Up next: Firestone 4-a-21x

[Schanz]

Picked this up at an antique mall. Talked then down considerably but still probably paid too much for it. I really wanted it though. Should fix up really nice. Doesn't appear to have been messed with.


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[dieseljeep]

Originally Posted by Schanz

Picked this up at an antique mall. Talked then down considerably but still probably paid too much for it. I really wanted it though. Should fix up really nice. Doesn't appear to have been messed with.


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Should perform great, when restored.
Built by Stewart-Warner, seven tube, power transformer chassis.
Found in Rider's #15.

[stusnyder]

I got one just like it, but it says airline on it....got it from Reese last weekend. Thanks Reese

[Reece]

Nope, Stu. It's a Firestone AIR CHIEF. These hunky sets are I believe made by Stewart Warner, as were many Firestones.

[Reece]

Yep, made by Stewart Warner: here's the S/W equivalent to the Firestone:

http://www.radioatticarchives.com/radio.htm?radio=3125

[stusnyder]

My bad....it does say firestone on it...

[Schanz]

This radio is coming along but I may have an issue. The transformer atop the field coil is open and it looks like it was a replacement already. The field coil is showing 1000 ohms which matches the schematic. I assume open is bad? Any hints on how I replace that?
Other than that, the rectifier was already toast and another one is on the way. Going to replicate the can capacitors and that should be all the repairs needed except for the transformer.

[dieseljeep]

Originally Posted by Schanz

This radio is coming along but I may have an issue. The transformer atop the field coil is open and it looks like it was a replacement already. The field coil is showing 1000 ohms which matches the schematic. I assume open is bad? Any hints on how I replace that?
Other than that, the rectifier was already toast and another one is on the way. Going to replicate the can capacitors and that should be all the repairs needed except for the transformer.

The transformer, you're referring to, is the audio output transformer. A lot of firms mounted them on the speaker, instead of in the chassis.
You have to find one the match the audio output tube.

[Reece]

If you have any spare output transformers, you could test them with the procedure outlined on the link below. It's a bit involved but no worse than doing your income tax. You have one known value already: the output tube is a 6K6 and that requires 8000 ohms primary load impedance, per the chart towards the end of the attached. We don't know the speaker voice coil impedance but it's probably between 4 and 8 ohms. An approximation would have you measure the VC resistance (disconnect one wire from the transformer so the latter's resistance doesn't enter into the reading) and then multiply by 1.25 for the impedance. Recall that resistance and impedance are both measured in ohms but represent different characteristics: impedance is "sort of" AC resistance.

Once you get through all that, and make the tests on any output transformers you have, you might have one that works. Or at least you know what turns ratio you need to order. One other note: read the note about audio feedback on the schematic: it tells how to hook up the transformer to avoid feedback.

www.radioremembered.org/outimp.htm

[Schanz]

Thank you. Let's see if I'm even close.
I disconnected the output transformer (sorry dieseljeep, my terminology ain't so good).
I measured 4 ohm across the speaker leads. multiplied by 1.25, that's 5 ohms. Is that a plausible value? Given the 8,000 ohms for the 6K6, I get a winding of 1:40 (8000/5 = 1600. sqrt of 1600 = 40). Given a voltage of the 6K6 of 230 and a winding of 40, I use e/r = I. 5.75 / 5 = 1.15 amp.

[Reece]

Looks good to me! Did you have any to test? If not, someone here may have one suitable. You can also check Playthings of the Past; I saw a couple on there that would work cheap but they have $20 minimum, I believe.

http://www.oldradioparts.com/pg2a23d.htm

[Schanz]

OK, now I may have a dilemma. I have found additional pages to the schematic. This says that the impedance of the voice coil is 3.5 ohms. It makes a little difference but maybe not enough to worry about.

[Reece]

This speaker also has a hum-bucking coil in series with the voice coil, so that may have been added into the mix? Don't think it's going to hurt in this instance. I'd go with the 5 ohms since that's what the secondary of the output transformer is going to see, per your measurement.

[Schanz]

edited: power transformer appears OK, at least the radio works with a new output transformer rigged up. I haven't had time to check the voltages yet.

[Schanz]

Thought I'd post a pic of the final product. Sounds really good. Absolutely nothing comes in on short wave with the internal loop antennae but I haven't had time to worry about that.

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