My Radio Update: 12-25-09 to 1-13-10

I've had this radio for a few months but finally started working on it Christmas Day.

It's this one:



And from the back,



I'm working on the amplifier first. This radio cost $800 in 1942; in today's money that would be around $10,000. Only the best and most expensive radios of this day had dedicated chassis for each section; this one has a dedicated amplifier and power supply, a dedicated tuner, and a phonograph. The models one or two steps up (up to costing $2,000 in 1942) had dedicated amplifiers for both woofers and tweeters instead of a single amplifier for both. Most radios, however, cost around $50 back in the day and had a single chassis for tuner, amplifier and power supply which was considerably less powerful overall.



The schematic for the amplifier section. Not too bad, yet...

To restore an old radio, the most important thing is the capacitors. They just weren't built the same back in the '40s as they are today; they will almost all have gone bad and connecting a radio with bad capacitors can destroy the power transformer, expensive tubes, burn out resistors and coils, and generally wreck all kinds of trouble all over the place.



A sampling of capacitors I took out. I'm replacing them with modern replacements from Mouser:



Partially replaced (all the bottom-side capacitors):



The top-side capacitors are wired inside of metal cans. I'll be doing this to them:



which ultimately comes out looking something like this:



I'm measuring the capacitance of this old electrolytic capacitor. The "1" means it's a dead-short, connecting it in this state would probably have caused the power transformer to overload and melt something.



That's what the bottom of the amplifier looks like after replacing all capacitors.

Now, I move on to start testing:



I put the tubes back in, brought out the speaker (15" coaxial two-way), and hooked it all to the Variac with my multimeter to verify output levels.

Unfortunately, after firing it up, I got nothing. So I check the power transformer out, and it's not looking so hot:





Showing signs of having overheated previously. This is confirmed by the fact that the primary is shorting to the iron core; one of the 4 secondaries is also open. It's probably toast. I've tried checking the different windings. Several people suggested I had the leads labeled wrong, but after spending about 2 hours with the schematic, I've triple-verified that is not the case and the part is actually dead.

Hammond Manufacturing produces a transformer (for tube guitar amps) that I can use as a near drop-in replacement with adding just a couple of resistors or running at 115V instead of 117V. So I'm probably going to buy a new one of those.

Here it is opened up a bit:



This transformer is odd because it doesn't have "taps" so much, the coil windings just extend downward and become the tap leads. Not the most serviceable design, that's for sure.

Readings, if you're interested:

My Lead #               Type               Voltage              Ohms
1,2                     Primary            Input (117)          OPEN
6,9                     Secondary          345                  69.2 Ohm
5                       Sec (Center)       345                  OPEN to 6,9
3,4                     Secondary          5                    0.6 Ohm
7,10                    Secondary          6.3                  0.8 Ohm
8,11                    Secondary          6.5                  OPEN

Incorrect Continuities: 2,8 -- 0.02 Ohm; 1,5 -- 2 Ohm

While I wait for parts, I'm moving on to the tuner. It has 35 capacitors, but not all of them will need replacing as some are a silver-mica design which basically never goes bad.





That's a molded mica capacitor. (Not to be confused with a "Micamold" capacitor, which is actually just a paper capacitor inside of a plastic housing molded to look like a mica capacitor.)

I'd say I'm approaching the 50% mark on this project.