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Atwater Kent model 20 Big Box - Printable Version

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RE: Atwater Kent model 20 Big Box - TTech - 04-26-2021

Chas,

What a bunch of wonderful reading. Went through the speakers A through Z. Some of them I could have picked up and not even guessed they were speakers. Others were the same as old singer sewing machines of that time. Absolutely beautiful.

Finally found through Radiomuseum, that my Perfectone was made by the Teleradio Engineering Corp.

RE: Atwater Kent model 20 Big Box - TTech - 04-29-2021

Found a small selection of A.K. model 20 parts on Ebay. Bid the minimum amount and won. Received the parts in the mail today which included these two audio transformers. Both check good and match the resistance values listed on Leigh Bassett's new schematic for a model 20 big box.

They may be from a model 20C. All dimensions are the same as the dimensions of the open one still in my radio, with one exception. The bottom flange is wider, plus about 1/3 of the mounting flange has been removed. Perhaps allowing them to fit in a smaller cabinet. At least thats my guess.

Next step is adding pigtails to the wires, then down the road install on the chassis.

RE: Atwater Kent model 20 Big Box - Chas - 04-29-2021

A lucky find!

Those will be fine. It appears the slice from the flange is such the transformer fits other models too.

Clean the wires carefully to identify the color coding, replicate the coding with the additional wires...  chas

RE: Atwater Kent model 20 Big Box - TTech - 04-29-2021

Chas,

Your right lucky find. When I saw them would have bet both were duds. Then at least I'd have something to stuff new transformers in. When I found both were good couldn't believe my good fortune. Carefully exposed stranded wires, fanned out the strands and cleaned them up using a small steel brush in a variable speed dremel. New wires are now soldered in place and they both still check good. 

Another spare part that came in was the rheostat plate. Originally both rheostats checked unstable readings from k ohms to meg ohms. Found they were easy to disassemble. Cleaned all contact points with 1500 grit sandpaper, sprayed out with contact cleaner, then air dried. Now both have min of .6 ohms and a max of 9.8 ohms. Smooth control now. On/off switch got same treatment. It was nice learning on a spare, somewhat prepared when I do the same with the one installed on the radio.

RE: Atwater Kent model 20 Big Box - Chas - 04-29-2021

The A-K recommended rheostat lube is petroleum jelly. Lube the shafts and wiping contacts. Lube for the tuner shafts is NuJol a brand of U.S.P. Mineral oil.

Nothing fancy as A.K viewed his radios for the world.

Again LUCKY, if there is distortion it will not be the audio transformers....

RE: Atwater Kent model 20 Big Box - TTech - 04-29-2021

Once again thanks for the added info Chas. Lets hope the luck continues.

RE: Atwater Kent model 20 Big Box - Phlogiston - 05-01-2021

Those two interstage trans were likely from different radios, probably of different production runs. The radios I have seen either have trans with the wrinkle finish OR the smooth finish.

I believe that the two audios were often 4:1 and 3:1 (possibly 5:1). A radio that used a 4:1 as the first trans usually had a 3:1 as the second/ Using two 4:1 (or 5:1) might induce a lot of distortion (though on these radios, some might never notice).

The paint color on them is a purplish-brown, not always exactly the same from unit to unit.

Just FYI

RE: Atwater Kent model 20 Big Box - TTech - 05-01-2021

Thanks for the heads up Phlogiston, I'll check further before install and wiring.

RE: Atwater Kent model 20 Big Box - TTech - 05-02-2021

Have a question to throw out there about the audio transformers in this radio. I have a copy from an A-K data sheet, page 8-1. It does verify that both audio transformers should have the identical primary and secondary resistance. Primary 1700 ohms, secondary 3250 ohms. The data sheet is for a Model-20 #7570.
They next state the same audio transformer is used in the model-20 # 4640, (what I have). They state for radio model 20 # 4640 transformer part# 7661 is used in both the 1st and 2nd stages.

They also state that the transformer for the radio 20 model# 4640 is part# 7661.
The transformer for radio 20 model #7570 is part # 4779.

They go on to say even though each model has a different transformer part numbers, they are the same audio transformer.

So... all that as a prelude to my question.
Can two different transformers with the same primary and secondary resistance 1700/3250 ohms have a different turns ratio if one of them was wound with a larger diameter wire?

