The Zenith model 103 Ultra


The 1932 Zenith 103 Ultra "Hyper-heterodyne"

was Zenith's top-of-the-line model for the 1932 model year. Even though the Great Depression was in full swing, Zenith managed to manufacture and sell expensive well designed radios.
By 1931, RCA began licensing manufacturers to be able to use their Superheterodyne circuit which they had previously kept tightly under wraps for the past decade. Zenith (as well as most other manufacturers) wasted no time jumping on the bandwagon.
Zenith pulled out all the stops for this Superheterodyne model. In fact, they dubbed it a "Hyper-heterodyne" as shown on the model tag at the back of the chassis.

It uses 14 tubes with two RF amplifier stages and two IF amplifier stages.
The way the RF is tuned isn't quite what I expected. The 4 gang variable cap tunes two Preselector stages before the firts RF amp. Then the first RF amp is coupled to the second RF amp without any tuning, then the input to the Mixer (or first detector as Zenith called it) is tuned. Of course, the 4th variable cap section tunes the oscillator.

This particular model 103 Ultra is from the Niagara Falls area of NY State and is a 25 Cycle model with a huge power transformer. As if the chassis wasn't heavy enough...
For the most part, it is in very good condition, but it is clear it saw humidity of a damp basement or somesuch. Not so much in the cabinet, but in the chassis and speaker.

The 103 shares a dial arrangement with only the smaller models 91 and 92. No other Zenith models used this big "Cyclops Smile" dial assembly. It uses a tuning meter which Zenith did not use again until 1942.
It also incorporates the Marvin Automatic Tuning mechanism which Zenith had been using since 1928.

Here is the chassis pretty much as it looked when I first removed it from the cabinet. Then another after cleaning and with the shield removed exposing all 14 tubes! The first image shows that the Regulator ballast tube was missing. It should have one, but Zenith conveniently designed the 103 to allow operation without the ballast by moving the main fuse to a different tap of the power transformer.

Here is a pic of the front of the chassis showing the 4 gang tuning capacitor and Automatic mechanism.

All in all, this chassis is a BEAST! It is also cadmium plated and had significant dusty oxidation which I cleaned up outside with a compressor, brush and face mask. This also means I must wash my hands after handling as they clearly smell of the oxidation afterwards.

Here is the speaker as it looked once removed from the cabinet. I'm not sure who manufactured this speaker, but with the help of Mike Simpson, the Midwest radio collector, we've decided it was manufactured by Wright-Decoster of St. Paul Minnesota.
The speaker has a very brittle cone and has some attempts of repair with cellophane tape. The cone also has sunken into the basket and will need to be reset. I'm not sure, but i suspect the cone is not original. I may be wrong, but it seems to be a replacement.
The first thing I did was address the electrolytic capacitors. Honestly, I do not like electrolytics as they have a life span. They are guaranteed to eventually fail. Perhaps not in my lifetime under proper use, but they will fail. As I am stuffing new capacitors inside the old can, I like to use modern day poly caps.

The capacitors I have chosen are Solen Fast Capacitors as they are decently priced at Antique Electronic Supply, are small enough to easily fit inside the cans and are available in a wide range of values. The 103 uses a 6 uf and two 8 uf capacitors in the power supply.

These electrolytics are the wet type and are originally full of liquid (Boric acid) which usually leaks or dries out rendering the cap bad. They are made of very soft aluminum and crimped together. I find the best way to open them up is to chuck them gently into a metal lathe and use an Xacto knife to cut them open.

Of course, some actually have some liquid inside, so a cup is needed to catch it as it drains out.

Inside, there is a spiral of perforated aluminum to make a large surface area for the formation of the insulating film. The spiral aluminum is the anode while the electrolyte itself becomes the cathode both insulated by an extremely thin insulating oxide layer.

The inside electrode is held in by an insulating rubber plug. It takes some pulling, twisting and digging at the rubber, but it comes out without too much effort. The rubber gets cleaned from the hole and a 1/4"-28 bolt gets installed with a plastic spacer and rubber washer for the positive connection. Thanks to the advice of ARF member dtvmcdonald, I was able to solder to the aluminum can to make a good electrical connection.

Once the new Solen Fast cap is soldered to the head of the bolt and to the side of the can, it is ready to have the top glued back on with clear RTV Silicone rubber and installed back into the radio chassis.

After checking all of the transformers and chokes, I discovered the field coil of the speaker was wide open. There is not even an indication of what the resistance should be on the schematic. The Audio output transformer also had an open winding.


The speaker got pulled apart.

You can see the corrosion and oxidation in the pot assembly. I'm not sure why the speaker field got the brunt of moisture, but the end is covered in cadmium powder. A damp paper towel too care of that.

Fortunately, the break in the field coil was right at the junction of the fine magnet wire and the lead. It had a nice green spot of corrosion, so I reconnected the wires and taped it back up.

Unfortunately, I neglected to take a photo of the field while I was repairing it.

As it turns out, the field coil is right at 2400 ohms.

