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Zenith 5679

Mystery Radio

This chassis was brought over to my shop recently. The owner also brought along the speaker as I had requested.

It looks like a fairly well built radio. At first glance, you'd think it was possibly a Grunow. But there isn't a bit of identification on the chassis.

The dial is quite attractive, and the telephone dial mechanism seems well built. The setting procedure for the buttons is ingenious and it took me a few minutes to figure out how to do it.

If you look closely at the bottom center buttons, you can see a small piece of metal towards the rear of the button on the upper left. If the button is pushed part way in, this metal part can be pushed to the right. Then the button can be pushed inward past the normal stop, and can be rotated to change the dial setting. Once it's dialed in on the station, the button is released part way out and the metal part is pushed to the left.

Another unusual feature is the voltage selector plug in the rear. It's labeled 177/234 volts.

The tag inside the cabinet also shows the radio can be connected to 117 or 234 volts. Also unusual is that the radio can be used with 40 to 60 cycles. Perhaps they has export in mind.
Again, no clue to the manufacturer. The dial escutcheon had Fidelitone written on it. Fidelitone was a major manufacturer of phonograph needles, but as far as I know never made radios. They weren't listed in the Riders index.

I did have a clue. Years ago, I had a console radio with no identification at all. It was kind of weird - the owners manual had statements like "For parts, contact the factory", but no clue where the factory was.

I don't remember now how I figured it out, but I found the set under Gamble-Skogmo. This is a company no one has ever heard of. But the odds are you've seen one of their radios with someone else's name on it. They were a pretty strange company - they made a lot of different radios, then put other people's names on them. A lot of companies like department stores, or companies that normally didn't produce radios, but wanted to be able to sell them.

I looked for the model number (A1-MB) under Gamble-Skogmo, but didn't find it. So I had to do it the hard way - look through all the Gamble-Skogmo schematics in Riders. The easiest way is to match up the tube complement.

The radio looked like about 1937 or 38, so I started in volume 7. There were 92 pages here, but many were "catch-up" - earlier radios that didn't get covered before. Most sets had earlier style tubes.

Volume 8 was a little more promising. And finally I found it on page 8-34. It's a model 762. The tube layout diagram was identical to the one inside the cabinet.

August 10:

Zenith model 5679

This is a unique radio. One of my long term customers brought it in recently. He said he got it from this old guy named Bruce. I immediately knew who this was - Bruce Gibb, one of the "old-timers" in the radio club. In fact, he was one of the founding members of PSARA. He's about 91 now, so doesn't get down into the basement to work on his radios much any more.

Bruce likes Zenith radios above all. In fact, he worked as a salesman in the late 30's, selling Zeniths.

So this has come full circle. I've known Bruce since the 70's, now one of his radios has come back to me to be rebuilt.

It's a pretty radio. The styling is unusual, especially in the area of the speaker. The grille cloth wraps around, and the speaker mounts at an angle, facing forward and up. The grille cloth is better than this photo makes it look.

Here's the unusual feature. It's a farm radio. It's made to run on either 110 VAC or 6 VDC.

I'm guessing it's about a 1940 model. It's pretty deluxe for a farm radio, what with the pushbuttons and the tone buttons.

The dual voltage input complicates the rebuilding process. The power supply is contained in the box at the left rear of the chassis. Filter capacitors are on the main chassis, but there are still some capacitors inside the box that must be replaced. One of them is a buffer capacitor, connecting between the rectifier plates. It would be suicide to leave it in there.

The "box" must be completely removed from the main chassis. There are seven wires coming out the bottom of the box into the main chassis, that connect to the terminal strip or tube socket.

Once the wires are all disconnected (remember to make good diagrams!), the box can be taken loose from the chassis. Once it's off, then the box can be disassembled and the capacitors replaced.

Again, I went by the tube complement to find the schematic. I looked through several Rider's volumes, from 9 through12. Finally I found it in volume 11.

The tube complement is pretty strange. It has two 6G6's for the outputs. The 6G6 is a low current drain power pentode - the filament current is only 150ma. But they aren't connected in push-pull - they are connected in parallel. It gets even stranger - the slide switch on the back panel is a low power drain switch. For maximum battery life, the switch cuts off the filament to one of the 6G6's so that only one is operating.

All the tubes are low current drain. Instead of a 6X5 rectifier, it uses a 6ZY5 (300 mil filament). The other tubes are all 150 ma replacements for common tubes - 6T7 instead of 6Q7, 6S7 instead of 6K7, and 6D8 instead of 6A8.

I'm not sure about the dial lights. The 5654 schematic shows two dial lights and a 13 ohm resistor in series across the 6 volts. They show the light bulbs as 2.9 volts 170 ma. The Sylvania tube manual shows type 292A with this rating. Don't think I'll find any at Radio Shack. The radio has 47's in the sockets. I don't think they'll light very bright with less than 3 volts across their filament.

I guess I could rewire them in parallel. But I'm a little worried about current drain. The radio has a 6X5, 6Q7, and a 6J8, all normal current drain. With the correct tubes and dial lights, the current drain would be 1.22 amps. With the tubes presently in the radio, the current drain will be 1.95 amps. I'm not sure how much reserve capacity the power transformer has. I think I'll feel better with the correct tubes. They're not expensive - I guess there isn't a big demand for them.

August 11

Once the wires are disconnected, the power supply box comes out easily. I took the bottom cover off and realized I'd underestimated the number of capacitors inside. There are nine. A couple are going to be hard to get at, but it doesn't make any sense not doing them all - as much trouble as it is to get at them.

Several of the capacitors are for the vibrator supply. I probably could leave them out, but as long as I'm goingthis far, I might as well do them all.

August 12:

There's another funny feature about the radio. On the rear of the chassis, there's an outlet jack that looks like you should be able to plug in a line cord plug. My first thought was the 3-way portables of the 50's, where you plugged in the line cord into a receptacle on the chassis when you were running off of batteries.

I was wrong. Looking at the schematic, it shows the outlet connecting to the 6 volt line, and it's labeled "Lamp". Probably, Zenith sold an accessory lamp that could be plugged into the back of the radio when it was running off a 6-volt battery.

The power supply box is back on the chassis, but i haven't connected the wires yet. There are still a bunch of capacitors in the main chassis.

The chassis support scheme also came from Bruce. I used 1X3's for the vertical supports and 1X2's for the horizontal braces. Cost of materials is very low, and it's amazingly effective. It's really nice to work on a chassis and have it stay steady.

Visible on the back panel of the chassis are the low power drain switch and the socket for the lamp. The knob selects between 6 volts and 110 volts.