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Old 04-02-2017, 06:29 AM
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1920s battery radios....how do they work?

Decided to take a leap backwards in my learning and try to understand how a 1920s battery radio works....and thereby learn to repair them and even soup them up a bit if possible

Let's take this 1925 Resas Tone-A-Dyne.....I now have two of them. Since I can't seem to get any form of schematic on it (and even if I could, 1920s radio schematics are horrible compared to the Sams I've been spoiled on using with TVs), I'm having some difficulty understanding how the radio works.

Let me start by saying that I do understand the A, B, and C voltages and what they do, and both radios do work. They use five 01A tubes. No real parts inside except the coils, two transformers (no idea what they do), and one plug-in 1.5 meg grid leak resistor (again, no idea what it does).

Resas #1 works fine, but the volume and frequency tend to drift around a little and it requires adjustment every few minutes. Resas #2 is much more stable, but the audio is more muffled. Since I operated both on the same set of 01As, this leads me to believe that there are components I can replace to get them performing better.

The 1.5M grid leak resistor on Resas #1 measures 2.6M, so that needs replacing. The one on Resas #2 measures out at 1.7M, so that's closer. What a grid leak resistor does I'm not sure.

Picked up a restored Atwater Kent big box model 20, and that thing makes the Tone-A-Dynes sound like the dog's blanket....so either the A-K is just a superior radio, or the Resas(es) need work.

Incidentally, I started figuring out how to tune them by hooking up a signal generator and dialing them in with it, then cutting the power when I found the desired frequency. Since all of these radios seem to love being hooked up to the signal generator and get much better reception when hooked up to it even when it's off, until I get a larger antenna I just run them that way

Any insights and advice are always appreciated!

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Last edited by Celt; 04-02-2017 at 04:11 PM.
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Old 04-02-2017, 01:04 PM
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TRF sets are basically an LC filter tune-able over the AM band followed by a stage of amplification (rinse and repeat a stage or three), a grid-leak detector (google for more on that I forgotten how those work), and an audio amp stage or two.

Regenerative sets are a complex way of making minimal tube count work like a big TRF....Basically the RF is fed back in a positive feedback loop to J-U-S-T shy of the point where it oscillates (you'll know when it does)...IIRC some managed detector/audio gain function from the same single tube.

There were also other schemes like one where there was a single detector tube and the TRF stages did double duty as audio amps.

There were a few more designs, but my knowledge is rather limited. 20's sets have always been stupid expensive, hard to get parts for, and hard to power putting them out of my reach. I've owned/worked on 3 early AC powered TRF sets, and just got my first restorable bat set a week ago.
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Old 04-02-2017, 02:06 PM
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Grid leak is used to minimize tube capacitance problems.
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Old 04-02-2017, 03:04 PM
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Tube capacitance problems were addressed by neutralization (negative feedback), as in the Neutrodyne scheme. A bit later, an xtra grid was added between the G1 and plate (screen grid) to minimize G1-plate capacitance, as in the famous 24A tetrode.

Grid leak resistor is used to bleed off ('leak') excess space charge (the cloud of electrons boiled offa the filament), otherwise the tube would self-bias to cutoff. It also provides detection (demodulation) of the RF signal via the nonlinear characteristic of the response curve.

Last edited by old_coot88; 04-02-2017 at 03:36 PM.
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Old 04-03-2017, 09:07 PM
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I've amassed a gaggle of 20's sets over the years; it frustrates me that a device so simple can befuddle me that much! A lot of the issues seem to be open coils, bad connections, bad grid leaks. I've had mixed results but do have a set or two that perform okay.
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Old 04-03-2017, 09:44 PM
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Quote:
Originally Posted by Kamakiri View Post

Incidentally, I started figuring out how to tune them by hooking up a signal generator and dialing them in with it, then cutting the power when I found the desired frequency. Since all of these radios seem to love being hooked up to the signal generator and get much better reception when hooked up to it even when it's off, until I get a larger antenna I just run them that way

Any insights and advice are always appreciated!
The signal generator is probably acting as a coupling devise to your homes AC wiring, which is acting as a makeshift long wire antenna. The down side to using your homes wiring is that the radio will hear all kinds electrical noises riding on the power line. Even the ground wire to your homes electrical system may act as an antenna at radio frequencies.
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Old 04-04-2017, 02:04 PM
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There are two small transformers in the set, does anyone know what they do?

With a television, I can look at a block diagram and schematic and generally figure out what's going on. With this it might as well be magic or something, because with five of the exact same tube I'm not really sure what anything inside this thing is really doing.

Until I get some sort of grasp on how the radio works, it's like knowing how to replace a fuel pump in a car but not knowing if it's even bad, or knowing how a fuel pump makes the car run.

I assume these are some sort of interstage transformer, and since there were so many 5 tube battery sets made there must be some sort of spec to them. I just can't find anything other than crude diagrams that look like Abraham Lincoln drew them on the back of a shovel with a lump of coal......
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Old 04-04-2017, 03:23 PM
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Usually air core = RF trans, iron core = audio trans. If you know where the antenna and speaker terminals are that helps tell what is what. Most sets with a straight line of tubes have signal flow/stage cascading linearly along that line. If you know which end is audio and which end is RF, and then can figure out which one is the detector then you've pretty much chopped it up into it's 3 types of stages. Usually the detector will have some meg ohm resistor feeding it's grid instead of an just an RF or AF transformer grid winding.

