12 tube chassis 2038

The 617 is one of the higher end radios of the 1933 series. It uses the 12 tube chassis that was considered one of the "Interocean" designs.
Above is a photo of my 617 sitting next to my 619. These radios use different chassis, but the circuitry used is almost identical between the two.

Back in 1983, while visiting Fair Radio Sales in Lima Ohio, I discovered Morgan E. McMahon's book "A Flick of the Switch". On page 188 is where I found the BEAUTIFUL Zenith 441 radio. I had hopes of finding one ever since. The 617 shares the same cabinet as the 441.

Of course, looking up the model 441 schematic showed that the chassis used (2033) was Zenith's best chassis for the 1933 line.
It took less time to find the 619 which has this chassis along with the twin 12" Jensen D9 speakers it uses.

But this page is about the 617. It uses the "Interocean" 2038, 12 tube chassis which as it does use pretty much the same circuit as the 2033, the chassis isn't near as fancy.
The 617 uses a pair of 10" UTAH brand speakers which are very similar to the UTAH speakers used pretty much across the 1933 line.
The circuitry is a conventional Superhet, but has a seperate AVC #57 tube wired as an amplifier. It also has a #57 used in a QAVC circuit which is essentially a squelch curcuit similar to that used on two-way radios. It was used as an interstation noise supressor which basically muted the audio between stations.
The QAVC threshold is adjusted by the bottom center knob on the front panel. While the audio is muted, the Shadowgraph tuning meter is still functioning and will indicate when tuning across stations.

The output stage uses a pair of 59 output tubes running as Class A triodes. These are driven by a 3rd 59 again, wired as a triode.
Zenith called it a "Class AAA" circuit, which today is called Class A2. It achieves a higher power output by driving the grids of the two output tubes positive. The power required to stay distortion free is significant when driving the grids positive, so a power driver tube is needed. In some ways, this is similar to a Class B amplifier in the fact that the driver stage and audio interstage transformer is similar. But this is purely Class A.

Here is my 617 loaded in the van just after I bought it.

It showed up on Craigslist near Baltimore Maryland. Many thanks to a friend on the Antique Radio Forums for pointing it out to me! (Thanks Rocklandman!)
On Friday October 3rd, 2014. I made the trek out to get it.

The fellow I bought it from claimed it was electrically restored by another radio collector, but upon inspection under the chassis, I found it has been pretty much untouched! THIS is the way I like to find them!
I'm not sure what was done, but the few replacement parts inside are all 1930s/40s vintage. I've confirmed the caps have not been restuffed, so that remains a mystery. Not that I'm complaining, thank you!

Unfortunately, the Bradleyometer volume control was replaced... Perhaps this is all that was done?
It does have a rather nasty looking cap strapped to the power transformer that is certainly not original.

Must find a Bradlyometer control. That is something I prefer to have original in these radios. Part of why I like the 1933 models. :)

Cosmetically, it needs cleaning, some new Grill cloth and a little repair work on the wooden grillwork at the bottom.
This cabinet is absolutely amazing in the design! I wish I knew what style you would call this? The 619 is considered a Louis XVI style of furniture. The 617 is similar, but yet different. Perhaps a mix of styles?

The cabinet cleaned up well. The top was speckled with tiny dots of paint. I suppose it didn't get covered up properly when the owner was painting the room? A good going over with Gojo hand cleaner removed all the specks and left it nice and smooth. It also cleaned up the dirt around the controls as it was clearly used a LOT!

It cleaned up rather well

The speakers are a bit rusty. Hopefully no open fields as one of them is center tapped to provide voltage to the screens of the tubes.

Interestingly, the tag on the back makes absolutely NO mention of manufacturer. Usually the 1933 models have either Zenith or Interocean Radio Corp. printed on them.
This one only has a small I.R. in the lower right corner. Must be for Interocean Radio.

These tags Zenith used in 1933 seem to be confusing. Of course, they didn't use a Zenith branded tag on an Interocean branded radio, but I've seen an Interocean tag used on a Coronado 530 which is the version of the 617 radio sold by Gambles department stores.
To make it even more confusing, I've also seen another Zenith 617 with Zenith model 530 on it's tag.
I do have an Interocean branded model 530, which is the same 2038 chassis in a completely different cabinet.

