The Zenith model 37A Radio/Phonograph


The 1928 Zenith 37A was one of two radio/phonograph combinations Zenith made for 1928.
It and its Big Brother, the large 40A were the first combinations of such available from Zenith.
There were two versions made. One with 8 tubes using a #10 output stage, the other using 9 tubes and a #50 in the output stage. The circuit was typical of the time, a standard AC tube TRF with 3 stages of RF amplification, a detector and two or three stages of AF amplification depending on which version.

This particular 37A was an unfortunate victim of someone who wanted a more modern phonograph and didn't care about the radio function.
The tone arm was replaced by a late 1930s style arm with a crystal pickup, the chassis was stripped of most of the RF circuitry and rewired to be a multi-stage resistance coupled amplifier and lastly, the original speaker was replaced with a FADA brand unit.

The following images are as I found it on Saturday April 28th, 2012.

As you can see, the phonograph compartment is a mess. Missing the original tone arm and the platter.
The speaker is from a FADA

That is a pleasant thing to see. The 50 is still intact!

Nope, the 37A did NOT use a 2A6 which is a 6 pin tube.

With the chassis removed, I could see the extent of the carnage. All but one RF coil is gone along with their shields. All audio transformers are gone, the detector grid-leak is gone as well as the detector socket.
Instead, is a haywired audio amplifier...

Well, first off, all that must go.

Once all that has been removed, I'm left with a bunch of old crumbling rubber covered wire.
Well, I don't want to use that either, so this poor thing will be mostly stripped down.

Here is the schematic of the radio. Everything that is highlighted in yellow is gone. Strangely enough, I could see the ghosts of five transformers on the chassis. It turns out the schematic does show FIVE separate audio transformers.

I drew this diagram depicting where the transformers appeared to be. The one in the dotted lines I wasn't too sure about, but it is the only place it could go.
Fellow Zenith enthusiast, Martin Blankinship, was able to confirm my suspicions by taking a photo of the underside of his 37A chassis.
Well, as it turns out, I was fortunate to have an extra chassis from a Zenith model 34P. It uses the exact same RF coils and shields as the 37A, however it has different audio transformers. bummer... Well... At least I have a start!

I started by tearing out what was left of the rubber wire and transferred the detector grid-leak, socket and coil. Also started wiring the heater wiring for the 27s.

Next, made a new cable harness to go between the chassis and power supply. Added one of the other RF coils.

I quickly noticed that the rivet holding the connection point of the coils was loose on all coils. Better to get these secure before it causes any problems.

This is a list of the Zenith radios in my collection. It may become this page one day with a link to each and every one, but that is a rather ambitious undertaking.

More to come