I wish I had time to track down a modern redraw of the Atwater Kent model 40 I used in a resto (it's on the net somewhere)...It gives a good idea of the anatomy of a typical 6 tube TRF receiver. A lot of 20's schematics in addition to being poor scans of riders (that is a topic in and of it's self), are drawn less as a schematic and more as panel wiring diagrams with no real standard...It was the wild west days of schematics and it takes some imagination, patience, multicolor highlighters and sometimes redrawing the thing in a non-bassackwards way to follow them...

Most 40's and later sets I can usually grab an iron and go blind, and if needed read important info off the schematic at a glance, older stuff often has moments of staring at the print and or chassis and tracing things out to make sure I'm not seeing things wrong.
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Old 04-04-2017, 03:37 PM
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Yes, these are interstage audio coupling transformers. Typical use would be one transformer between the gridleak detector and first audio stage, and another between the first audio stage and the audio output stage. Since the simple triode tubes of the era (01As 199s and the like) had pretty low gain, the transformers "step up" ratio actually was used to boost the signal between each audio stage. A 3:1 ratio was common, but other ratios were used as well.
What are the tubes in your set?

jr
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Old 04-04-2017, 08:17 PM
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Five 01As. All I'd need to do is see a simple decent schematic of a similar set with values and part identification to start the process of understanding this. Only thing I've run across is poorly drawn pencil schematics with nothing but symbols.
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Old 04-04-2017, 08:21 PM
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Quote:
Originally Posted by Kevin Kuehn View Post
The signal generator is probably acting as a coupling devise to your homes AC wiring, which is acting as a makeshift long wire antenna. The down side to using your homes wiring is that the radio will hear all kinds electrical noises riding on the power line. Even the ground wire to your homes electrical system may act as an antenna at radio frequencies.
You're 100% right because I have a wonky socket on the bulb on my workbench light....when it cuts out I can hear it on my radio 20 feet away.

Now, that said, I'd have to think there's a simpler way to make such a coupling device to do just that rather than use my signal generator. I tried coiling a wire around an AC cord thinking that might create a capacitive coupling, but no luck. I've searched out references to an .001 cap at a high voltage connected to the AC line somehow that accomplishes this, but can't find any direct references.
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Old 04-04-2017, 09:13 PM
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There is a fairly decent generic schematic in this wikipedia article. The diagram shows three audio stages rather than two and uses the less efficient resistor capacitor (rather than transformer) coupling between two of the stages. If you eliminate the second audio stage and put a transformer between the detector stage (grid leak) or first audio stage and the audio output stage, it should fairly closely resemble your set.

https://en.m.wikipedia.org/wiki/Tune...uency_receiver

hope this helps,
jr

Last edited by jr_tech; 04-04-2017 at 09:21 PM. Reason: add info
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Old 04-04-2017, 11:39 PM
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This might be closer:


jr

Would you like to go over it stage by stage?

.

Last edited by jr_tech; 04-04-2017 at 11:48 PM. Reason: add question
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Old 04-05-2017, 11:06 AM
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Here's a whole slew of Atwater Kent schematics redrawn to modern standards. Take a look at the model 20.

http://www.atwaterkent.info/TechData...wingIndex.html

For your indoor antenna try connecting the radio's ground to the third wire ground(green) of your homes AC wiring, which in theory goes to earth ground at your service entrance. Then try a short length(10-15 feet of whatever wire) stretched out across the basement floor. That short wire should pick up whatever radio signals are radiating off your homes AC wiring. I have very good reception from local stations using this method, but I also hear every appliance in the house turn on and off, as well as well as all the crappy wall wart switching power supplies that most every modern day electronic devise use. Nothing can compare to a 50-100 foot wire strung up outside far away from other electrical interference. I have about 50 foot going from my shop to a tree with a grounded shield coax(75 ohm) leading all the way to my basement. Ideally you want to earth ground the coax shield only where the antenna enters your home, then you can use the shield as the radio ground at the other end of the coax. Yes there are loses using coax for lead in but they are negligible relative to the overall clean signal you receive.
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Old 04-05-2017, 11:24 AM
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I'll give that a shot.....

See, the reason for all of this is that I'm going to be working on a Radiola III. Might be some nights when my wife is doing whatever, that I might want to emerge from the basement and sit in the living room and recline on the couch with the radio on a small table and put the headphones on. That's not going to be very easily done if I have to string a huge antenna....and my living room is on the driveway side of the house and the front. Not really conducive to a long antenna, and what if I decide I want to move to the other side of the couch or another room? This is why I want to make it more "portable".

And I'm guessing, why more people don't express an interest in these dinosaurs. If there were an easier route here, I'd be willing to bet that more people would express an interest in operating one.

In that case, adequate is good enough because the station I'd want to listen to is a short distance from me.
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