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 Post subject: Detrola TRF '32? 4-D?
PostPosted: Apr Thu 09, 2020 12:41 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 469
Location: Terre Haute, IN USA
I took this 1932 tombstone in for repair. The cabinetry is in fine condition. However, it looking at the wiring versus the TWO schematics on Riders 5-1, I am seeing some major differences. I realize that in the 88 years since this was made, it HAD to have been worked on, perhaps several times. The sockets are original, and have the tube number embossed into the wafer. This one uses 2 #224, 1 #247 and 1 #280 as its tube compliment. One of the 224s looks original, but I can't be sure. All 4 test good on my Hickok. Here are my concerns: #1 the TRF '32 schematic shows a #35 as the RF amp--not a real big deal. #2 the power supply shows the speaker field coil in the power supply return to ground. In my case it is in series with the positive lead from the rectifier. #3 The volume control/ RF gain pot on BOTH schematics on that page---the TRF '32 AND the 4-D show it shunting the antenna lead. In the one I have it is a control for the DC voltage on the RF amp's screen. Does anyone have any OTHER source for a printed schematic or schematics for this model? From what I have read, this was the FIRST radio Detrola made. Any thoughts are welcome. Thanks.

Jerry


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 Post subject: Re: Detrola TRF '32? 4-D?
PostPosted: Apr Thu 09, 2020 4:50 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 26115
Location: Detroit, MI USA
There is no relationship between the 1932 4-tube TRF and the 4-D, as the latter is a midget set with a chassis half the size of the other one. Both schematics ended up being on the same page in some publications. You are correct, the '32 4 tube TRF was their first household radio.

I've never found schematics for all the versions of the '32 4-tube TRF, but there are definitely a few variations on them. You might be better served by drawing the schematic of what you have, and see if it seems likely to work by comparing it to other manufacturers designs from around the same time. I'd expect better performance with a 35 in the first socket but it would work with two 24's.

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Dennis

Experience is what you gain when the results aren't what you were expecting.


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 Post subject: Re: Detrola TRF '32? 4-D?
PostPosted: Apr Fri 10, 2020 1:35 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 469
Location: Terre Haute, IN USA
Attachment:
Detrola TRF32.jpg
Detrola TRF32.jpg [ 317.26 KiB | Viewed 213 times ]

Dennis:
I took quite a while, and carefully made a schematic from what I have been given. If you have a moment, please take a look at this, and give me your opionion. It has more than likely been modified in the last 80+ years. But I do not see anything all that unusual. I've NOT put any power on it yet. And of course, I WILL put new power supply electrolytics in it prior to powering it up. Thanks for your input.
Jerry


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 Post subject: Re: Detrola TRF '32? 4-D?
PostPosted: Apr Fri 10, 2020 1:51 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 26115
Location: Detroit, MI USA
It looks fairly reasonable, with a couple of things that are questionable.

First, the grid resistor on the 47 is too high and probably should be about 500K to 1 Meg.

Second, there's no provision for bias on the 47 tube. Typically sets which did not apply a fixed negative voltage to the bottom end of the grid resistor would instead have a wirewound resistor between the CT of the 2.5 volt filament winding and chassis ground to raise it enough positive (maybe 16 to 18 volts) to make the grid significantly negative with respect to the filament. Try about 350Ω and 5 watts as a starting point.

I'd like to see the coupling cap between the 24 and the 47 larger than it is, but it will likely work with what's there for the time being.

I agree with your thinking, after changing the power supply electrolytics, and maybe the .1 on the screen grids of the 24's, I wouldn't hesitate to cautiously apply power and see if it works.

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Dennis

Experience is what you gain when the results aren't what you were expecting.


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 Post subject: Re: Detrola TRF '32? 4-D?
PostPosted: Apr Fri 10, 2020 3:05 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 469
Location: Terre Haute, IN USA
Dennis: Yeah, the grid leak resistor on the 47 bothered me, too. It is colored as a 2.0 Meg. But in measuring it, it shows 2.9 Megs on 2 different digital meters. But as I said, It's obviously been worked on before I got it. As for the coupling cap between the 24 and the 47, I also thought it was quite low. But it is a mica---and usually I've not seen too many mica caps drift in value THAT much. Unfortunately, it has NO discernible markings as to its original value. But if I'm not getting enough audio to the 47, it won't be difficult to add some capacity there. An interesting side note: ALL the .1 caps were part of a 4 x .1 'bathtub' cap, mounted on the rear apron. I plan on using new bypasses for them as well. Thanks for your input!
Jerry


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 Post subject: Re: Detrola TRF '32? 4-D?
PostPosted: Apr Fri 10, 2020 1:36 pm 
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Location: Beaver Falls, PA. USA
A more reasonable value for the coupling capacitor would be 0.01 mfd. As Dennis said, the 47 needs a bias to work properly; either a resistor from the center tap of the filament winding to ground, or a negative voltage from the power supply.

You can't always assume that the wiring and parts in an old set are correct. Parts were hard to get during WWII, and repairmen did whatever they could to keep them going.

The schematics of those TRF radios are all very similar. Just find another set with the same tube lineup and use it as a guide.

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 Post subject: Re: Detrola TRF '32? 4-D?
PostPosted: Apr Fri 10, 2020 2:38 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 26115
Location: Detroit, MI USA
In addition there were lots of undocumented production changes particularly among the smaller companies of the era, either they figured out something simpler, something that worked better but didn't cost any more to build, or something that saved money and worked roughly the same.

Interesting side note on the Detrola 1932 4 tube TRF, I was told by former employees that those early sets were built in a rented garage in Ecorse, MI, and each radio was wired from start to finish by one person, there was no assembly line. Not sure how many assemblers there were, but you couldn't fit all that many people into a garage. Some of them were high school students who came in the afternoons and worked several hours. I'm picturing a pile of chassis with tube sockets riveted in already, and boxes of power transformers, capacitors, resistors, and tubes. They quickly moved to an assembly line for the next generation of sets they built.

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Dennis

Experience is what you gain when the results aren't what you were expecting.


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