Introduction: Fixing a Mystery Radio

I acquired this radio many years ago and never tried to get it working because I didn't know what make it was (no stickers, name plates or model numbers). Not having any of this information made it virtually impossible to find a schematic diagram for it. In the past couple of weeks I have had a lot of time on my hands convalescing from an illness so I was determined to do some detective work and find out what I could about this radio.

Firstly, I noted the tube numbers and dated them, they are 43, 44, 77 and 25Z5. The 43 tube came out in Sept. 1932, the 44 tube came out in April 1932, 77 tube in March 1933 and the 25Z5 in April 1933. This means that the radio can't be any older than April 1933.

The style of the radio is similar to Crosley radios that came on the market in the 1932-33 and 1933-34 model years and even has the Crosley style dials, but I am sure it isn't a Crosley because they didn't use this tube compliment. All the capacitors are Micamold brand whose manufacturing plant at that time was located in Brooklyn, New York. The variable capacitor was also manufactured by another company in New York. There is a good possibility that the radio was manufactured by one of a number of New York radio manufacturers that disappeared in the depression.

Taking a look at the radio innards there are no IF bins, just a two gang variable capacitor and two coils that are peaked with two mica trimmer capacitors, one on top and one on the bottom of the chassis. This radio is a TRF or Tuned Radio Frequency a style that was popular in the 1920's and only seen in low-end radios by 1933. This radio was probably selling for around $19.00 when new, which was a week's wages for an average worker. This radio was low priced for the time, but there is nothing cheap in its construction, it has a sturdy metal cabinet and quality components excepting the Micamold capacitors which were considered to be poor quality by technicians. Manufacturers made radios cheaper by using simpler circuits and saved on a power transformer by having the B+ coming directly from the power line through a rectifier, and filter consisting of two capacitors and a coil around the neck of the speaker and a "bucking coil" (which cut down on the hum by being wound in the opposite direction from the field coil. The tubes were in series adding together to produce 72 volts of a voltage drop with an additional 40 volts dropped in the resistance wire in the power cord. Radios of this type are called "curtain burners" because the power cord gets slightly hot when the radio is running.

Step 1: Parts, Supplies and Tools Needed

1) Long nosed pliers,
2) Electronics Solder and Soldering gun (If soldering gun is used it should be at least 40 watts)

3) Digital multimeter (I used a Vacuum Tube Voltmeter that I restored in an earlier Instructable)

4) Side cutters,

5) Wire strippers

6) Various types of insulated hookup wire. (green, red, blue, black)

7) Assortment of 400-600 volt rated capacitors, .005, .01, .02, .04, .05, .1, .2 uF.

8) Electrolytic capacitors rated at minimum 250 volts and 6-15 uF. 9 )Rivet gun and rivets.

9) Small wrenches, assorted screwdrivers

10) Solder sucker

11) Wire strippers

Step 2: Evaluating the Radio

Considering that this radio is almost 90 years old, it's in extremely good original condition. The faux walnut burl paint job is still shiny with a minimum of chips. It has its original grill cloth on the front and the back of the metal cabinet and the box is designed to come apart with the cord detaching via a plug in the back. The metal cabinet has four brass colored legs that unscrew and so that the chassis can be removed from the box. The metal chassis is in good shape and the coils are solid and have been undisturbed. The variable capacitors are very clean but the potentiometer has been replaced and wired in wrong so that it doesn't control the volume, it plays only at full volume. It doesn't fit the dial very well so it will need to be replaced. In the past somebody started to restore this radio but gave up. The filter capacitors have been replaced but all of the capacitors will need to be replaced before the radio will play properly. The speaker cone has been repaired with speaker cement but seems to have come away from the metal part of the speaker. This will need to be glued. I checked all the tubes on a tester and they are all good. The cord is in good shape except for one small section where the fabric has come off exposing the internal wires. This isn't an electrical hazard but more of an aesthetic issue.

