Modern Radio Laboratories
No. 2-A Crystal Set

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MRL 2A crystal set
A "No. 2-A Crystal Set" built from plans published by Modern Radio Laboratories.

MRL 1963 catalog entry
MRL catalog entry from January, 1963.
The circuit was developed in 1933 by Elmer Osterhoudt. Thousands of kits were sold until his death in 1986.

MRL 2A crystal set
Because MRL once sold the 2-A as a kit, Paul Nelson of MRL wanted me to state that this is a "reproduction." However, this set was built from plans written by Elmer Osterhoudt before Paul Nelson was even born. It's a matter of semantics. If it's not an MRL No.2-A crystal set, what is it? Inquiring minds want to know. Only Paul Nelson and his lawyers know for sure.
MRL crystal radio
MRL crystal radio
This set was built in October of 2020.
The parts and layout were adhered to as closely as practical.
All parts are mounted to the front panel except the antenna and ground connections.

Instructions for building this set are found in MRL HB-2 and DP-22A (click for the links).
The coil form is from an actual MRL No. 2 coil made in 1984 by Elmer Osterhoudt. It was mounted in an MRL No. 2 crystal set. Over the decades it had been baked and frozen repeatedly in a garage and the celluloid shrank slightly, causing the winding to come loose. Since this particular No. 2 wasn't a kit from MRL, or even built to the MRL layout, I had no qualms about removing the coil. Besides, a 2-A is basically a No. 2 set, though Elmer stated that he had not been able to pick up shortwave on the 2-A.
The coil of wire was removed and the form was polished until it looked new. I couldn't help but think that the last person who touched the form under the wire was Elmer himself. I didn't find any fingerprints on it, or it would be in a display case right now.
A new coil was wound using the instructions in HB-2. A close inspection and comparison with one of Elmer's actual coils reveals it is missing a piece of black tape on the left side. Except for this, it is indistinguishable from an MRL coil made by Elmer.

The coil form is 2" in diameter by 4.5" long. The coil is 90 turns of #22 cotton covered wire. Taps are at turns 5, 10, 16, 23, 40, 50, 61 and 73.

Thousand of these coils for the No. 2 and 2-A were sold by MRL. What was the formula Elmer used for the taps when he made the first ones in 1933? After analyzing the coil I found he used a ruler! The coil is tapped at 1/4, 1/2, 5/8, 3/4, 1
, 1, 2, and 2 inches.
I caused myself a lot of grief by putting the switch points too close together, If solder lugs are used, they touch each other and short out the coil. The "fix" was to solder a small loop on the end of each wire.

I caused myself more grief when it was time to mount the variable capacitor. I followed Elmer's instruction for making a template. Then the panel was marked and the holes were drilled. When it was time to mount the capacitor I found I had put the template on the wrong side of the panel! The holes were a mirror of where they were supposed to be.

Well, between the switch points being too close together and the holes drilled in the wrong place, I was about to chuck the panel into the trash. Unfortunately, it was the only panel I had. 
No.2-A crystal set
No problem; MRL made two different size dial scales! I used a larger one, which was serendipitously found in a box of odds and ends. It was scanned and "cleaned up" in a photo editor, then a brand new scale was printed. Naturally, I kept the original.
The brass screw holds the coil. It is the only mounting point. Elmer's instructions say to use lock washers to keep the coil from moving. He wasn't kidding, they are absolutely required.

The schematic and pictorial. The "BS" switch cuts out political ads. (Actually, the "BS" switch is for "Broad" and "Selective" tuning.)

In DP-22A Elmer said to mount Fahnestock clips on the back of the crystal detector stand so you can use a diode. I didn't want to do this, because it meant cutting the screws. When you're using these old parts you don't want to damage anything, even a screw. I outsmarted myself and used different screws! The Fahnestock clips are 1/2 inch. The diode is a 1N34A.
The instructions say to make a wooden box for the radio, sand it, coat it with white paint and then give it a coat of Nu-Enamel.

