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 Post subject: Stewart-Warner R3271 rebuild/restoration questions
PostPosted: Feb Sat 11, 2017 5:27 pm 

Joined: Jul Wed 22, 2009 3:07 pm
Posts: 723
Hello Everyone,

I was asked by one of our friends to rebuild/restore the above tube based car radio.
The radio is for a '40 Packard, the schematics can be found here:

The car was converted from a 6V positive ground system to a 12V negative ground system.

The radio looks like a nice high-end radio, it is unmolested, but needs a rebuild.
I enclose a couple of pictures.
I told the owner that I will not make an authentic restoration, but will save and enclose all the parts, components removed/replaced, in case someone later on wants to do a "proper" restoration. He is fine with that.

Now the questions. It is a bit long, please bear with me.

Conversion to 12V.
Looks like the vibrator based B+ PS had its time, the sealed transformer spilled its tar. My guess would be that the vibrator was shorted, one of its contact welded shut, and that overheated the transformer. So far I have not disassembled/removed/measured anything, did mostly just visual inspection.
As the vibrator and its transformer is not meant for 12V operation, I am thinking of making/using a modern DC-DC converter for the B+ supply. I estimate the radio's B+ requirement as 250VDC, 70mA - 100mA.
Is my estimate correct?
(First when restored, I plan to run the radio on a bench PS, I will measure those values.)
I was looking at low-noise resonant Royer converter circuits (Linear Technology has a number of good application notes about those), but I also found the following ready made converter:
I can not beat the price, build quality, delivery time of that. It is a zero voltage switching (ZVS) converter.
Anyone familiar with such type converters? How noisy are they? I would house it in a metal box, next to the head unit.
Any comment on the idea of using a modern DC-DC converter?
The tube filaments can be rearranged and strung up for 12V, there are an even number of 6.3V filament tubes with identical filament currents.
Any advice on the stringing, which one would be closest to ground, which one closest to +12V?

I did check the tubes on my Eico-667 tube tester. The tubes are fine, except the 6R7 detector/AF amp tube has a 1 MOhm h-k leak.
My intention is to bring up the radio and then see how does it work with that particular tube.
Any trouble to expect due to the leakage?

I will replace most of the components, all the capacitors, except the ceramic tube capacitors. Any comment on those molded square mica capacitors? Do they need replacement?
Any guess about the capacitors in the IF-cans? Are those mica caps and are those subject to deterioration?
How about the resistors? The resistors look like those skinny ceramic tube based components, with the legs covered in green corrosion. I will test those, but what is the experience with such resistors?

Any other comments/suggestions, do-s and do not-s?

It looks like it will be a slow restoration. I asked the owner to order the caps, he can get those delivered to him cheaper, and it seems he is not in a hurry.

Thanks, Peter

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 Post subject: Re: Stewart-Warner R3271 rebuild/restoration questions
PostPosted: Feb Sun 12, 2017 4:03 am 
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Joined: Dec Sat 24, 2011 9:17 pm
Posts: 2868
Location: Vancouver Island, Canada
I can't answer your PS question, except that I would check the existing transformer for continuity of the windings. Melted wax or tar might not mean it is toast. If good, a 12 volt neg ground digital vibrator would go well with it. Be sure to replace the capacitor across the output of the PS with a cap rated for 1600 volts.

Re: the tubes, I don't know if this has been tried by anyone here, and I've not tried it but... could you hook up the heaters of the existing 6 volt tubes in pairs? You say they all draw the same current.

Car radios tend to be more robust, so I would expect the IF cans to be OK. Hope for the best, and good luck.

Watch the doughnut, not the hole.
Burl Ives, RIP, oldtimer.

 Post subject: Re: Stewart-Warner R3271 rebuild/restoration questions
PostPosted: Feb Sun 12, 2017 6:17 am 

Joined: Jan Thu 01, 1970 1:00 am
Posts: 36954
Location: Livermore, CA
You could wire filaments in pairs but most of those tubes have 12 volt equivalents. For 6R7 need to use 12Q7.

The transformer would need to be replaced when using 12 volts even if it's good.

This type of vibrator is different than most. It can be powered directly by applying voltage to the two large pins unless the vibrator is a replacement? Most vibrators have a set of points which short across the coil dropping the magnetic field. This one has points that open to drop power to the coil unless it's been replaced.


 Post subject: Re: Stewart-Warner R3271 rebuild/restoration questions
PostPosted: Feb Tue 14, 2017 3:03 pm 

Joined: Jul Wed 22, 2009 3:07 pm
Posts: 723
Thanks for the replies!

Yes, I am planning to wire up the heaters in pairs!
I also did order that converter, it is very affordable for what it is. I will see how it works out when I get there, and might revisit the idea of building a quieter power supply if it turns out to be too noisy.

I will keep you guys posted!

Thanks again, Peter

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