Feel free to respond in a manner that lets me know I'm being dense. Just trying to get a handle on having two transformers that match the factory data sheet resistance wise. But may be incorrect ratio wise. It would be nice to have some clarification before I go to the trouble of installing them. 

Like any newby, I might have my jock strap on backwards and don't even know it yet.

RE: Atwater Kent model 20 Big Box - Phlogiston - 05-02-2021

Well, you are not "dense" unless I am too.

The turns ratio on the same iron, laminated crore is going to determine the impedance ratio. It does not guarantee that the resistance in ohms is going to be the same ratio. Resistance measurements are just a good way to guess. If resistance is way off or open or shorted, the trans is broken.

In some of these radios the first trans is for voltage gain ( the tubes themselves were only good for a gain of 9 - 15 unlike later tubes that could have gain in the hundreds or thousands. The second trans can be center tapped for radios that have push pull outputs (2 tubes). The gain for these can be as little as 0 or in this case 1.5 - 2 per side. The transformer's first job is to provide the phase shift required to run tubes in the config. - one going + at the same rate as the other side is going - ). A lot of gain in this stage can drive the output into distortion, but they  - generally - do provide some voltage gain. These push - pull trans age generally a bit larger than the 2 that you have.

Russ

P.S. the "core" of thes early AK trans is usually a bunch of iron wires twisted into a "core". Kind of messy, but it works. Also, yours is not push-pull (center tapped secondary), but a lot of these trans were made for that service.

RE: Atwater Kent model 20 Big Box - TTech - 05-03-2021

Got it Thanks. Guess I'll install what I have and then see what I've got sound wise.

RE: Atwater Kent model 20 Big Box - TTech - 05-04-2021

    Audio transformers installed and soldered into place. Remains of some of the old  transformer wiring was not soldered in placed. Just a hook of bare wire loosely crimped in place then taped with cloth tape. Thought I had never worked on a radio this old. Evidence shows I've worked on this one before. 

While no transformers were in the chassis, removed the rheostat panel and cleaned all contact points along with on / off switch. Looks like the antenna tap switch is put together similar to the rheostat panel. Will give it a clean up next.

RE: Atwater Kent model 20 Big Box - TTech - 05-04-2021

Bottom view of transformer transplant.

RE: Atwater Kent model 20 Big Box - Chas - 05-06-2021

Hi again TTech,

I notice that the color coding of the transformer lead wires has been lost?

Since a resistance measurement can differentiate between the primary and secondary the only "difficulty" that can arise is that the start/finish of each of the windings has been lost. That may seem unimportant and in truth may not be in this radio. What can happen is the transformer can pass the phase of the signal without change or invert the phase. Further, an alteration of the phase not as designed could result in the audio stages oscillating. The solution is to exchange one pair of wires from either the primary or secondary on one of the transformers.

Chas

RE: Atwater Kent model 20 Big Box - TTech - 05-07-2021

Hey Chas,

You caught me. Since the replacement transformers had all wires cut within 1/4 to 1/2 inch of tar sealant, and all wires had turned the equal shade of black. I did not know how to determine start/finish of each winding. Was there originally a colored thread in the wire cloth to determine which wire was start or finish? Also, even if I did know the start and finish ends of the winding, what determines the connection point in the radio?  If the 1st audio transformer start/finish wires are known for the primary, does the start end connect to the detector tube plate or +20 supply?  

Phlogiston was kind enough to school me in turns ratio determining if the audio will distort. Here again I didn't know how to determine turns ratio to a sealed transformer.

I did try a little researching. Internet,  and the books, Elements of radio servicing, and Robert Shraders Electronic Communication 3rd edition. If the information was in anything I read, was unable to translate it to answer my questions. 

So I simply read resistance, determined which was primary or secondary windings. Used white wires to identify primary winding and black secondary. Thought as long as primary  and secondary wires are connected according to the schematic, at least the radio should work at some level and not smoke out of the gate. I have found some mistakes in repairs. For instance the Detector phone capacitor was connected between ground and #2 tube, pin 4 filament, and not between ground and 90volt B+.

If there are ways to determine the answers please share the details. I have a four channel oscope and 8640B sig gen, a variety of multi-meters and know how to use them. Just need a little technical hand holding and direction to identify the Whats,Where, and Hows.  
I can re-do all that I've done if needed.

Thanks Again

Tim