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 Post subject: Zenith 6-S-52 SPKR.Field 1000 Ohms
PostPosted: Aug Tue 07, 2018 3:58 pm 
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Joined: Aug Tue 07, 2018 3:38 pm
Posts: 5
Hello Folks! I'm new to the restoring hobby, but I've been an electronics tech for decades. I've been given the task of restoring a Zenith 6-S-52 console radio and making it play again. I have a schematic for chassis 5619, which matches what's in the cabinet. I've ordered new caps for it, and a new grill cloth and dial belt kit. And I've got someone who can do work on the cabinet. So far so good.

My problem is with the Speaker Field Coil. This looks to be the filter choke for the power supply. Its located between caps C16 and C17, and has C9 across it. The schematic indicates it has 1000 ohms value, I'm thinking this is ohms of inductive reactance. In my radio, this has burned open. The iron form is in two pieces, and the wire coils are loose in the immediate vicinity of the choke.

How are people replacing this part? If it were simply a damaged coil winding, I could rewind that. But since the form itself is burned in two I'm not sure how to proceed. Since the wire isn't broken into two lengths, I'm thinking the form got hot and melted. But I'm not familiar with older construction techniques of these things, so my analysis could be flawed.

I'm open to thoughts and suggestions. Thanks! Bill R - Hamatuer


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 Post subject: Re: Zenith 6-S-52 SPKR.Field 1000 Ohms
PostPosted: Aug Tue 07, 2018 10:44 pm 
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Joined: Nov Tue 14, 2017 5:09 am
Posts: 1773
Location: Austin, Texas
Bill,

It would help if you can post a picture of what you have. Note that you need to size the picture to no more than 600x600 pixels.

If the speaker is otherwise OK, you may be able to rewind the field coil.

You can also change to a new permanent magnet speaker. In that case, you will need to use a power resistor in the B+ filter and a much larger second filter cap.

Edit: I forgot to mention that nearly all of the measurements in the older schematics are DC resistance.
The cap across the field coil indicates that they were running it as a parallel resonant circuit at 120Hz. With the small electrolytics available at the time, they needed to get as much filtering as they could from the inductors.

Jay


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 Post subject: Re: Zenith 6-S-52 SPKR.Field 1000 Ohms
PostPosted: Aug Tue 07, 2018 11:49 pm 
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Joined: Jan Mon 16, 2012 4:15 pm
Posts: 5670
Location: Near Brandon, Iowa
Unlike most other radios, the loudspeakers used in late 1930s Zeniths were bolted together such that it is
fairly easy to remove the field coil winding. If this is the case with the set you are working on, the core can be rewound with little difficulty. The ohms given are resistance at room temp so you can calculate the number of feet of wire of a given gauge needed to achieve that value.

While it is certainly possible to replace the original ED speaker with a permag one, if the original speaker is in good shape otherwise (good cone) it would probably be the "path of least resistance" to just rewind the field coil.

Another option would be to snag a decent Zenith ED speaker on eBay. 1000 ohms is a typical field coil resistance for the lower-tube-count sets and these show up fairly regularly on eBay.


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 Post subject: Re: Zenith 6-S-52 SPKR.Field 1000 Ohms
PostPosted: Aug Wed 08, 2018 3:12 pm 
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Joined: Aug Tue 07, 2018 3:38 pm
Posts: 5
Thank you gentlemen. I have posted a picture of the part. It is mounted under the chassis, not on the speaker.

The speaker cone seems to be in good condition. There is free movement of the cone within the magnetic coil when slight pressure is applied. So I'm electing, at this point anyways, to stay with the original assembly.

Someone has been into this thing before. The candohm has been replaced with a series of series resistors. The original electrolytic filter caps have been replaced with axial caps mounted under the chassis. These appear to be 50's vintage. I've ordered new electrolytics and paper caps to replace them all.

The broken part might be a wirewound resistor? In that case 1000 ohms at say 5 watts or so? The wirewound would have inductive properties as well as resistance. To me the power supply looks like a capacitive input Pi filter. 120 hz would be proper since it comes off the common cathode of the dual rectifier 5Y3.

Hope this additional info and picture helps.

Thanks!

