Wednesday, December 25, 2019

New Life For An Old Radio

Back before I got into 3D printing, I used to collect and restore old vacuum tube radios and phonographs.  I was particularly fond of wood cabinet table-top radios from the 1930s because they often used very beautiful, artistically bent plywood for the cabinets, sometimes with exotic wood veneers. The circuits were not too hard to repair once you learned which type of parts failed the most.

Believe it or not, before TV became popular, and for a little while after, there used to be shops where people actually earned a living by selling and repairing radios.  Those shops used to subscribe to manual publishers that provided schematics and tuneup instructions for new radios as they were released by the manufacturers.  The two main service manual companies, Rider's and Beitman's, produced multiple volumes of radio repair information, all arranged by manufacturer, model, and date.  I have a couple volumes of the old paper manuals in a box somewhere.  You can easily find them on the web- here's a link and here's another, and you can download a full set of the Beitman's manuals via the internet archive, here (finally, something you can download legally with bittorrent!).

One of my favorite restorations was a Zenith 5S-127 radio made in 1937.  It has a large round dial of a type that is very popular with old radio collectors.  IRIC, I paid about $35 for it at an auction about 25 years ago.

When I restored old radios, I would restore the electronics first, then the cabinets.  Repairing the electronics was sometimes a little difficult and tedious, so I used the cabinet restoration as a motivator to keep going on the electronics.

Some of the service info for the radio from the Rider's manual.

A schematic diagram from the Beitman's manual.

A schematic from the Rider's manual.

My restorations weren't museum quality- I didn't try too hard to preserve the look of the original components and chassis.  I was more concerned with restoring the function of the electronics, and then getting the cabinets looking nice again.


Old radios had all sorts of parts that were prone to failure.  Electrolytic capacitors in power supplies often dried up and shorted or just stopped working as capacitors.  There were also a lot of paper and metal foil capacitors, usually dipped in beeswax, that would fail shorted because the paper would decompose.  They used a lot of carbon composition resistors that would drift upward in resistance over time.  That would cause the bias voltages on tubes to drift out of range and the radio would quit working properly.  Tubes would fail by becoming "gassy", developing shorts, or just burn out.  Even wire would fail because it had cotton insulation that would get chewed on by insects or rodents, or rubber that would harden and crack.  Speakers were often destroyed when something poked through the cloth cover and tore the paper cone.  Water damage often wrecked speakers and cabinets if the radio was stored in a barn or basement.

It's pretty easy to find modern replacements for all those parts that will last a lot longer than the originals did.  Modern electrolytic capacitors are made better, but will still probably always be the most likely parts to fail.  Old radios from the 20's sometimes used electrolytics caps that were just a couple uF.  Those and the paper and foil type caps can be replaced with non electrolytic mylar film caps that will probably last hundreds of years. Carbon composition resistors can be replaced with modern thin film or metal film resistors. Wire can be replaced with modern stuff that uses PVC insulation, but I often use PTFE (Teflon) insulated wire that I expect to last a lot longer.

Transformers can be hard to replace, and sometimes speakers are also tricky, especially those that had electromagnets instead of permanent magnets.  The magnet coil in the speaker was often used as a choke in the power supply, so if you replace the speaker with a modern, permanent magnet type, you have to keep the old speaker's magnet wired in to keep the power supply working properly.


Tubes are still relatively easy to replace - they were made by the millions, but you need a tester to check them.  Back when I was doing radio restorations, I found a military surplus TV-7 tube tester at a swap meet for $50.  Shortly after I bought it, tube mania hit the audio world and suddenly tube testers were in big demand by tube audio fanatics. Those people have deep pockets, and they drove the prices up, so now my TV-7 is worth about $800.

You can still find reasonably priced tube testers that can test commonly used radio tubes, and you can still find reasonably priced tubes to test. 


One of the ways that radio manufacturers tried to distinguish their radios from the competition was to use fancy dials that sometimes used complex arrangements of pulleys and cords (and even motors) to move the dial pointer when you turned a knob or pushed a button.  Zenith radios are very popular with collectors because they really went all-in on the dial design. 

Here's a video of one of Zenith's "shutter dial" console radios that can sell to collectors for thousands of dollars:

And here's the high point of Zenith radio design, a 1935 Stratosphere console with 16 tubes!

Dial Belt Replacement

My Zenith radio used a cloth belt that was somehow rubberized, and of course, was broken when I got the radio.  I replaced it using the common technique at that time (20 years ago, not 1937)- I cut an o-ring to the appropriate length and glued the ends together.  I think it lasted a couple months.

The radio has been sitting on a shelf in my office for years, gathering dust, staring at me, and daring me to try to repair the dial again.  Today, I could resist no longer, so I took the radio apart and measured the o-ring, then drew a replacement in CAD:

CAD rendering of the belt.  Do you think you could handle something like this?  3mm wide, and 1.2 mm thick, 71mm inside diameter.
... then I sliced it:
Yeah, a pretty complicated slice, too.  Not for beginners!
...and printed it using TPU filament.  It took three attempts to get the size just right.  Once I had the belt fitting properly, I dusted off the chassis and reassembled the radio.  TPU is good for this application because it is flexible and will stretch a little under tension.  It is super strong and will not break, ever.

Printed TPU belt (orange) mounted on the radio.  The radio is geared so that turning the tuning knob turns the tuning capacitor slowly and moves two dial pointers at the same time.  Pay no attention to the dust on the chassis.
When I powered it up I found that it was working quite well and the dial calibration was even pretty accurate.  The whole process probably took about an hour.

3D printing can be used to replace all sorts of hard or impossible to find mechanical parts in old radios.  Knobs, pushbuttons, dial parts, etc., can all be replaced with printed replicas. 

Maybe it's time for me to start restoring old radios again.  The purists who go for museum quality restorations will never approve, but us regular folks, who want the radio to look and work like new, won't mind if there are some replacement parts inside.


  1. My old GE Model 202 has play for many years but now has no sound at all. All the tubs light up & the dial does also, but no sound. Any idea what the problem could be? Thanks much..bill.

    1. Start with the usual suspects- electrolytic and wax/paper caps, then check resistor values. It's probably a good idea to check the tubes, too, if you have access to a tester.

      This page has links to service diagrams, etc.:


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