Modern Radio Laboratories ®
No. 10 All Wave Crystal Set
(Replica of the MRL No. 10 kit designed by Elmer Osterhoudt)

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MRL no.10 Crystal Set
MRL® No. 10 All Wave Amplified Crystal Set replica.
MRL No. 10 crystal set
Front View.

Originally developed and sold as a kit by Elmer Osterhoudt of Modern Radio Laboratories, the MRL No. 10 All Wave Amplified Crystal Set first became available sometime around 1957. This replica was built in March of 2022. The price in 1957 isn't known, but in 1972 the kit sold for $8.50, which is about $58.00 in 2022. Considering what I spent acquiring parts for this, the $58.00 price makes me feel a little better. Let's just say that this little crystal radio cost me about $16.00 in 1972 dollars.
MRL crystal transistor set
Though a few changes have been made in the layout (it has a base and the original didn't) the front panel is a duplicate.
MRL No. 10 crystal set
Rear View. The variable capacitor and batteries are mounted to the base. In the original, everything was on the front panel.
MRL No. 10 crystal set
The one transistor amplifier is on the left. Near the top is a 1N34A diode which can be switched into the circuit from the front.

Bottom view.

Modern Radio Labs No.10
 There is one page of instructions to build the set (MRL Detail Print 34), which can be found here.

MRL No.10
MRL No. 10 crystal set
  The layout from the plans.   An actual surviving example from 1972.  
Diode Switch
Without the photo of an original set, there would have been no way to know this label was part of the kit.


diode transistor kit
The No. 10 kit purchased from MRL would have looked similar to this. The panel would already be drilled, painted and have the switch points mounted. The coil and switches would be made for you. A battery, hookup wire, and instructions were supplied with the kit. It even came with solder. All you had to do was put it together.

The last of the original kits was sold sometime prior to 1987. 35 years later it isn't easy or cheap to acquire these parts. The little box with the crystal detector was 50¢ in 1986. Today, they are $60.00 and they don't even come with the crystal.


plastic tubes
plastic tube
The coil forms are 2" in diameter by 4.5" long. These clear plastic tubes were from ebay and are exactly 2" outside diameter. Each tube makes 2 coil forms. They were cut by scoring them all the way around with a hacksaw, as opposed to sawing right through them.
cut end of tube
coil form
After cutting the tubes, the ends were very rough. They were polished by sanding the ends with an electric palm sander, then wet sanding them with 800 grit sandpaper. The result was excellent. On the right are two coil forms and two reinforcing ends. The reinforcements are purely for cosmetic reasons and aren't needed with these tubes.
MRL #10 city coil
The completed coil. 90 turns of 22 gauge Double Cotton Covered wire tapped at 5, 10, 20, 40 and 65 turns for the primary, and 30 turns of 26 gauge enameled wire for the secondary. Notice my taps are sticking up in the air. I should have wound it much tighter, but the cotton covered wire is so rare and expensive I didn't want to attempt to rewind it. Instead, I painted it with diluted white glue (which was what was going to happen to it anyway).

In the MRL catalog this coil is the #10-A. The plans give instructions for a "city" or "country" set. The country version uses the #10 coil.
Coil mount
The MRL method of mounting the coil is also a single point of failure. If the coil is not securely mounted, it will droop and come into contact with the variable capacitor. Lock washers are used at both the front panel and on the coil.

I gave a scrap piece of coil form the "crush test." A nut and bolt were tightened down in an attempt to get the plastic to crack, which it did not. I was then able to mount the coil with two small wrenches as tightly as practical.


MRL switch
MRL switch
Here are two switch levers made by Elmer Osterhoudt. He "invented" this type in 1952, after the supply of antique commercially made switches was depleted. Replicas are needed for the No. 10.
MRL switch
MRL switch
MRL switch
An original MRL switch was carefully measured to get the dimensions. Knobs were made from 3/8" diameter dowel cut into 9/16" lengths, drilled and painted. The brass lever is .01" brass sheet from an arts & craft store. The holes must be drilled in the brass first, then the brass is cut with heavy scissors.

I magically ended up with two extra brass pieces. I could have doubled the amount if I had thought to measure the lines so they were equally spaced.
crystal set switch parts
Enough parts for six switches.
MRL switch
Six replica switches. I don't know how Elmer made them, but I know how he DIDN'T make them. He didn't cut the dowel with a hack saw and throw half the knobs in the trash. He probably cut a bundle of ten dowels with a band saw and made 100 in the time it took me to make one. I bet he cut the brass with a heavy paper cutter and made hundreds of levers at once.

He sold the completed switches for 25¢. Mine cost more than that just for the screws.
Two switches
Switches in use.
MRL switch
The switch as seen from the back of the panel. These switches are the ultimate in form and function. There is no actual spring on the back, the "springiness" of the brass creates the required tension on the switch points. You bend the brass slightly, insert the switch into the panel, set the nut on the back till the knob turns smoothly, then lock it in place with the smaller nut.