S-38C Restoration – Filter Capacitors

S-38C Restoration – Filter Capacitors

When the radio got to me, I looked all around it to see if there was anything that looked dangerous. Not seeing anything outstanding, I decided to give it a whirl. I took the power cord, unwrapped it from the little coil it was in, shoved it in a close by power receptacle and turned the power on. And…Woh-la! The radio buzzed like a buzz saw. So I instantly knew I had a filter capacitor swap-out ahead.

Multistage Filter Capacitor
The Old, Multistage Filter Capacitor – AKA “buzzard”

So the first thing I decided to do was make a list of the capacitors I needed to replace.I wasn’t exactly sure if I just needed to order all the capacitors or if there were just a few I needed to replace. So I decided to look around on the Internet to see what I could find. Surly some other soul has done this restoration and would have a list to shortcut this task.

Right!  Well…it looks like I need to come up with the list on my own.

One of the pages I read did said to leave the mica caps alone so that really helped a lot. I went to the S-38 guy, Brian Ripley’s pages and he said that he replaced seven tubular capacitors and four filter caps in his restoration. He had a little picture showing what he did, so I set out to duplicate his process. Problem was for me, was that he didn’t provide a parts list, and even after I blew up his picture, I couldn’t read the values so I gleaned as much as I could from his text. And I did snag the schematic and parts list off a site and took a gander at that as well.

First of all, I have to say that his page was probably was the most helpful in my restoration. I don’t remember anywhere else where anyone was talking about a CW problem being created if you simply move a wire that doesn’t look like it’s connected right or you ‘adjust’ a couple of other wires that look like they are simply holding a wire in place. He further goes into the explanation that that wire and all the other wires are mounted specifically that way because they create the BFO circuit for the CW. Any movement of that wire or any of the others associated with it can render the CW switch useless. Who’d thunk it!

Here’s his link: http://www.the-s38-guy.com – Thanks Brian!

Looks like he replaced the four section multistage electrolytic capacitor (the big beige tube with colored leads coming out.) I pretty much followed what he did here since I was sort of in a hurry and really wanted to see if the radio did anything except buzz. He replaced the large filter cap by clipping the original wires at the body of the capacitor and used the wires to attach to a terminal strip he installed. Sounded like a great idea, so I bought some terminal strips and cut the capacitor wires right at the base of the capacitor. That’s really cool as the wires are already going to where they need to go, and all I need to do is mount the new capacitors, which are smaller, and solder the wires to the correct places.

Filter Capacitor Strip
Filter Capacitor Strip

Here’s my value list for the  four section filter capacitor:  and the values of the  axial electrolytic capacitors I replaced it with in parentheses.

1. 60 mfd @ 150v (80 uf @ 450v)
2. 47 mfd @ 150v (47 uf @ 160v)
3. 47 mfd @ 150v (47 uf @ 160v)
4. 20 mfd @   25v (22 uf @ 450v)

Dang…I meant to take a picture as I was building the filter capacitor strip, but I didn’t. Here is the completed strip with the capacitors mounted and in place. There is a fourth, smaller, 22uf capacitor under the large one seen on the back.

I bought a terminal lug strip similar to the ones of that period and soldered in my four replacement capacitors. I needed a six terminal lug strip and could only find some with five of the style I wanted, so I bought them and cut two terminals off one of them and soldered it to the end of one of the others. I decided to run the wired of the capacitors through the holes of the lug strip leaving the top lugs for the wires. This better facilitated adding the partial strip to extend it since the wire helps hold the two strips together when soldering them. I built the terminal strip and capacitors out of the radio on the bench, so I placed all the capacitors on one side, to better allow access to the mounting lug.

I soldered the wires left from when I cut the old multistage capacitor out, to the lugs of the appropriate values on the strip. Once I did that, I could easily see where to drill the mounting hole in the chassis. I marked the location of the hole with a black sharpie and pulled the strip back away from the location. I selected a 1/8″ drill bit and sleeved it with a piece of plastic tube leaving only about a quarter inch of the tip of the bit exposed to keep the bit from suddenly dropping through when the hole was cut. I cleaned out all the shavings or chips from drilling with a magnet and proceeded to mount the strip. I placed a pop-rivet into the foot terminal lug strip mounting lug and fed it through the new hole in the chassis. With a “POP” the strip was attached.

Step 3: Now it’s time to replace the old cracked power cord.


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