In order to correct the "sunken in" cone, I took water and dampened the surround, then put pressure on the voice coil as it is exposed with the field coil pot removed. It sat for several days to make sure it stays in the new position. I also had pressure on the spider to get it back to being flat at rest.
Again, I neglected to take photos of this step. :-(

By now, the speaker is completely taken apart. I don't believe there is anything more I could remove.

I discovered that this particular speaker does not have a way to automatically center the pole-piece in the hole for the proper voice coil gap. I needed a way to keep it perfectly centered as I assembled the pot and bolted it to the basket. Well, once again, the lathe came in handy to fabricate a centering tool.

I now can assemble the speaker without worry of the pieces moving.

One issue with the cone is that the paper is very brittle. I worried that once the speaker was put into use, the surround edge would break apart. Ed App recommended Devcon Amber Rubber Adhesive to spread around the speaker surround and repair all breaks in the cone. This worked out very well as it holds the cone together but lets it flex.
I reinstalled the cone and got it nicely centered.
I found a suitable push-pull audio output transformer in my stash which has a close match to the impedance ratio. I will still keep the old one in case I get ambitious and want to rewind it. ;-)

Now the speaker is done, time to turn attention back to the radio chassis.

One problem I found was the volume control is not original. It had been replaced sometime in its life.
I was not happy with the one that was inside it, so I chose to replace it with something that looks at least somewhat close to the original.

This is what the original looks like as this image is from my model 91 chassis which uses the same part number volume control.

The Zenith models 91, 92 and 103 chassis went through 3 revisions. The first version has the volume control in the AVC circuit. Zenith changed the location of the volume control to the audio amplifier in the second revision as it was discovered that the tuning meter action diminished as the volume was turned down. Secondly, there is a significant delay in the volume control action as when it is changed, the AVC circuit must alter the bias on the RF and IF amplifiers. Much like the action of tromping on the accellerator of a Turbo Charged car, there is a delayed reaction and one must be careful in advancing the volume as it may blast!

Here is my solution. I modified a 5000 ohm wirewound Mallory pot to have an on/off toggle switch much like the original. A hole was punched into the back cover and the end of the pot shaft was drilled and tapped with a 4-40 thread. The brass piece is a pipe fitting machined to mount with a 4-40 screw and has a wire to actuate the switch.

The shaft was, of course, too short. Therefore I also drilled and tapped the shaft on the other end and mounted a shaft extension. This was epoxied on with J.B. Weld and then the flat was milled to accept the original knob.

All paper capacitors in this model are the metal "bathtub" type. The most difficult part is drilling the rivets so they can be removed.
Once removed, they are easy to restuff. Just unsolder the wires, heat with a heatgun and the wax-potted cap falls out!

They actually look like a fudge or chocolate bar! :-D

A majority of the caps are part number 22-107, which according to the service info are .1 uf. The capacitors themselves are marked .5 uf and my capacitor tester confirms this. So, I restuffed them with .47 uf @ 630V yellow mylars.

Most of the bathtubs have dates printed on them such as this one of July 14, 1931. Others are marked July 15th or 16th.
I would say it is a safe bet this radio was manufactured within a month or so of these dates.

I checked all resistors and trimmed them to proper value with small 1/4 watt resistors in parallel. There are two Muter Candohm resistors in the 103. One had two sections open and the third was intermittent, so I soldered standoffs on the lugs and mounted new resistors on top.
I know of no good way to restuff Candohms, so this will have to do.

Click on the image for a larger version.

The radio roared to life after being recapped and seems to be working fairly well. I started with an alignment, but discovered the IF transformers are somewhat intermittent with the adjustments. I suspect the trimmer caps have some oxidation and need to be cleaned.
After removing one of the IF transformers, I discovered they are assembled with a press fit and pretty much stuck together. After soaking the joint with some WD-40 and bolting the mounting bolts to a metal rod for use as a handle, I was finally able to work them apart.

It turns out the adjustable trimmer assemblies are loosely held together with rivets. The caps have a bottom plate against the porcelain base, a sheet of mica, another plate, another sheet of mica and finally the curved top plate which has a threaded piece for the adjustment screw to engage. The top and bottom plates are supposed to be electrically connected. Well, age and oxidation have caused the rivets to loose their connectivity, so I needed to solder things together. I also soldered a small loop of wire between the top and bottom plate to ensure the connection.

I discovered some markings on the base. It seems the porcelain was molded by "Isolantite Company" It also has DeJur-Amsco imprinted in the base. DeJur-Amsco was a capacitor manufacturer, so they most certainly had Isolantite make the bases, and they made the capacitor. The question is, did they make the entire transformer? That I don't know...

Another interesting thing I discovered. The ventilation cutout patterns on the bottom pan appear to have been stamped out by the same press that made the holes for mounting the IF transformers

Look! perfect fit!

After repairing the IF transformers, the IF aligned quite nicely.

The Zenith 103 is a very good performer! It does have a slight bit of distortion. I suspect this is because of the type of detector used. It is a plate or power detector which were never known for high-fidelity performance. Still, the radio has very good sound quality. I'm VERY happy with it!
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