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This is the chassis of my R-520 in its current condition. It arrived on February 19, 2004. From the pictures you can see that it definitely needs some work, and it will take some time to do it right. My first impressions The chassis is complete, but it is dirty and someone has made some modifications to it in the past. Notice the blue dial pointer.
Notice in this picture that there is a disconnected resistor and the dial lamp should be a neon lamp. Hmm... someone has been here before.
HORROR OF HORRORS, someone repainted the rear of the dial scale. It is supposed to be brown, but now is flat black!!! Some of the original paint is still visible on the edges, and the black is starting to flake off, so I think I have a good chance of reversing this mess.
I wanted to find out what the serial number of this radio was. Ordinarily the serial number was stamped on the asset tag on the front of the radio. Hoping to find it elsewhere, I turned the chassis over and around, looking for any kind of marking. In the SPCA manual there is an addenda on page five that indicates that R25 on the receiver was changed during production from 150 ohms to 47ohms. Serial numbers 1 - 2550 had the 150 ohm resistor, and serial numbers 2551 -7217 had the 47 ohm resistor. So I started looking for R25, but it isn't shown in the parts layout of the manual. Checking the schematic showed that R25 was connected to L13 and C35B.
Very interesting, R25 on my set is 150 ohms!!! That leads me to believe that this set was probably manufactured in late 1952 or early 1953, and had a serial number below 2550. Does anyone know if there is any other way to date this radio??? If so, please e-mail me. Restoration notes Ok, while waiting for various parts to arrive for the cabinet restoration, I decided to start on the chassis. To keep the chassis authentic, I am only going to replace the electrolytic capacitors and the selenium rectifier. There is not a lot of extra space underneath the chassis though, so I will have to use a little ingenuity. I started with the 4 section filter capacitor, designated C45 A, B, C, and D. I removed the capacitor from the chassis and marked all the wires and components so I could get it back in correctly, but in doing so, I broke the insulated twist lock mounting plate. Oh well, good thing that I have a parts chassis. Once the capacitor was out, I carefully removed the yellow plastic shield.
Now that the capacitor was off the chassis, I hauled out my trusty Dremel Moto-tool, and cut off the top about an inch from the bottom. Then I removed the old guts, cleaned out the shell with some pumice orange hand cleaner, and installed 4 new caps. A little hot melt glue and a rolled up piece of cardboard secured the top back on. If you look carefully, you can just see the cut line in the metal can below the lettering.
Once the four section filter capacitor was re-installed, I replace two other electrolytic capacitors, then tackled the selenium diode. These diodes are known to fail, and they give off a fairly foul smell. To be on the safe side, I replaced it with a 1N4004 silicon diode. Now there is a fair amount of extra space under the chassis.
Now that all the necessary components are replaced, I started cleaning the chassis. All the switches got a good dose of contact cleaner, and the chassis was sprayed down with industrial electronic cleaner and scrubbed with an old tooth brush. Someone had also scratched up the rear of the chassis and applied some white paint. I carefully removed the paint with some goof off and wet sanded the heavy scratches out. Upon careful examination of the scratched area, I noticed that the words "US Gov't Prop" had been engraved by hand. Perhaps when this radio was decommissioned by the army, part of the process was to scratch out these words.
The hardest part of the chassis restoration was reversing the black painted frequency dial. Based on some input by other collectors, I set the dial in a container with just enough ammonia to cover the back of the dial without any flowing over onto the dial lettering. I let it soak for a day then scrubbed it down with a tooth brush and some pumice hand cleaner, then wet sanded all the nooks and crannies. After a couple days of soaking and sanding, muttering and complaining, I got most of the black paint off. Then I sprayed it with some metallic copper paint, and it looks pretty good. Unfortunately, some of the white lettering and markings flaked off during the process, so I am looking for a new frequency dial. I also lost some of the pictures I took of this process, so I only have a "before" and "during" photos to post.
Before I plugged this radio in for the first time, I decided to use a 1R5 tube instead of the 1L6 converter tube, just in case something happened, and run it on battery power. I powered up the radio, but nothing happened!! I did a voltage check on all the pins of all the tubes and noticed that on the 1R5 tube, pin 4 was 2.3 volts instead of 1.9, pin 5 was at 2.3 volts instead of 42.5 volts and pin 6 was at 1.8 volts instead of 20 volts. Hmmm..... that isn't quite right. I did some head scratching and measuring, and discovered that C11, a .01mfd capacitor across pins 1 and 5 was cracked open and was a high resistance. I quickly replaced it but still nothing. I tested the 1R5 in my tube tester, it was ok. Then I decided to check the specs of the 1L6 and 1R5 in my tube manual. It turns out that pins 1 and 5 in a 1R5 are internally connected together. Ok, so this tube doesn't work as a substitute for the 1L6 in the R-520 radio. I installed a 1L6, and bingo, the radio fired up! It is very sensitive and selective, so at the moment, I will not be doing an alignment to it (actually, I don't really know how to do an alignment, looks complicated, so I will ask one of my friends to give me a hand with that). I let it run for a while on the batteries to burn in, then tried it on AC with my isolation transformer, and it worked too.
With the chassis restoration complete, I started on the RF shield. The metal part of the shield is made out of aluminum, 7" long by 4 15/16" high. Mark P. was kind enough to send me a tracing of his shield, which I will post in the links section soon. I traced out the design on a Hammond chassis cover plate, and cut it out with a jig saw (the dremel is broken at the moment). I filed down all the sharp edges then sanded the surfaces and sprayed it with two coats of flat clear lacquer to prevent corrosion and to dull the bright shine. It looks pretty close to original, except for the pop rivets I used to attach the plastic part of the shield. I will change that when I find the correct type of rivet tool.
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