Bill R , Hamatuer


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 Post subject: Re: Zenith 6-S-52 SPKR.Field 1000 Ohms
PostPosted: Aug Wed 08, 2018 4:05 pm 
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Joined: Jan Mon 16, 2012 4:15 pm
Posts: 5670
Location: Near Brandon, Iowa
Your initial assessment of the failed part's function was incorrect: it is not the speaker field coil; nor is it a choke. The field coil/choke is of functional necessity mounted on the speaker itself and provides the static magnetic field essential for speaker operation.

The failed part appears to be a wirewound resistor.

The REAL speaker field is indeed electrically located between caps C16 and C17 as you initially stated; however the field coil is, as noted, on the speaker, not under the chassis.

I can only think of two reasons why someone would put a wirewound resistor across the speaker field end nodes: the first reason would be to bridge a failed speaker field. This would also necessitate replacing the factory electrodynamic speaker with a permanent magnet unit. Does your speaker have a permag field magnet or not?

The other reason for the presence of the resistor would be that the original ED speaker failed for whatever reason, and another nonidentical ED speaker with a higher-than-1000 ohms field coil resistance had to be substituted. A shunt resistor was needed for current-splitting purposes to avoid overheating the higher-impedance field coil.

Since the part burned up, you can bet that a significant abnormal load exists on the "downstream" end of it (this would be the end that connects to cap C17). Could be caused by a shorted C17 or shorted paper/foil B+ bypass cap such as C9 (the C9 hanging off the B+ bus in the middle of the schematic, not the C9 that shunts the speaker field coil... curse Zenith and their stupid use of duplicate component identifiers).


Last edited by lorenz200w on Aug Wed 08, 2018 9:35 pm, edited 1 time in total.

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 Post subject: Re: Zenith 6-S-52 SPKR.Field 1000 Ohms
PostPosted: Aug Wed 08, 2018 7:06 pm 
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Joined: Aug Tue 07, 2018 3:38 pm
Posts: 5
Thank you Lorenz. Right now your comments seem to fit my situation most closely.

I believe I have the original speaker system, non permanent magnet. The speaker has a part number of 49-118A0(?), 1000 ohms.
The large coil mounted directly to the speaker at the voice coil has the number 875 on it.
Mounted to the speaker frame is what looks like a small transformer, mounts with two screws by the frame that holds the laminates together. Has 4 wires coming from it.

The 875 coil has 4 wires: 2 wires that go to the plug that goes into the chassis socket. Black and what may have been Red. This pair goes to the center 2 pins of the plug. This pair measures open when measured by an ohm meter.

The 875 coil also has a second pair of wires. Short and black. One wire goes to a tie point that has only a wire to the speaker cone. The other wire goes to the other side of the speaker to a tie point. This point has a wire from the speaker cone and a wire from the *&% coil winding. Black.

The other smaller free standing transformer like device has 4 wires. 2 go to the outer pins on the plug that goes into the chassis. The other 2, one goes to the speaker cone tie point, the other to the 875 unit. There is continuity across the outer 2 pins. That winding is OK.

So it appears the radio lost a winding in the 875 winding. The decision here now is to abandon the idea of using the original speaker and associated components.

To convert this to a modern 4-8 ohm speaker, what is needed? I believe I need a output transformer, maybe a universal transformer? How much power does a 6F6 produce? Plate impedance is above 10K? Is there a posting I could look up and see what has been done before?

My parts have shipped today. So I will be re-capping soon!

Thanks for every one's help and patience with me. Its a learning process!

Bill R, Hamatuer


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 Post subject: Re: Zenith 6-S-52 SPKR.Field 1000 Ohms
PostPosted: Aug Wed 08, 2018 9:34 pm 
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Joined: Jan Mon 16, 2012 4:15 pm
Posts: 5670
Location: Near Brandon, Iowa
You can re-use the output transformer from your present speaker. Disconnect the voice coil wires (enamel-insulated wires coming from the transformer and going to the speaker. Ohm across the two wires; if they measure a couple of ohms or less (but not zero), the transformer secondary is OK. Then ohm across the two wires coming from the speaker plug to the output transformer; these connect to the transformer's primary winding, and should measure a few hundred ohms across them. Very low or very high readings indicate a defective transformer. If this transformer ohms out OK it can be removed from the old speaker and bolted to the new one. Zenith speakers of this era tended to have low impedance voice coils (i.e. on the order of a couple of ohms). A permag speaker with a VC impedance of 4 ohms should work OK. Audio power output of this radio is low, on the order of 3 watts or so, so the speaker needs to be a high-efficiency unit.

