Como/General Radio IF transformers

     Here is a nice little receiver that I purchased a few years ago. It’s a Como breadboard superhet. The kit itself, or possibly factory wired radio, had six each UV-199 tubes. There is a separate set of binding posts on this part of the radio. Accompanying the six tube kit is a three tube Daven resistance coupled audio amplifier that uses UV-201A tubes, which makes a nine tube receiver out of the entire affair. The two meters add a lot of charm to this old set. Here are some of the particulars of the set:

1. This radio operates on the standard 1920s superhet circuit.
2. Como 30 KC filter transformer, oscillator coupler, antenna coupler, and bypass capacitor.
3. General Radio IF transformers (type 271), rheostats, and tube sockets.
4. Daven three stage resistance coupled audio amplifier.
4. Claratone meters.
5. Tuning caps are unknown but they are high quality (possibly General Radio?).
6. Front metal panel is 16” long by 4” tall by 1/16 inch thick (continues under the set for a base panel).
7. Baseboard or breadboard measures 26” long by 12” deep by .5” thick.

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     General Radio advertisement from the June, 1924, QST Magazine.

     Unfortunately, none of the General Radio 271 transformers were good. This type of transformer seems to have a high failure rate, most likely due to the hard tar-like substance that they are potted with. In addition, the Como filter transformer was peaking at 55.5 KC instead of 30 KC. The transformer is tuned by the primary inductance and a fixed primary capacitor, so I looked at those components first. The primary capacitor was marked .00025 and it measured .000312 mfd. The value was a little bit high but the capacitor was probably good. The primary inductance measured 11.5 mH (secondary measured 142 mH). If the primary coil is good then a capacitor of about .0024 mfd will be required to resonate at 30 KC. Was the value of the primary capacitor off by a factor of 10? That is exactly what I think the problem was (which rules out factory wiring). I tested the transformer with a .0026 mfd capacitor and it worked perfectly! This type of problem seems to be fairly common in the 1920s superhets. Something is wrong with the frequency determining components of the filter transformer and the original builder does not have the right equipment to measure it. The table below shows the bandpass measurements of the filter transformer with the original .00025 mfd capacitor and the .0026 mfd capacitor. The transformer was measured out of the circuit with no grid leak components and A- as a secondary reference.

     At right is a photo of the Como 30 KC filter transformer. This transformer defines the Como superhet perhaps more than any other part does. The transformer warmed up during use and some of the sealing wax oozed out of it. Its cleaned up a little bit in this photo but you can see the wax in a couple of the other photos. The transformer is still good.

     Below is a copy of the factory wiring plans for the Como superheterodyne (courtesy Dale Davenport). The plans are for a nine tube set with push pull audio amplification. My set is identical to the first six tubes part of the set, but then my set uses the Daven amplifier instead of the push pull amplifier. Compare the top view of my set above with the factory wiring layout plans below. This is an interesting set that doesn’t appear to be very common. I do not plan to restore this radio. I’m just going to preserve it and enjoy it for what it is.

     Como bypass capacitor.

     The General Radio type 271 IF transformers.

     Como antenna coupler.

     Above is a topside view of the set. When I got this radio one of the brass-base tipped 201-As was missing, and another one was broke, leaving only the brass base. The tube had been broken for a long time, which makes me believe that all the tubes were the original tubes. The set has eighty-five years worth of dust and soot, but it’s in remarkable condition considering it all. It must have been in some type of protected environment for all those years.

     Como oscillator coupler.

     Daven three stage resistance coupled audio amplifier.

     Claratone meters.