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The innards of that AK 20 Big Box set - Printable Version

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The innards of that AK 20 Big Box set - ccomer1955 - 06-17-2015

Thanks for the advice on the cabinet earlier. I'm pretty sure how I want to go there, now. I've done my cleaning, desoldering, and measuring on the set itself. The rheostats check out good, and the RF coils as well. The three tuning condensers turn great and I show no shorts on the plates. I replaced the grid leak resistor, grid resistors, and the .3 mfd capacitor. Oddly, the primaries on both interstage transformers were bad, so I ordered the p-t156 transformers from AES and they came in today. I put the transformer cases in the freezer overnight and the old transformers popped right out with no problem and no damage to the cases. So far all is well. I'm a bit terrified of putting the transformers in wrong since this is my first time to attempt it. My issue is I'm puzzling over the right way to hook up the transformers. The transformers each have a red and a blue wire on one side, and on the other side is a black wire, and two green wires. AES sent the transformers without a schematic drawing.
I'm sure I can disregard the black wire as a center tap, but what are the correct primary and secondary coil wires? I looked up color coding and found this:

From the ARRL Radio Amateur's Handbook:
Plate finish lead (pri.) ------- blue
B+ (power supply) lead --------- red (whether center tap or not)
Plate start lead (pri.) -------- brown
Grid finish (sec.) ------------- green
speaker return (sec.) ---------- black (whether center tap or not)
Grid start (sec.) -------------- yellow

http://www.atwaterkent.info/TechData/Drawings/Images/AK04640sch.pdf

Am I correct in thinking on the first transformer the blue goes to the plate of the detector, the red goes to the power switch, green should go to the grid of the 1st audio tube but there are two green wires?
The connection points for that transformer should be the power switch and the grid of the 1st audio tube across the primary winding, and the 22v post and the plate of the detector tube across the secondary winding.
The connection points on the second transformer should be the 90v post and the plate of the 1st audio tube across the primary winding, and the power switch and the grid of the 2nd audio tube across the 2ndary winding.

So if I figure out which wire pairs constitute which windings, and how to keep them in phase (probably not the right term), I should be good?
I'm sure several of you guys have done this many times and could just tell me which colors go where, but I'd like to learn from it. Perhaps someone knows where I can find a color code guide for the wires, although from the code above it looks like blue is plate, red is B+, and green is grid...or do I ignore one of the green wires and use the black? As you can see, I'm a little lost in this.

RE: The innards of that AK 20 Big Box set - ccomer1955 - 06-17-2015

Actually, is it the same as the Hammond 124A? If so, is this schematic the one I need? http://www.hammondmfg.com/pdf/124A.pdf

RE: The innards of that AK 20 Big Box set - Ron Ramirez - 06-17-2015

(06-17-2015, 06:17 PM)ccomer1955 Wrote:  ...Oddly, the primaries on both interstage transformers were bad, ...

Not so odd. Actually, bad interstage transformers in 1920s battery radios are the rule rather than the exception, and it has been this way for decades. That is great that you were able to "freeze" out the insides of the originals!

There is a simple solution to your dilemma. If your transformer wires have the same color coding as the Hammond 124A, then simply measure with your multimeter set to ohms, across the red and blue leads. If you get around 400 ohms, that is your primary. Measure across the two green wires and see if you get around 1500 ohms, and measure from the black lead to each green lead (in turn) and see if you have continuity. If so, you've found the secondary. Then, take notes so you won't forget which wires are for the primary and which are secondary...and proceed with the installation.

Good luck! It should be a really nice radio once you are finished. Icon_thumbup

RE: The innards of that AK 20 Big Box set - Radioroslyn - 06-17-2015

Yup you are correct. Red B+ to the det, Blue plate of the det. Green to grid of 1st audio other green to C-. There is no polarity for the green wires. Black is unused. I use a couple of those winding on a Crosley Trirdyne worked well.
http://philcoradio.com/phorum/showthread.php?tid=7614&highlight=Crosley

Terry N3GTE

RE: The innards of that AK 20 Big Box set - ccomer1955 - 06-17-2015

Perfect. Thanks to both of you. I'll let you know how things go on reassembly. I have three other try radios: radiola 18, Philco 77, and a Freed Eismann NR-6. None of them needed an interstage transformer.