Zenith did some very confusing things. It is clear there was plenty of mixing and matching going on. But to what end?

Before I started digging into this radio, I decided to draw up a diagram of the location of all capacitors within the 2038 chassis. As I have two other Zenith radios that need restored which use the 2038 chassis, it should come in very handy with them as well.

Please click here for a larger size version

Also, here are the pages from the Zenith service manual on the 2038 chassis (NOTE: They are marked "Interocean Radio Corp.")
They include some notes as I've worked on the radio.


One thing I am very happy about is that two of the ORIGINAL electrolytic filter capacitors are intact! This gives me an idea of what they look like and maybe I can reproduce the missing capacitor.
As I absolutely do not like restuffing capacitors with electrolytics, I have found modern film capacitors of similar value and voltage ratings indeed fit the old 1930s electrolytic packages.
While at Midwest Surplus in Fairborn Ohio, I found some 18 uf @ 250V Mylar capacitors. The two in series would give me 9 uf @ 500V which is PERFECT! The leakage on such capacitors are almost nil, but I wanted to add a couple resistors across the two to help balance out the potentials across each one, lest they have an imbalance of charge and one takes on more voltage than the other. Two 5.1 meg ohm resistors help keep the balance while only adding 50 uA of leakage current. Well below most electrolytic caps.

Perhaps this is uncessesary... I will agree, but it certainly doesn't hurt, and I don't want to have to tear the stuffed cap apart at a later date to replace a failed cap.

Here is the main filter capacitor as removed from the chassis and cleaned up.

Here is what I found inside!

And here is the assembled capacitor I put inside to replace the original. (2 sets, I need to make a 3rd)

And the capacitor after being rebuilt. Big difference, eh? ;-)

The electrolytic capacitors are made by Aerovox and Mallory, but the paper caps are all Cornell-Dubilier "Cub" capacitors.
These early C-D units are built with a wooden dowel down the center. The dowel has the wax paper and foil wrapped around it to form the capacitor. The foil is then crimped underneath metal end caps onto the wooden dowel.
This makes it very difficult to restuff, especially if you want to keep the doewl intact, as I do!
I tried the idea of using several smaller film capacitors arranged in a staggared pattern around the dowel. They still would not fit within the cardboard tube.
I finally looked into the idea of using modern surface mount capacitors as their voltage/capacitance ratings are really improved over recent years. Unfortunately, high voltage surface mount chip capacitors aren't cheap. But, they WILL make rebuilding C-D "Cub" capacitors possible without loosing the wooden dowel!

I selected C0G temperature coefficient capacitors as they are the most robust and hold their value no matter the temperature or voltage applied.
Perhaps overkill, but as this is one of my favorite radios, I'm willing to give it the best!

My solution after disassembling the capacitor and removing the wax paper and foil from the wooden dowel.
Put two strips of 1/4" wide adhesive backed copper foil onto the dowel. Solder seven .015 uf @ 450V C0G 5% 1206 size ceramic capacitors around the dowel. This makes a .1 uf @ 450 volt capacitor which there are over ten .1 uf caps in the radio.

There is one single FAT .5 uf Cub in the radio. It bypasses the screen supply to the RF, Mixer, IF and QAVC tubes.
I opted to buy some .1 uf 450V C0G caps. I bought 10 of them which came to $27.05 plus tax and shipping.... what one must do to save a dowel...

I then covered the assembly with two layers of heat shrink.

Then covered as much as I could with copper foil.

This should simulate the "outside foil" as most 1930s capacitors were marked. The intent was to ground this end to reduce the chance of stray coupling. Many times there seem to be gremlins in radios which plague the servicer. Oscillations, "Motorboating" and such instabilities can arise when replacement capacitors don't have the outside foil end grounded.

The chances are low, but it does happen and can drive one batty trying to squelch the demons.

Finally, the completed capacitors.

I used beeswax to seal the ends. A slab of beeswax can be purchased at Hobby Lobby which helps greatly in rebuilding these wax-paper capacitors.

I opened up the capacitor from the "Ground" or "Outside Foil" end as it goes toward the chassis.
I called it the "ugly end" but the wax covers a multitude of sins.