The field coil winding has an "extra" small winding grouped with it: this is the "hum bucking" winding and is connected in series with the voice coil. Neither the field coil or the humbucking windings are needed for a permag speaker conversion.

To eliminate the field coil, you will need to bridge the C16-C17 nodes with a 1000 ohm 10W wirewound power resistor. This will take the place of the burned-up resistor discussed earlier. The resistor will get very hot in normal operation so you may want to invest in a metal-cased heatsink variety that can be bolted to the chassis. You may also need to increase the value of C17 (not C16!) by a factor of two or three, possibly even more, in order to compensate for the loss of inductive reactance filtering action from the removed field coil.

Don't install this mod until after you discover and eliminate the cause of the B+ bus overload... otherwise
the next time you power up you may lose the rectifier tube and/or the power transformer.


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 Post subject: Re: Zenith 6-S-52 SPKR.Field 1000 Ohms
PostPosted: Aug Thu 09, 2018 11:08 pm 
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Joined: May Sat 30, 2015 11:49 pm
Posts: 804
Location: Keystone Heights, FL, USA 32656
I think you meant to say that the part, C9, between C16 and C17 has burned up, not the field coil? I am not sure what that part is, from your photo. It would not make sense to replace the field coil with a resistor and then leave the old speaker in place; wouldn’t work.

I see the speaker cable plugs in to the chassis (yes/no?). How many wires on that plug, I guess 4. If so, 2 wires will be the field coil and the other 2 will be the primary of the output transformer. Unplug that connector and check resistance there.

Jeff

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Jeff
“Nothin’s worth nothin ‘till somebody wants it.”—Irv Metter


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 Post subject: Re: Zenith 6-S-52 SPKR.Field 1000 Ohms
PostPosted: Aug Mon 13, 2018 4:21 pm 
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Joined: Aug Tue 07, 2018 3:38 pm
Posts: 5
Thank you Lorenz and Jeff. I agree, need to see if there are any hidden problems. The transformer needs protection. My parts have arrived. I will now begin the re-capping process. I have a variac that I will use to bring up the radio power supply slowly and I can measure current.

Thanks for the hint about the output transformer. I'll study that in detail. Also the resistor with a heatsink. I'm familiar with the type. Know what we need there.

As I get to know this old girl I suspect the reason it was put in storage was that something failed. I was told it was "working" by the person who donated it to us. I think there was a catastrophic failure in the past, and that's why the unit was placed in storage.

I'll get busy now and see where I wind up.

Thanks again all for the help and tips. I've got enough to be dangerous now! Update soon.

73, Bill


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 Post subject: Re: Zenith 6-S-52 SPKR.Field 1000 Ohms
PostPosted: Aug Mon 13, 2018 5:35 pm 
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Joined: May Sat 30, 2015 11:49 pm
Posts: 804
Location: Keystone Heights, FL, USA 32656
Some suggestions to do first:

1. What is that part that burnt up? We have a good guess but not conclusive answer. A zoomed out picture of the same area would help.

2. Where is it located, electrically, on the schematic? http://www.nostalgiaair.org/pagesbymodel/037/M0025037.pdf

3. After determining 1 and 2, can we figure out why it burned up? (maybe, maybe not)

4. Is the field coil good or bad? Unplug the speaker plug from the chassis and measure resistance between all pairs of wires. There should be one pair with about 1000 ohms and another pair with about 7-6k ohms, according to the schematic.

These are some things I would check before starting any replacement of capacitors, to get an idea of what failed, causing it to be put in storage.

Jeff

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Jeff
“Nothin’s worth nothin ‘till somebody wants it.”—Irv Metter


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 Post subject: Re: Zenith 6-S-52 SPKR.Field 1000 Ohms
PostPosted: Aug Mon 13, 2018 9:41 pm 
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Posts: 34393
Location: Maryland 20709, USA
As lorenz said, a pair of wires from a coil in front of the field coil normally connect to a hum-bucking coil. But the schematic does not show such a feature in this set. Perhaps the field coil was replaced at some point with a coil from a different radio. That might explain the paralleled resistor, to drop the over-all filter series resistance to the desired value.