This box is made from 1/8" plywood from a craft store. To cut it, you just score it (deeply) on each side and snap it.

Elmer wouldn't have said to use Nu-Enamel if he didn't use it himself. It wasn't an advertisement; he sometimes used brand names in his literature, and they are clues into his personal life. For instance, he said to scrub the front panel of the MRL 1-Tube DX radio with Ajax. That tells us the Osterhoudt's used Ajax Cleanser.

Cabinet color
Cabinet color
Cabinet color
Using the computer to decide the color. I didn't use any of these colors.
MRL 2-A repro
It looked good painted white, so  I decided to leave it white because the paint came out smooth for once.
Smallscrews holding radio
I thought of several methods to hold the radio in the cabinet. Obviously with the radio in a cabinet you can't access the diode behind the back panel, and you need a way to easily access the diode. One idea was to use tiny Neodymium magnets epoxied into the base, which would stick to small "L" brackets in the cabinet. That way, you could simply press the radio out of the cabinet.

Since it worked so well with the cat whisker, I just removed the diode and screwed the base into the cabinet with tiny screws. Translation: It was easier and I'm lazy.
Cabinet color
  Large label Small label Click image to download.  
Labels - The labels are printed on heavy paper and glued on with an Elmer's Glue Stick. This Glue Stick idea came from Mike Peebles. Before Mike retired in 2020 he was the Elmer Osterhoudt of the 21st Century. I actually argued with Mike over this Glue Stick idea, saying it would never hold up. His final comment was, "I never had any trouble with it." I tried it, and years later I can also say, "I never had any trouble with it."
Here's a short video. It is just as boring as the previous video on Page 1.
WNPV 1440 AM
Missing from the video is the local "flamethrower," WNPV AM 1440. Transmitting with 50,000 watts, this station came in so loud you could pick it up with a piece of wire and a jelly donut. After serving the area for 60 years, it went off the air on April 30, 2020.

The station has five 165 foot antenna towers on 13 acres of land near the North Penn High School, in Lansdale, PA. Also on the site is the transmitter and a building housing the studio.

Just prior to the station closing, the North Penn School District, crying broke as usual, raised the North Penn area property taxes by 3.1 percent. This violated the 2.9 percent cap imposed by the state of Pennsylvania, but they just seem to do whatever they want. They weren't even holding classes at the time due to the virus. In July 2020 the school district magically found 2.3 million dollars in its "reserves" to buy the WNPV property.

According to a report by ABC news, Channel 27 WHTM, North Penn was one of 12 school districts who played a "shell game" of moving money around so they could always raise taxes, though they had millions of dollars in their General Funds. Ergo, 2.3 million dollars seemingly materialized out of nowhere for purchase of the radio station.

This is a good thing, right? A high school with an actual radio station, not some obsolete collection of low power equipment connected to a wire antenna on the school roof. Think of the educational possibilities, the revenue stream from advertisers, and the fact that they could make it into a cool station that would rival the two college stations in Philadelphia, WRTI and WXPN.

Their plans are to demolish the station and use the property as an athletic field and a parking lot. Now mommy and daddy can park in the new parking lot and watch Johnny kick a ball, and guess who has to pay for the parking lot and the ball. In a newspaper article, the miscreants on the school board actually commended EACH OTHER for spending the money on the WNPV site. Well, they weren't going to get a pat on the back from any taxpayer, that's for sure.

UPDATE: As of March 2023, WNPV is back on the air as part of the North Penn HS communications program.
Don't like the cabinet? I made this one in 2023 after I refurbished an actual MRL 2-A sold by Elmer Osterhoudt.
MRL 2-A with cabinet
MRL 2-A with cabinet
MRL 2-A with cabinet
Elmer said to make slots in the sides and top so the front panel can slide into it. The bottom screws on.