Over Easy: The Art and Technique of Radio Restoration
Radio restoration or rather the restoration of antiques radios is hobby unto itself and there are many who engage in this past time, not all of them geeks or techies. It often encompasses more that a knowledge of radio and electronics. Mechanics and wood working skills often required as well.
I have just finished the mostly complete restoration of a communications receiver, a Hammarlund HQ 129X manufactured in the late 1940s, after WWII. The procedure I use for most of these projects follows with safety as the primary consideration.
First thing to bare in mind is that these radios are HEAVY ! This one weighs in at excess of 50 pounds in the cabinet. So get help and be careful. Getting it up on the bench, I had to make sure there was nothing obstructing my route or view. The next step was to remove the chassis from it cabinet and set the chassis on a separate work table and visually inspect [and sniff] for any thing that might have failed. Transformers and resisters and capacitors can and often do give off a unique aroma when they over heat and fail. They also leave visual traces of this. Carbon and dark brown stains around the part(s) or oozing on the part near on the chassis. Make note of any parts that “look” like they may have failed, also any that may be missing. Knobs as well, they can be hard to find the correct ones. One person to restore an old Zenith – which used wooden knobs – turned down a complete set on his lathe.
If looks like and smells like the power transformer has bit the dust, this generally not a big problem as replacement power transformers are usually available from many sources, even an exact replacement. If necessary the original can also be rewound from people who still do this, also available on the web. If you are planning to restore an antique Philco or Zenith or RCA or some other consumer radio it’s a good idea to check the speaker at this time as well. It may need to be reconed. And be advised that many of these radios, if fact most, had a field coil on the back to supply the magnetic field for the speaker to work. What is known as an electro dynamic speaker, very common since large permanent magnets necessary were difficult to make and expensive as well. Making sure it has not gone “open” by checking it for continuity. You simply cannot just replace the speaker with a new current one. The field coil on the speaker was an integral part of the radio’s power supply and oft times had what was known as a “hum buck” coil as part of it to suppress any AC hum in the audio.
Also you never know what has or may have taken up residence under the chassis while it has been in storage. My Hammarlund had cobwebs and what looked like small spider eggs.
After being satisfied that there had been no major failure the next step was to consult the schematic diagram from the service information that I downloaded from the web. In this case it was a very high resolution copy Photofact from Howard W. Sams. There are original service manuals available from the Boatancher Manual Archives but these often are of poor quality and the schematics difficult to read. At this time I make note of the number and value and voltage rating of the paper and electrolytic capacitors used since I will be replacing them all. Paper capacitors are just that, they use paper as the dielectric between the conductive foil. Paper was the common type of capacitor dielectric for values of .001 MFD to 50 MFD and voltages up to 600 Volts but the paper would decompose over time causing failure and electrolytic capacitors used for filtering in the power supply will fail as the dielectric electrolyte dries out and the foil sheets corrode.
These types of capacitors where superseded by ones using special ceramics and plastics in the early 1960s and haven’t been manufactured since. I uses ceramic capacitors rated at 500 volts or more through out. And made up a replacement for the multi-section electrolytic that was originally there with new type high voltage filter capacitors.
After this was completed I did a final check with the rectifier tube removed to make sure the power transformer was OK and check the vacuum tube hear voltage. All OK. Replaced most of the vacuum tubes with known good ones and plugged it into a variac so I could bring up the line voltage gradually. All voltages appear to be on the money but still no joy. Seems while replacing the capacitors I knocked a resister loose that had been attached very badly by a previous repair. Decided to simply replace the resister. Now it works and at fist listen quite well.
It’s at this time I did some updates to it by adding a 3-wire grounded power cord and an AC line filter to eliminate any power line carried noise. For tuning purposes I also added a socket attacked to a buffer for the local oscillator so I could attach a digital frequency read out. I am not that into originality like many restorers are.
Some getting VERY intense into the originality part indeed. Either buying or making their own multi-section electrolytic capacitors by gutting the one(s) that were there and stuffing the aluminium cans with new capacitors and even denuding the original paper capacitors and stuffing new ones into the cardboard sleeves and sealing them up again. I saw where one enterprising individual removed ALL the parts and completely rebuilt the whole radio after cleaning and polishing the chassis !. Even rewinding all the coils.
And did I mention mechanical and wood working skills. Here a restoration of a Philco 1938 floor model. We had this model at one time. It was from my great grandpa Hofner. It played great and looked like the “after” pictures on that site.
I still have to get some gloss black paint to paint part of the front but I was fortunate that the HQ 129X was in decent shape physically for it’s age. 70 years old. That I would look so good at that age.