Schematics normally show ohms readings for most inductors. These are DC resistance values, presented to aid troubleshooting. They generally do not relate to circuit performance.

- Leigh

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http://www.AtwaterKent.info
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 Post subject: Re: Zenith 6-S-52 SPKR.Field 1000 Ohms
PostPosted: Aug Tue 14, 2018 5:25 pm 
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Location: Leo, IN or Zellwood, FL
When you replace the original speaker with some PM speaker you seriously devalue the radio. Even if that field coil is open they are not that difficult to rewind the field coil and fix that speaker. From the field coil area you will find 4 wires. Two of them are rather stiff wires. They are the hum-buck coil. The other two are much more flexible and small stranded wire. Connect your ohm meter across those two wires. A 1000 ohm field coil is a hot reading. None of them ever read over about 900 ohms cold with an ohm meter. That said. IF you get infinity on your meter then yes the field is open. Before you get all excited you need to try one more thing. Do you have a working radio that has a DC power supply in it of around 300 volts or so? If so take about 80 to 100 mfd of electrolitic capacitors and parallel them. Now load them up with your working 300+ volt supply. Be careful this thing will bite you if your not careful. Now discharge that loaded up capacitor across the two small flexible field coil wires. If you see a spark then the connection through the field coil was made. If not try it again. Now read the ohms of that winding and see if it reads around 900 ohms. What happens many times in these Zenith speakers is the solder connection between the stranded flexible wires and the enameled wire the field coil is wound from become cold solder joints and don't make the connection. If you zap it a couple times many times it will reflow that solder and the speaker field coil will be just fine. I have several that has happened to and 20 years later they are still working perfectly.

Or if that doesn't work then rewind the field coil Search this section of ARF for speaker rewind several well written threads in this forum on rewinding field coils. The 900 ohm aim for this speaker is not critical when you do the rewind. No need for a coil winder... a nice variable speed drill will work just fine to spin the bobbin to put the new wire on the field core form. The only critical when you put the field coil back in place on the back of the speaker basket is getting equal spacing so that the voice coil doesn't rub the center pole.

Or watch Ebay and for about 50 bucks you can find one that already works. Zenith used 1000 ohm field coil in many of their speakers.

John k9uwa

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 Post subject: Re: Zenith 6-S-52 SPKR.Field 1000 Ohms
PostPosted: Aug Tue 14, 2018 7:23 pm 
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As John Goller said, it wouldn't be too hard to find a Zentih speaker to fit this model. The 10" ones aren't very popular, and usually don't cost much. There is one on eBay right now, though it has a 1280 ohm field coil. https://www.ebay.com/itm/1938-1939-Zeni ... 3460707176

Bob

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 Post subject: Re: Zenith 6-S-52 SPKR.Field 1000 Ohms
PostPosted: Sep Mon 24, 2018 8:08 pm 
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Joined: Aug Tue 07, 2018 3:38 pm
Posts: 5
Well!!

I've got the radio playing and making audio using the original parts. I'm thrilled and very appreciative of all that offered help and suggestions. I never thought I'd be restoring anything like this, but never say never!!

OK, some details. The damaged part I was concerned about. This turned out to be the 250 ohm resistor at the bottom of the candohm string. I didn't identify it properly and its labeling on the schematic was mis-leading me. The candohm had been changed out previously by someone else. There's a string of larger wirewound resistors in a row under the chassis. I traced out all the taps from the resistor string and figured out the broken part was the 250 ohm resistor. So I replaced it.

I did use a variac with all the tubes pulled to see the AC and measure it. All seemed normal, so I put the 5Y3 into its socket and brought it up again still on the variac. Nothing smoked and B+ voltages looked OK. Next all the tubes went in and I removed the variac. Plugged it in and all the tubes lit except the 6F6 audio output tube. The filaments were open. A friend supplied another 6F6 and the radio worked first time. Heard a couple of local radio stations.

I'm waiting for the restored console to be delivered the end of the week. Pictures then.

Thanks again for all the help. I'm glad to know of this site and to have met some of the friendly folks who hang out here.

Bill R


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