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 Post subject: Stewart Warner 950 Line Ballast Problem
PostPosted: Apr Tue 29, 2008 8:51 pm 
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Joined: Oct Mon 02, 2006 5:18 pm
Posts: 1838
Location: Alger, MI
Hi all,

I have a Stewart Warner 950 Series A.C. radio to restore. I have determined that the line ballast is open. It is a plug-in unit that looks like a resistor inside a perforated metal can. The schematic shows a p/n, but no resistance value. Do any of you have some info on this radio or a way to tell what the line ballast resistance is?

I jumpered out the ballast and powered up the radio through my Sencore PR-57. According to the voltage chart on the schematic, the voltage on the plate of the 45 tubes is 260 vdc and the filament on the 45 tubes is 2.3 vac at a line voltage of 115 vac. These conditions are met when the Sencore is set to 85 vac and 1.1 amps current draw. A quick application of ohms law yields a ballast resistor of 27 ohms at 32.5 watts is needed. Probably 27 ohms at 50 watts would make sense. For those of you with a good feel for these things, does this sound correct? Am I doing this right?

By the way, the radio plays beautifully with no hum. I need to check the paper filter caps yet to see how much leakage they have. Maybe they are OK? This is one of the radios I purchased at the MARC swap last Saturday. If you didn't see my post in the Clubhouse, here is a pic of the radio.

Stewart Warner 950 and 445-A Speaker
Image

Thanks for any insight you can provide.

Ed


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 Post subject:
PostPosted: Apr Tue 29, 2008 9:29 pm 
Silent Key

Joined: Jan Tue 16, 2007 11:48 pm
Posts: 9664
Location: Hueytown, AL
I get a bit different assuming a 120 volt line. Need 35 volts drop @ 1.1 amps = 32 ohms. Power = E sq. R = 35 sq. divided by 32 = 38.3 watts. It's my understanding that these old ballasts had compensating effect which a resistor will not provide. Don't really need it as long as line is stable. Line variations is why some mfg. used these things but 85 volts is what Majestic transformers were and I assume yours is same.


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 Post subject:
PostPosted: Apr Tue 29, 2008 9:55 pm 
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Joined: Oct Mon 02, 2006 5:18 pm
Posts: 1838
Location: Alger, MI
Jkaetzjr,

OK, Thank you. That confirms that my method is correct. :)

My line voltage is always 125 - 127 volts. So, I guess I could incorporate enough resistance in the ballast to drop the set input down to 85 volts or I could restore the ballast to original and then add an additional resistance or buck/boost transformer to drop from 127 volts down to 115 volts as I do on most of my sets.

Regards,

Ed


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 Post subject:
PostPosted: Apr Wed 30, 2008 1:38 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 2257
Location: Warkworth, Ontario, Canada.
Hi Ed,

I restored an identical speaker some time ago. It did not have any name or number on it. I take it that your 445-A speaker was also made by Stewart Warner.

Ed.


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 Post subject:
PostPosted: Apr Wed 30, 2008 2:46 am 
Member

Joined: Oct Mon 02, 2006 5:18 pm
Posts: 1838
Location: Alger, MI
Ed - Yes, the 445-A speaker has a Stewart Warner tag on the back of it. It says 'to be used with Stewart Warner A.C. sets'. It was definitely sold by Stewart Warner. I'm not sure if it was manufactured by Stewart Warner or if it was jobbed out to one of the more common speaker manufacturers.

Jkaetzjr - I took the ballast apart to attempt a repair. It uses nichrome wire wound on what looks to be a mica insulator frame. I found it was broken in two spots. The nichrome wire is very brittle and not repairable. I was thinking of winding 36 ohms of #32 enamel wire on the insulator. It would take about 222 feet. Or, I could buy some power resistors and stuff them inside the insulator frame.

Ed


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 Post subject:
PostPosted: Apr Wed 30, 2008 5:31 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 3445
Location: Nr London, England, SS1 3PT
Hi Ed

I recently restored a 950: see pics

Image

Image

Image

Shame your has had a toggle switch fitted! And you can see my finish and estcutheon is not quite original. Sometimes you just have to do the best at the time.

Mine is an export set and was in pretty poor shape so it was stripped and the chassis replated and then rebuilt. I can send you some info if you PM me an e-mail.

I'm suprised yours worked and it makes me think someone has worked on it before? I had the following O/C items: interstage Xformer, second choke, ballast resistor and speaker field coil. Plus all the caps (except micas) and most resistors needed sorting!

My ballast (for 240V supply) was OK once I took it apart and sorted the connections. If I was you I would make an ali heatsink and mount those metal clad resistors all inside the can.

As to the wax paper cap box I rebuilt that. You may be able to get the caps out without disturbing the chokes, they (the caps) are at the top if I remember correctly, unfortunately I didn't take a pic. If you take out the lot I bet the second 1K3 choke wont like it (it may then need a rewind!)

Gary


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 Post subject:
PostPosted: Apr Wed 30, 2008 8:16 pm 
Silent Key

Joined: Jan Tue 16, 2007 11:48 pm
Posts: 9664
Location: Hueytown, AL
I don't think I would use copper because it would take so much and such a "wad" probably wouldn't hold up long if it were to work. I have never repaired one of these but here's a possibility. AES sells nichrome wire in several sizes; #26 @ 2.57 ohms/ft; #28 @ 4.09 ohms/ft; #30 @ 6.5 ohms/ft. You could estimate what length would be needed to fill the old frame and choose a size that will give the closest to the proper resistance for that total length, that is, if you need 30 ohms and if 5 feet will fill the form then #30 would work. You could even do this to get close then "trim" out with an externally mounted supplementary resistor. But frankly, unless you are looking for "authenticity" I would be tempted to see if I could stuff regular resistors in the old enclosure. The goal is to get as close as possible the resistance needed spread over the longest length of wire so as not to overheat the wire.

