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Antique Electronics Autopsy: RCA Radiola 16
11/14/2013

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The RCA Radiola 16.
The RCA Radiola 16.

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Susan Rambo
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Beautiful!
Susan Rambo   11/14/2013 4:03:25 PM
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This radio is a thing of beauty! Nice shots, Caleb. Thanks for this.

Bert22306
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Electro-mechanical
Bert22306   11/14/2013 5:18:20 PM
What strikes me most is how mechanical these old radios were. From the mechanical tuning capacitor, to the tubes, to the volume control. And yes, those wood cases seemed to be meticulously built.

But on the other hand, one reason electronics were this way is because they had no options. If they had plastics, they wouldn't have used wood. If they had solid state electronics, they never would have bothered with these touchy and expensive electro-mechanical tubes. But they are fun to watch as they glow.

Those mechanical tuning capacitors could be problematic. We had an old radio where that tuning capacitor became vulnerable to the slightest vibration, emitting loud thumps, and was scratchy too. Tubes changed characteristics as they aged, or just as they warmed up with each use, were an egregious waste of energy, and were so bulky that the electronic design always had to be compromised to keep the number of tubes required in check. And so forth. These old electronics are fun to look at, but we have also come a long way.

In spite of the common wisdom, solid state electronic products can be far, far, far more long lasting than these old radios or phonographs. I'm still using an amp I built in 1980, for instance. Hard to do that with these old tube jobs. The tubes would need changing, plus the heat they emitted caused other components to fail as well.

Graham Bell
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Schematic Courtesy the Museum of Broadcasting
Graham Bell   11/14/2013 6:17:11 PM
I found the schematic for the Radiola 16 at the Museum of Broadcasting .  Click on the thumbnail for a larger view.   Notice the the use of a grid-leak detetctor. Enjoy!



http://www.museumofbroadcasting.org/16schemLo.jpg

Susan Rambo
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Re: Schematic Courtesy the Museum of Broadcasting
Susan Rambo   11/14/2013 6:47:43 PM
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@Graham Bell, thanks for the Radiola schematic and link to Museum of Broadcasting.

Bert22306
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Re: Schematic Courtesy the Museum of Broadcasting
Bert22306   11/14/2013 7:32:25 PM
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Hey, thanks for that, Graham.

Holy smokes, it was simple. You have three consecutive transformer-coupled RF amps, each one tuned in synchronism with the other two by virtue of the mechanically ganged tuning capacitor. Then the envelope detector, nothing more than a low pass filter. Then two series connected audio amp stages. All transformer coupled, class A, no feedback, no separate voltage and current amp stage in the audio amp.

The first audio output has a high-pass filter, to cut down the squealing no doubt. They didn't like anything above 5 KHz audio in those days. And the really odd thing is the volume control. It operates on the voltage appplied to the *heaters* of the RF amps. How weird is that? As opposed to being a voltage divider just before the audio amp. I wonder if that was prevalent back then?

Graham Bell
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Re: Schematic Courtesy the Museum of Broadcasting
Graham Bell   11/14/2013 7:53:09 PM
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I have seen the volume filament control on other early radios.  It worked nicely on strong local stations; you could burn just enough battery power to get a decent sound to your headphones or perhaps an early loudspeaker. And with the lower gain, meant later stages were not overloaed from a strong signal.  However if you were trying to pull in a far away station, and getting the most gain out of the tubes, you would shorten your battery life.

Bert22306
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Re: Schematic Courtesy the Museum of Broadcasting
Bert22306   11/14/2013 7:56:42 PM
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Good point on battery life. I would also expect that the tuning might drift as you change the volume setting, because those RF amp tubes would change characteristics as they heat up or cool down.

Did you notice that the speaker outputs are both sitting at 135V when there's no signal? It looks like the plate bias voltage to the last audio tube is fed throught the speaker! Or earphone. Hmmmm.

rick merritt
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Re: Schematic Courtesy the Museum of Broadcasting
rick merritt   11/14/2013 8:02:16 PM
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A classic, like my grandma's old tube radio or even my parent's old Motorola radios and TVs.

Think of that, if those old beasts are still around they would technically be Google TVs and Google radios ;-)

WKetel
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Old electronics and things electrical
WKetel   11/14/2013 10:20:33 PM
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Caleb, I was recently given an old set of Audel books, copyrighted 1917 to 1923. They include some sections on electronics and radio, and a great deal on electrical power in general, both large and small. It is indeed interesting to observe how things were done back when labor was cheaper than materials. The effort required to add an outlet to a room in a house was a whole lot more than it is today. And a lot of the explanations for how electrical things work are interesting inn that they assume no previous exposeure to electrical anything. Really a fun read book set . 

David Ashton
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Re: Schematic Courtesy the Museum of Broadcasting
David Ashton   11/15/2013 3:19:58 AM
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@Bert...I would think the "speaker" would be a 2kohm set of headphones or maybe a high impedance speaker.   Fancy listening to the radio with +135V that close to your ears?

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