RE: The innards of that AK 20 Big Box set - Alan Douglas - 06-18-2015

What, an NR-6 with a good transformer? Call Guinness.

RE: The innards of that AK 20 Big Box set - morzh - 06-18-2015

I just did. It is in my mug now.

RE: The innards of that AK 20 Big Box set - ccomer1955 - 06-18-2015

I didn't say it was a Freed transformer, though.

RE: The innards of that AK 20 Big Box set - ccomer1955 - 06-27-2015

One more question about the AK 20 Big Box set: I have just finished putting the AES power supply kit together, and when I measured the outputs on the 'B' terminals, the 90 was just under at 88v, but the 22 was higher at 23.5. Should I add a small resistor to the 22 terminal to bring it down to 22.5? I have a fear of hurting a 201a tube.

RE: The innards of that AK 20 Big Box set - Eliot Ness - 06-27-2015

(06-27-2015, 05:40 PM)ccomer1955 Wrote:  One more question about the AK 20 Big Box set: I have just finished putting the AES power supply kit together, and when I measured the outputs on the 'B' terminals, the 90 was just under at 88v, but the 22 was higher at 23.5. Should I add a small resistor to the 22 terminal to bring it down to 22.5? I have a fear of hurting a 201a tube.

No, you should be fine with those voltages.  The 23.5 will probably drop a little when you hook it up anyway.  Back when the set was new those voltages were not that precise because of the batteries, eliminators, chargers, etc..  I would verify the A voltage going to the filiments before hooking up the power supply.

RE: The innards of that AK 20 Big Box set - ccomer1955 - 06-27-2015

I can set the A voltage on the power supply. I thought I might hook it up without tubes, first, and measure the voltages for the filaments and all the B+. If all is well where I can measure, then try it again with the tubes in.

RE: The innards of that AK 20 Big Box set - ccomer1955 - 06-29-2015

Update on the progress here: I measure resistance/continuity across the primaries and the secondaries of the two RF coils between the antenna coil and the detector, my grid leak is good, and my by-pass capacitor is good (replaced both). I built and set up my AES power supply, made a cloth covered cable in correct AK colors. I also put an extra terminal on the power supply that connects to the green ground on the plug for the power supply (no connection to hot and neutral on that terminal of course--my household power sockets all test good for hot, neutral and gorund connections). I set the A battery voltage to 5v. Hooked it all up...and not much.
I discovered that if I placed the antenna on the screw that has the lead to the grid leak resistor on the tuning gang next to the detector, I could get a tunable signal. I touched the antenna to the grids of both RF tubes and got nothing. I was using a 20' antenna inside the house.
I then ran a lead to my outside 100' antenna, and repeated the experiment. I got a very satisfying loud signal that was tunable to different stations from the screw on the tuning gang by the detector, but I got a much quieter signal from the grid on the 2nd RF tube, and an even more quiet signal on the grid of the 1st RF.
Thoughts?

RE: The innards of that AK 20 Big Box set - ccomer1955 - 06-29-2015

I should also mention that I'm not sure about the relationship between the tuning dial calibrations and the actual tuning gangs. Should they all be in the same physical position (say all plates fully engaged) when the dial calibration markings are at the same number? The dial for the gang next to the detector is not in synch with the other two gangs.

RE: The innards of that AK 20 Big Box set - Radioroslyn - 06-29-2015

Check on the back of the rf tuning caps (not the detector) there is a wire wound resistor. Bet you've got and open one or two. Can use carbon resistors to replace. They are usually a pretty low resistance. Value not very critical anything below  1000 ohms or so should work fine.

Terry

RE: The innards of that AK 20 Big Box set - Ron Ramirez - 06-29-2015

(06-29-2015, 02:14 PM)ccomer1955 Wrote:  Should they all be in the same physical position (say all plates fully engaged) when the dial calibration markings are at the same number?

Yes.

Now, when you get this set going and attempt to tune in stations, the dial settings may vary. In other words, when you tune in a station, they may not all read the same numbers. This is normal.