Finally, the cathode bypass caps for the detector, 1st audio amp and driver stages.
These are Mallory brand 8uf 50Volt electrolytics. Again, I avoided using an actual electrolytic and paralleled two 5.6 uf 100Volt poly film caps which fit easily into the package.

There will not be one single electrolytic in this radio. All done with surface mount C0G ceramic or polyester film capacitors.

OK... I put this part off long enough.
The dial cord needs to be replaced as the front cord was broken before I bought it. Dial cord restringing can be one of the most frustrating parts of restoring old radios. These Zeniths have a two-cord arrangement with a front and back pulley on the tuning shaft. The two cords are then wrapped around the dial drum. Therefore as one pulley takes up its cord, they other releases its cord to the drum.

I decided the only way to do this without driving myself NUTS was to remove the tuning capacitor/dial drive assembly from the chassis.
After removing, I went to work figuring out the proper length of cord needed, tied the knots and restrung the front half.
Oh what fun.... but I succeeded.
The knots on the ends were given additional help with some service cement.

Oh, how about a photo of the chassis without the assembly. It looks strange, don't it?

Thanks to Earl Hennagir (who owns the Coronado version of this radio) I now have three 22-167 capacitors plus an extra for the 619. Thank you Earl!
He sent the Mallory version of the 22-167. Interestingly, the inside is not built to the standard of the Aerovox. Nonetheless, it will work to fill the empty slot.

And the two together side by side all rebuilt with modern capacitors!

For the audio coupling capacitors, I chose to use three .068 uf caps to make .2 uf @ 450 volts.

Here are 3 Cubs. original, taken apart, and rebuilt.

Alright now, here is the chassis after being recapped. Look at all those new modern capacitors, eh? OK.. yeah, just being silly.
The volume control is back to the one it came with temporarily. The loudness tap has a network on it also in a temporary fashion. I need to do some repair to the Bradleyometer I have.

Click on the image for a larger version.

Now the speakers...

I discovered the audio output transformer has an open primary on one side. Still, I could try the radio out, so I connected it up and gave it a whirl. It came to life and was working well enough, but the speaker without the transformer was not making any sound.
Turns out one of the tinsel wire flexible leads was open. Replaced that and got both speakers working.

You can see the corrosion on the wire.

The other issue of the output transformer I will attempt to deal with later, but for now, I pulled out a speaker from my Zenith 775 which uses the same exact speakers. It has a good output transformer, so why not give it a shot?
I quickly found that the field coil was completely open... ugh... well, can I save it?

After pulling out the coil, I find a big burned spot on the side. That is NOT good...

I spent much time carefully peeling away the paper cover over the field coil, trying not to break any delicate wires. A very tedious and nervewracking task, but I was successful.

I managed to find the burned spot was on the outermost part of the winding. There were several broken/melted strands, but I found the winding end and lost only 50 or so ohms of the 5000 ohm coil.

I decided to wrap the field coil in DuPont Kapton tape as it is a good insulator and won't melt at the tip of a soldering iron.
The 3 termination contacts are held down with another strip of Kapton tape.

Finally, all connections are made... WHEW!

I finally attached lead-out wires, held them together with heat shrink, then wrapped the entire thing in black electrical tape.

It turns out the coil is pretty much 5000 ohms with a tap about 2/5ths from the bottom. B+ goes to the 3000 ohm end, the tap provides power to the tube screens and the 2000 ohm section teminates at the QAVC pot.

After reassembly, the radio came to life, but the audio just wasn't quite right. I noticed if I removed one of the 59 output tubes, the bass tone would improve, but... the radio would motorboat as the tone control was advanced.
While doing some voltage checks, I discovered the problem. I connected the pins wrong on the speaker and had the output transformer wired with B+ going to one end! I'm surprised the radio worked as well as it did!
I then corrected the wiring and found it is not a bad performer at all! Produces nice sound! Not as bassy as say, my 12-S-266, but not bad at all!
It needs a good alignment. The dial calibration is WAY off and the performance drops towards the upper end of the dial, so that will be next.

After an alignment, I made a video of my 617 in operation.


Thank you for reading!