High Voltage and Old Radios – Recapping The National SW-54

photo of The National SW-54 at Bussey's Flea Market

Marilyn and I enjoy the odd flea market ramble, and a trip to Bussey’s in Schertz, Texas was fun. It was our first time to this meet, and we didn’t really know what to expect. I wasn’t looking for anything in particular, but old tools, radios or cast iron always get my attention.

photo of surface rust on the radio
Cosmetically good with just a little surface rust

To my delight, there was one vendor who specialized in old radios and phonographs. Some of them were really old, and a bit rich for my blood. There was this one shortwave radio however — a National SW-54. The price was right, it was cosmetically pretty good, and appeared to be complete. I sensed a new trip down the rabbit hole!

graphic of National SW-54 Schematic
National SW-54 Schematic

Once home, I set to work researching my new toy. Made between 1950 and 1957, it was popular in its day and covered the AM Broadcast and shortwave bands up to 30 MHz. It is an “all American five” design — meaning it uses the same five vacuum tubes that nearly every other AM Broadcast receiver of the era used. It has CW (morse code) receive capability and a stand-by switch — perfect for the beginning ham in the 1950’s and a good radio for my small collection.

It languished in the shop for several months (always too many projects and too little time!) but I was finally able to give it some love. Old tube equipment can be tricky, and is almost guaranteed to have problems. This radio is the AC/DC “hot chassis” type, meaning it was made before modern shock hazard protections like polarized plugs were in place. One can get a lethal shock by simply plugging the radio in and turning it on.

photo of radio interior with parts that need replacement
Pretty clean, but the waxy paper and electrolytic caps will have to go

Looking inside, I found what I suspected were failed capacitors. Being at least 60 years old, the electrolytic filter capacitors were likely dried out, and the waxy paper capacitors were probably also faulty. I decided to replace them all before even turning the radio on. Luckily, I was able to find a user manual for the radio online, and it contained not only a schematic, but also a parts list. I made a list of the replacement capacitors I would need.

The radio has a multi-section electrolytic filter capacitor – often hard to find since they haven’t been in common use for several decades. The waxy paper caps are often replaced with polypropylene “orange drop” capacitors originally made by Sprague and now made by Cornell Dubilier. I found not only a suitable substitute multi-section electrolytic but also “orange drops” at Amplified Parts online.

photo of radio interior showing tight spaces
Tight spaces

There are at least two schools of thought on the process. In this case, there were 11 caps to change, and some of them were in very tight spaces. In a perfect world, one would remove the old capacitor by desoldering it from the terminal strips — leads and all. Given the tight spaces this can be difficult, and the other strategy is to simply clip the leads close to the old capacitor, form the remaining leads into small hooks and connect them with matching small hooks on the new caps. This is substantially easier but a little less tidy. Since this radio isn’t especially rare or valuable, I decided on the “hook” method.

One thing to note: non-polarized capacitors can still have a “correct” orientation. If you look closely at the old wax-covered paper capacitors, you will notice they have a black band on one end. This indicates the outer foil side of the capacitor and is usually oriented to the ground or lower-potential side of the circuit. The new orange-drop caps also have an outer foil side, but it is not indicated reliably. It’s easy to determine the orientation using a method described in a “Mr Carlson’s Lab” video. It was a new idea to me when I watched his video, but he makes a persuasive argument.  Take a look and decide for yourself.

photo of new capacitor installed
New capacitors installed

I was done in a couple of hours. This radio is very clean, but I spent some time disassembling the case and cleaning the parts. The plastic tuning window for example really benefitted from a little extra attention. The isolation stand-offs — intended to separate the radio ground from the case and therefore  avoid shocking the operator — seemed intact. Good news, limiting the extra work needed to make the radio safe.

Turning it on for the first time was fun but a little tense. To my relief, the tubes all lit up, and in a few seconds there was white noise issuing from the speaker. A quick-and-dirty wire antenna resulted in the reception of at least one AM radio station, but the radio seemed a bit deaf. Possibly a weak tube. The next step will be a new tube set and an alignment. Good practice for the Hallicrafters and Hammurlund I plan to work on next!

photo of Cleaned, recapped and ready for alignment
Cleaned, recapped and ready for alignment

2 thoughts on “High Voltage and Old Radios – Recapping The National SW-54

  1. Have you considered uni-directional polarity? With an old timer like this, it may provide more clarity in sound.

    1. Yes. The new caps are first tested (using the method mentioned in the article) for the shield side, which is then connected to ground (for bypass caps) or the “low” side of the circuit (for caps that transfer signal between stages). This minimizes the introduction of hum into the circuit, and in theory, improves fidelity. We’ll see, as this is my first experience with the practice.

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