This is my first project, a 1940 Coronado model 907. This was originally a “high end low end” receiver: it only receives the AM broadcast band, but has three tone settings and mechanical push-button presets. It also uses 7 tubes when most common low-end sets used only five or six; more tubes = better quality. As you can see, it was a bit on the dusty side when I found it, having sat in someone’s basement for decades until they passed away and it was cleaned out at an estate sale. It cost $20, I didn’t want to spend much for something at this point.
Fortunately all the tubes are inside. When a set stops working properly due to “old age”, passive components wearing out, the tubes generally are still good. In this case 6 of the 7 checked out just fine and I had a spare handy for the other.
The most common mode of failure in an antique radio happens also to be the easiest to fix: bad capacitors. Manufacturing technology from the early days of electronics left a lot to be desired and after some time these would dry out and start leaking electricity from one side to the other. When this happens, the set will not play music, only playing a loud 60Hz hum independent of the volume level. If the radio is allowed to continue to run in this condition, will rapidly cause the transformer to short out and potentially destroy other components with it. Never, never plug in an antique radio if you don’t already know for a fact it works or has been reconditioned lately or you could cause permanent, expensive-to-repair damage. Or worse, start a fire. Even if you unplug it before it burns up, antique radio smoke can have some nasty stuff in it and smells horrible.
Moved to my workbench. The chassis is pretty once taken out of the dirty cabinet; it was well stored. Many antique radios have become rusty or infested with rodents at some point during their lifetime, but this one is remarkably clean.
The under side of the chassis is a bit less pretty to look at. The dark-tan cylindrical things are the old capacitors, which are almost universally leaky at this point. It’s a good idea to replace every capacitor in an old radio, if they haven’t failed yet they will soon. The colorful components are molded “dog-bone” resistors. The bright orange squares are new, modern capacitors I’ve put in to replace some of the old ones. Seen in this shot to provide a contrast between old and new.
Here, even more capacitors have been replaced with modern versions. These “orange drop” metalized polypropylene capacitors will probably not need to be replaced again during my lifetime, especially as the duty cycle on them is going to be very, very low.
The old parts I removed – capacitors and some extra solder. The largest one is a multi-section capacitor used in the power supply; the others are small signal capacitors used to help route electricity around the radio to different tubes depending on what task needs to be accomplished at the time.
Repaired and ready to put back in the case!
On the shelf, ready to display! I managed to get most of the grime off using a vigorous rubbing with Goop, followed by furniture polish – but it still needs to be stripped and sanded. That’ll be a project for better weather, Seattle winter in an apartment is not the best for doing work on furniture.
After repair, the radio plays loudly and with great tone. It’s a solid performer. There are some resistors that will need attention, but I was rushing this one through because I wanted to have it done to show off. I’ll probably do a follow-up to this one later to finish the job.
RETROVOLTAGE 
Cost Engineering in the 1930s
It looks like cost engineering was already happening back in the 1930s:
In the foreground, the destroyed original power transformer from the 1939 Zenith 7-S-363 radio; the rear, an equivalently rated replacement built in October 2010 by Edcor Transformer. In order to squeeze out a few cents cost savings per chassis, Zenith under-rated their power transformers and ran them right at the limit. In certain modes of tube failure, or just age and components wearing out, the power transformer’s razor-thin safety margin would be exceeded resulting in overheating, shorting and melting. Which is exactly what’s happened on the one I’m repairing at some point in its life.
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