AK Model 37 Condenser Block Rebuild

[Winky Dink]

After my post on my Atwater Kent Model 37 restoration I was asked about the filter condenser block, but at that time I hadn't done anything with it. Although the radio worked well, the experience of many restorers seemed to indicate that bad things might happen if 85-year-old papers caps are left alone. So, I decided to have a go at it and post some photos showing what lurks beneath the tar.

The Model 37 power unit comes in two separate cases; one for the transformer and one for the filter condensers and chokes. Previously I had only replaced the external wiring.



To preserve the original crinkle finish, I did minimal heating to the outside of the case. I expected that the case to be tightly packed with only a small amount of tar, but this was different. I removed one and a half pounds of tar just to expose the components and wiring. There's a metal plate dividing the case into two sections. One section has two coils; the other has one coil and four paper condensers. To be sure that I would be able to rebuild it properly, I wanted to keep the whole assembly intact, but I anticipated breaking some of the connections, so I color-coded them before removal.



After softening the remaining half-pound of tar, I pulled out the filter condenser assembly, part number 13315. There are two versions of part 13315--early and late. This is the early type. The late type is the same except for the addition of one more condenser.



I made a color-coded a schematic of the assembly to help me reassemble the unit. Continuity and resistance readings through all three coils are good, so the rest of this job should be a piece of cake!...I hope.

[Reece]

Nasty! about a twelve-rubber-glove job.

[JBL GUY]

Very interesting. Good work.

Thanks for the pictures.

[Kevin Kuehn]

Fun job.

I did a AK 40 supply years ago and it was not the most pleasant experience. I've since bought one of those counter top convection ovens for slowly heating up tar cans like these. I now take the tar jobs outside in the summer.

[Winky Dink]

I'll have try our big toaster oven for tar-melting the next time my wife goes away to visit her sister. For this rebuild the last steps are to replace the condensers...



...and then replace the tar. That's just a piece of cardboard with some crumbled tar melted to a quarter-inch thickness.



Next I created a problem by fixing the problematic (noisy) volume rheostat. Found a used part on Ebay--different Bakelite color, but the same innards. Replaced the defective wire-wound resistive element and rewired it the same way as it was, but now it won't shut the volume down enough. The stronger stations are fairly loud with the rheostat in the starting position.

I'm sure I have it wired consistent with the schematic, the service manual description, and the original wiring. The manual says:

"...variable resistance connected between the antenna and ground posts of the set. When the knob is full 'on' the slider passes off the coil, cutting the latter out of the circuit entirely. As the control is turned back, less and less resistance is included between the aerial and the ground so that more and more of the incoming signal is shunted to the ground...thereby reducing the volume..."

But it doesn't reduce the volume enough. Here's the Atwater Kent schematic (for my "early" model without later additional connections for a shielded antenna cable) with my simplified version inset.



This is a snip from a modern schematic made in 2005 by W3NLB (stated for "all versions" of Model 37, but clearly different from the AK schematics).



What's wrong, and how do I fix it? Is it possible that I have chassis ground leaking some signal back into the RF amplifiers? I could rewire it according to W3NLB's schematic except for the center tap on the antenna choke.

Hoping for suggestions.

[Electronic M]

I have a model 40 that does that, and I blame the design. With the minimal shielding those sets have just because you short the signal at the input of the 1st TRF stage(with the 'volume' control) to ground does not mean that the 2nd stage is not picking up enough signal for it to play at a good volume.

[old_coot88]

Originally Posted by Winky Dink

...I created a problem by fixing the problematic (noisy) volume rheostat. Found a used part on Ebay--different Bakelite color, but the same innards. Replaced the defective wire-wound resistive element and rewired it the same way as it was, but now it won't shut the volume down enough.

Did it shut the volume off all the way before you did the replacement? Or maybe the control was too noisy/erratic to tell for sure(?).
Generally radios with that kind of vol control setup will exhibit the behavior you describe. Signals from strong local stations get into the RF stages directly, bypassing the rheostat. But if the set has an all metal case, it should prevent this. Does the AK 37 case close up completely?

[Winky Dink]

I understand. I could confirm that by rigging some temporary shielding. Thanks much.

[Winky Dink]

...or, I could just put the metal lid back on the radio.

[Reece]

These sets required a good external ground connection for the volume control to "kill" the antenna signal. Give that a try.

[Winky Dink]

Yes, the Model 37 is the typical all-steel bread box, and the condition of the rheostat may have kept me from noticing the phenomenon. I stuffed the chassis into the cabinet, and when I put on the lid the volume significantly decreased. The separate steel cover for the power unit may also help.

How come nobody suggested stuffing a sock in the speaker horn?

I've been considering establishing an earth ground. The electrical system ground is at the opposite side of the mansion; we have modern non-metal plumbing; and the heat/AC vents are connected with non-metal ducts. I am prepared to drill through an outside wall and run a line to the ground. Maybe the close-by A/C compressor cover is earth-grounded?

[Winky Dink]

But maybe I'm making this too difficult. And maybe I shouldn't write posts at 3 am. When I have the radio fully reassembled I'll try the center screw on an electric outlet first.
Thanks.

[old_coot88]

Yeah, running a dedicated earth ground is a lotta work justa maybe gain a bit more volume shutoff.

[bandersen]

Nice job I have the same volume issue with my Brunswick 5KR (Radiola 18). I think a rheostat on the antenna just wasn't a very good design.

[old_coot88]

Just part of the charm of the old sets.