PostPosted: Sep Sat 29, 2012 2:34 am 
Member
User avatar

Joined: Feb Fri 25, 2011 8:00 pm
Posts: 876
Location: Dublin, Ohio
Hey Bob, thanks for looking that up. My set does not have the huge 25 cycle power transformer with taps for the export version. I am guessing this , perhaps like others , is a Franken-Zenith or a result of some extended screwup at the factory. Interestingly, though, the R10 in my set is a 240K resistor like the schematic you sent rather than the 190K in the classic American version. There are other signs that mine was worked on before so maybe this is some blend of versions to get a working one. I plan to try the suggestions above from Terry and get the set to perform well and call it a day with the 1000 ohm speaker. I doubt I will find a 2125 ohm speaker that someone is willing to to give up.

_________________
Website - http://www.louhaskell.com/radio.php


Top
 Profile  
 
 Post subject: Re: Zenith 5S127 - wrong speaker?
PostPosted: Sep Sat 29, 2012 2:51 am 
Moderator
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 17494
Location: Berkeley, CA 94709
Hi Lou --

It also could be that Zenith put out a few of these when they were running low on parts for the American version. It wouldn't have been that hard to do. It would be interesting to know whether the other examples like yours were export versions. I would think that their owners would have mentioned it if they were, which makes me think that this was Zenith's handiwork. AFAIK the 49-165 speaker is rarer the the 49-143, so it would be hard use one to create a FrankenZenith. If I had to bet I'd say that this is how the radio came from Zenith. BTW does the speaker have a black crinkle finish? Many of the export model speakers were finished that way, and they look very nice.

Bob

_________________
"The stupider it looks, the more important it probably is!" -- J.R. "Bob" Dobbs


Top
 Profile  
 
 Post subject: Re: Zenith 5S127 - wrong speaker?
PostPosted: Sep Sat 29, 2012 1:17 pm 
Member
User avatar

Joined: Feb Fri 25, 2011 8:00 pm
Posts: 876
Location: Dublin, Ohio
OK, time for some experiments. I pulled out my trusty Ferret speaker box I bought here a few months ago and used that to try different field coil values. Very interesting results. I made a little table to show the options and resulting measurements:

Image

As you can see, the interaction of the current through the field coil and the current draw of the 6F6 tube as the grid bias changes is very evident. My speaker is low on winding resistance, so I also added a +1500 ohm option to kick up the FC resistance to 2383 ohms as another test. It would seem that the first option with no added resistor is not a good one in that the current draw is high and may cause the transformer to run hot. The last value results in a high grid bias and low overall current. But I still have to put a signal input onto the grid and make sure there is no clipping when driven. I will do that and report back.

A few other notes and items evidence. As I mentioned, the R10 is listed as 190K in the 5S127 chassis 5516 schematic. My radio has the 240K as shown in the export schematic that Bob provided. Also, my chassis had a .05mf for C15 just like the export version where the 5516 chassis calls for a 0.1mf. Very interesting... However, as Bob asked above the speaker is the Zenith gold and not black crinkle paint.

My view now is that at some point, the export radios were not selling well, so the production line put the simple American transformers in and shipped them as US versions. Because "a transformer is a transformer and the voltages are the same", there was no need to call the engineering staff. I would also believe that if you have one of these radios, your power transformer may be running hot. :shock:

I will add another post when I finalize my modified design and can check with an input signal to make sure there is no clipping, i.e. a grid bias is chosen well.

Lou

ADDENDUM:
I just also realized that the current through the speaker field coil will influence how well it operates. Enough current is needed to make the field coil a good enough magnet to operate the speaker. The magnetic field is linearly proportional to the current. Bottom line is I have more experiments to run for sure :)

_________________
Website - http://www.louhaskell.com/radio.php


Top
 Profile  
 
 Post subject: Re: Zenith 5S127 - wrong speaker?
PostPosted: Sep Sun 30, 2012 5:57 pm 
Member
User avatar

Joined: Feb Fri 25, 2011 8:00 pm
Posts: 876
Location: Dublin, Ohio
I did some final measurements with varying the grid bias on the 6F6 output tube. As expected, if the bias is raised beyond -16V to say -25, the current draw is lowered and the B+ voltage rises with less load on the power supply. The B+ actually went up quite a bit to maybe 320V, way too high. Plus the current through the speaker field coil was now lower. So, I decided to stay with the speaker alone and check the output waveforms and make sure the power transformer was not overheating.