Step 3: Resoldering All Connections and Replacing Capacitors

In a radio this old, the soldered connections cannot be considered to be trustworthy any more. You never know what type of flux they used in those early days. Many times I have heated up the solder joints in an antique radio and it smells like they used acid core flux. Over time, these solder joints oxidize and don't make good connections. By heating them up and adding some modern solder, you can make a good solder joint again. I resoldered all the connections in the radio and changed out all the paper capacitors with good ones. When I take apart old electronic equipment, I save good capacitors and resistors for this purpose. If I don't have the right value I will put in a modern equivalent. The voltage ratings should be at least 400 volts.

Note: The top right hand picture shows an old wax/paper Micamold brand of capacitor from 1933.

Step 4: Removal and Re-installation of Rear Socket

There were cold solder joints that I couldn't reach because the rear socket was in the way. In order to access them I needed to drill out the old rivets holding the socket in and solder all the connections around it. I then re-riveted the rear socket to the steel chassis.

Step 5: Checking Resistance Element in Cord

The four tubes have voltages that consist of 2-25 volt heaters and 2-6.3 volt heaters adding up to 72.6 volts at .3 amps. In order for them to be across the total line voltage, the resistance cord would need to drop 40 volts across it adding up to 112.6 volts. In addition there is a 30 ohm power resistor in series attached to the chassis. This resistor would drop an additonal 9 volts adding up to a total of adding up to 121.6 volts. The resistance of the cord should be 40/.3 = 133.3 ohms. I checked the cord resistance wire and it was approximately 130 ohms. With some more checks with the ohmmeter for shorts, I was now confident to plug the radio in. The tubes at first didn't light up and this told me that the plug end of the heater was connected to the wrong side of the plug. It and the tubes were in series and they needed to be wired from hot to neutral. The plug that plugged into the back of the set was wired so that that AC would flow no matter which way the two prong plug was turned but the filaments would light with the end of the resistance cord tied to only one of the prongs and not the other.

Step 6: I Found Out the Name and Model of Radio

During the process of writing this Instructable, I happened to be looking at radios on Ebay and saw the identical radio for sale. The seller had one that still had the nameplate on the front and model number on the chassis. It was made by a company called Simplex (which I was familiar with) for a company I was not familiar with. The radio was also sold by Simplex as their model V with the same circuit diagram. The company that it was made for was a New York based tube company called Goldentone and they sold this radio as their model 41 TRF. I don't know if this one is a Simplex V or a Goldentone 41. At least I now have the schematic diagram and information about the radio. I was right about this radio being a 1933/34 model.

Step 7: Replace the Potentiometer

The potentiometer that was in the radio was the wrong one and had been installed improperly resulting in it burning out. The result was that when I turned on the radio, it played at full volume. The schematic diagram called for a 100M potentiometer shunted with a .1 uF capacitor. The the old style of resistor measurement 100M was what is now called 100k or 100,000 ohms. I had a potentiometer that was a bit higher in value but was of the exact same style with an on/off switch. I tested it and knew it would work so I wired it in. All I had to do was cut the shaft to the same length as the variable capacitor shaft so that they would protrude the same amount out of the cabinet. The potentiometer worked fine once it was wired in according to the schematic. It varies the amount of cathode current that goes through the first tube. Not the best design for a volume control! The performance is what is to be expected of a radio of this simplicity of design but has surprisingly good volume considering a small speaker of this vintage.

Step 8: Gluing Speaker

This previously repaired speaker doesn't look like much but considering it's 87 years old and sounds pretty good, I decided to not do anything to it except gluing the cone back to the metal in the back. This gave it a bit more stability and gave it a deeper sound. Hadn't I visually inspected it, I wouldn't of known from the sound which wasn't bad at all.

Step 9: Evaluating the Radio's Performance

This radio originally came with 25 feet of antenna wire but now had just a few inches sticking out of the back. I used an alligator clip connected to a spool of wire that was reeled out inside a house for about 25 feet and the performance of the radio was quite good. I live near a large city with lots of AM stations so I could pick most of them up without any problems. The selectivity of this radio isn't very good so 1410 was difficult to get between 1470 and 1320 but with some adjustment of the tuner it was possible to hear 1410. The original advertisement says it can get 200 miles and I guess this would be possible with an outside antenna and less stations on the dial. Overall, the radio performs well considering the simple TRF design and its age. The tone is very good due to the wide bandwidth of the two tuned circuits.

Here's a short video of it working:

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