If indeed these old units lent some measure of "regulation" in the form of changing resistance vs current, you won't get that action with regular resistors or with the nichrome as both have very low temp. coefficients, again no problem as modern line voltages are relatively consistent.

Hope this is some help. PS: Terminations will have to be made with mechanical fasteners, bolt & nuts, rivets or such. Can't solder, won't work on nichrome and solder will melt.


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 Post subject:
PostPosted: May Thu 01, 2008 4:14 am 
Member

Joined: Oct Mon 02, 2006 5:18 pm
Posts: 1838
Location: Alger, MI
Gary - That is an absolutely beautiful restoration on the Stewart Warner 950. My radio is in pretty good shape. It plays very well and has no hum! Yes, it is to bad that the switch was retrofitted in such a way. But, such is life, I can't complain since I got the radio and speaker for only $25. By the way, I haven't pulled it out of the cabinet yet, but I don't think it has been worked on much. It looks pretty much original. The guy I purchased the radio from said it had been on the shelf in the garage for 30 years. It was his father's radio, who has since passed on.

I also bought another radio from the same guy on the same day. This one was his father's radio also. Here is a pic.

Remler 'Best' 8 Tube Superhet - A beauty!
Image
Image

I haven't tried to fire up this one yet, since I need to build a power supply to run it. It should be a fun restoration though. I will drop you an email to get whatever info you can provide on the Stewart Warner 950. Thanks.

Ed


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 Post subject:
PostPosted: May Thu 01, 2008 4:32 am 
Member

Joined: Oct Mon 02, 2006 5:18 pm
Posts: 1838
Location: Alger, MI
Jkaetzjr - Thankyou very much for the info. I did not realize that I could purchase small quantities of nichrome wire from AES. I have several other items I need to get from them, so the wire will fill out my minimum order. As far as the ballast is concerned, I'm pretty sure it was nichrome wire on it. Each turn is 4.25 inches and the form will hold thirty turns. It was about half full (apx 15 turns) originally. I need about 36 ohms, so I think I will buy 20 feet of #28 and also #30. That will give me a maximum of 43 ohms for #28 and 69 ohms for #30 on my form. It should work out well. Thanks again for the suggestion. By the way, the original nichrome wire was terminated at each end with a brass screw, washer and nut of apx 4-40 size. I should be able to re-use the same terminations.

Also, I'm undecided on how much recapping to do on this set. Since I'm not planning to use this for an everyday player, I may just install a fuse and let it go at that. It is going to be mainly a display radio. I have only restored about 25% of the radios I have here. Some are in the cue for restoration and others will probably never get restored by me. Anyway, I would be interested to hear your opinion on this.

Regards,

Ed


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 Post subject:
PostPosted: May Thu 01, 2008 4:37 pm 
Silent Key

Joined: Jan Tue 16, 2007 11:48 pm
Posts: 9664
Location: Hueytown, AL
I would go with the #28 for the ballast, especially if it is close Size-wise to the original wire. The smaller you use. the hotter the wire will run for a given needed resistane and the less mechanical strength it will have.

Re recap, looks like most of the caps are contained in two "blocks" or cans, 2 in one, these being the input filter and the intermediate filter. The other has 6, including the output filter and 5 bypasses that operate with some appreciable voltage on them. It's impossible to predict these things. If they are easy to disconnect so as to check leakage, then I would do so and if any check a megohn or less, I would replace those. The 1.5, 2, and 5 mfd filters can cause damage if they leak badly or short, especially the first two. It will be difficult to come up with a proper fuse size that will protect the power transformer if this happens. Frankly, since you can get the 1.5 and the 2 (2.2) in 630 volt polyester caps, probably pay to replace them. Further downstream if one shorts it probably won't cause catastrophic damage immediately.


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 Post subject:
PostPosted: May Mon 05, 2008 1:03 am 
Member

Joined: Oct Mon 02, 2006 5:18 pm
Posts: 1838
Location: Alger, MI
Update. The nichrome wire came in the mail yesterday from AES. I bought a length of #28 and a length of #30. I measured a piece of the original and it mic'd out at .006 in, which I believe is pretty close to #34. Based on that, I decided to use the #30. I needed 36 ohms, so I calculated that it would take 66.5 inches or 15.6 turns of the #30. I measured off that length of the #30 before I cut it and it did in fact ohm out at 36 ohms. I wound it on the original form and installed it in the radio. It worked beautifully. It did radiate a fair amount of heat, but I would expect it to, since it is dissipating 43 watts. It did not get hot enough for the nichrome wire to glow red, so I think it is OK. I played the radio for two hours this afternoon, with no apparent problems. So far, so good. :D Thanks again for your help.

Ed


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 Post subject:
PostPosted: May Mon 05, 2008 1:23 am 
Silent Key

Joined: Jan Tue 16, 2007 11:48 pm
Posts: 9664
Location: Hueytown, AL
Sounds like a winner to me! As long as you got the required length on the form, bigger is better as far as the wire is concerned as it, as you observed, will run cooler. Total dissipation is same, just spread out over more wire.


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