The output at the plate of the 6F6 looked good when driven. The usual distortion of any class A amplifier so that was fine. Sound good too at various frequencies and volumes. So I decided to stay with the values as shipped, i.e. the speaker FC alone at 1000 ohms (890 for mine) and R10 at 240K. The B+ is 240V with input at the listed 112VAC for which I will need to build a bucking transformer.
After running for an hour, the power transformer was barely warm.
So I aligned everything according to the directions and this radio is performing very well now.

Thanks again to all who helped in this little experiment.
Lou

_________________
Website - http://www.louhaskell.com/radio.php


Top
 Profile  
 
 Post subject: Re: Zenith 5S127 - wrong speaker?
PostPosted: Dec Thu 07, 2017 9:50 am 
New Member

Joined: Dec Fri 01, 2017 12:30 am
Posts: 6
louhaskell wrote:
The output at the plate of the 6F6 lSo I decided to stay with the values as shipped, i.e. the speaker FC alone at 1000 ohms (890 for mine) and R10 at 240K. The B+ is 240V with input at the listed 112VAC for which I will need to build a bucking transformer.
After running for an hour, the power transformer was barely warm.
So I aligned everything according to the directions and this radio is performing very well now.

Thanks again to all who helped in this little experiment.
Lou


Well, it’s been 5+ years since the last post on this thread, but I’m really hoping one of you could help me with my Zenith 5S119, which, I believe uses the same chassis as the 5S127 discussed here. My radio also has the 1000 ohm field coil where the schematic indicates 2125. I’ve done a complete recap on mine and replaced all out-of-spec resistors. After alignment, output is very weak.

Is it possible that switching R10 to 240K from 190K would fix the issue?

Referencing the “socket voltages” chart on the schematic, I’m still getting some anomolies and I can’t seem to figure out why. (I am using a modern Fluke which could explain some of the discrepancies.) I’ve double and triple checked all the resistors and capacitors. These are my measured voltages (in parentheses) compared the chart (all filaments measure 6.4V):

6A8: Pin 3: 240 (290), Pin 4: 85 (67), Pin 5: -1 (5) Pin 6: 166 (226), Pin 8: 4 (5.1)
6K7: Pin 3: 240 (292), Pin 4: 85 (64), Pin 5: 3 (0), Pin 8: 3 (0)
6Q7: Pin 3: 75 (152), Pin 4: .1 (2.3), Pin 5: .1 (2.3), Pin 8: 1.5 (2.7)
6F6: Pin 3: 230 (273), Pin 4: 240 (293), Pin 5: -5 (-18)
5Y3: Pin 3: 240 (293), Pin 8: 240 (293)

Any assistance greatly appreciated!


Top
 Profile  
 
 Post subject: Re: Zenith 5S127 - wrong speaker?
PostPosted: Dec Fri 08, 2017 1:43 pm 
Member
User avatar

Joined: Jun Wed 14, 2006 11:49 pm
Posts: 6687
Location: Leo, IN or Zellwood, FL
louhaskell wrote:
OK, got the recap complete and fired it up with the 1000 ohm speaker. I adjusted the input to 112VAC as indicated on the schematic for voltages. The B+ ran hot at 280 rather than 240. I measured the 6F6 bias at -15V. The spec says -5 volts. A tad off I thought. But then measured the total drop across the voice coil at 50V. So 50V across 1000 ohms FC gives 50ma total b+ drain. That is quite typical I thought. But if the FC were 2125, the drop would be over 100v, b+ down to what it should be, but the grid bias would be -30V approx. That does not jive.
It would seem that a 1000 ohm field coil is what was intended for this radio. Also, the tube manual recommends a bias more like -15V, not the -5v.
So it would be interesting to hear if anyone can comment on the B+ and grid bias voltages for a radio with either a 1000 or 2125 ohm field coil.


Lou as a temp put a 1225 ohm resistor in series with your field coil and remeasure your voltages.
John k9uwa

_________________
http://www.JohnJeanAntiqueRadio.com


Top
 Profile  
 
Post New Topic Post Reply  [ 15 posts ]  Moderators: Marcc, Norm Leal

All times are UTC [ DST ]


Who is online

Users browsing this forum: Electronut51, jimjimmunster, jkoebel, mrx, rebrands and 18 guests



Search for:
Jump to:  


















Privacy Policy :: Powered by phpBB