How To Restore A Rare Johnson Viking Mobile Transmitter

Getting back to my Thermatron Roots

A couple of years ago, I found a Johnson Viking Mobile transmitter on eBay. It appeared to be complete and in pretty good condition. The price wasn’t too unreasonable, so I made a bid and won the auction. It is about the size of a shoe box. I planned to ease into tube equipment restoration by starting with a relatively simple project.

The Johnson Viking Mobile is a 60-watt input, AM transmitter for 80 through 10 meters. Originally, it used an external dynamotor-based power supply, but I planned to use it with a solid-state 12-volt inverter. That was also a vintage project, since it uses a Triad toroidal transformer from the 60’s. You can read all about it here.

Work started on the transmitter by testing the eight tubes. Mainly 807’s and 6BH6’s, they all seemed to be good. Nice surprise. Examining the chassis, I found a number of old electrolytic and waxed capacitors.

Old, Waxy Capacitors

I don’t even bother testing waxed tubular capacitors any more. At an age of at least 65 or 70 years (like me), they are bound to be leaky. They were soon replaced with new “Orange Drop” fixed capacitors. Easy.

The electrolytics were another matter. They have a distinctive look, and might still be usable with a careful re-forming of the electrolytic film. This is one of the reasons equipment restorers will often bring the line voltage up slowly with a Variac. It slowly reforms the electrolytic capacitors when turning vintage radios on for the first time.

That would be hard to do in this case since the transmitter is powered by an external power supply. It was not yet built. I decided to use an old capacitor tester. It checks for excessive leakage by applying increasing voltage to the capacitor.

I had recently purchased and restored an old “Heathkit Condenser Checker” from the 50’s. It could apply up to 400 volts to capacitors under test, perhaps reforming them. It seemed to work, and they had reasonably low leakage, so we’ll call that done.

Check the resistors too

Next, resistors. Tube equipment from this era will have a lot of carbon resistors. Over time, they have a tendency to change value, usually gaining resistance. There are two distinct types of carbon resistors in this transmitter. Rounded edges with a clear molding seam are usually higher wattage resistors.The other type are those that have crisp corners where the tubular resistor is formed at the lead attachment.

The rounded end resistors are most likely to have changed value, but it’s worth checking them all. I ended replacing about a dozen resistors.

A little tank trouble

The Viking Mobile has a clever way of adjusting antenna loading in the output circuit. Each of the five bands have a pair of coils that “nest” inside of one another. The “coupling” knob on the front of the transmitter. It operates a rack that slides all five inner coils in and out their matching outer coils.

This means there is a tiny piece of copper braid on the moving sub-assembly. It connects it to the band switch. With use over time, the braid fatigues and separates. Two of the coils were completely disconnected and the others looked like they would soon.

Pondering my options for a while, I realized the answer was laying on the bench. Solder Wick, normally used to de-solder components, would make a good replacement. It comes in several gauges, and fortunately I had an exact match for the broken straps.

First Power Up

By this time, I had built and tested the new solid-state power supply. I was ready to test the transmitter. I plugged the supply onto the transmitter, powered it up, and watched, listened for signs of trouble. Nothing was smoking, so far so good.

Trouble was, there was very little power out. Further troubleshooting revealed a defective “drive” pot and the 6Bh6 driver wasn’t quite as good as my tube tester thought. A little shopping on eBay and replacements were on the way.

After that, I was starting to see a little power, but it was odd. I had drive and plate current, but it didn’t seem like the output tank would resonate and load. I struggled with this for a while on 80 and 40 thinking they would be the easiest to get working.

Then, in desperation, I tried all the bands. Amazingly, the transmitter had full output on 10 meters. What the Wha?

Removing Mods

Not knowing what else to do, I started comparing the transmitter with the circuit shown in the manual. And that’s when I spotted the problem: there was an extra inductor and trimmer capacitor in the 807 plate circuit. I had violated one of the cardinal rules of equipment restoration: Find and remove mods!

Janis Carson (AB2RA), AKA the “Wireless Girl” has a good introduction in her description of a Viking II project. Well worth the read, and her site is a wealth of information for tube gear restoration in general. “Don’t Start Here” is a good place to start.

The 11-Meter Connection

My guess is that the venerable old Viking Mobile had been modified for 11-Meter operation. Since that was the only band in use, who cared that it ruined operation on the other bands?

I removed the added components and restored the original configuration and voila — reasonable output on all five bands. I also removed a 1/4-inch jack someone had artlessly added to the front panel. It allowed keying the transmitter for CW. A hole plug helped restore the cosmetics. I now had a working transmitter with nice AM modulation.

Let’s talk accessories . . .

The Viking VFO

Now it was time to think about accessories. I don’t have a lot of 80 and 40-meter crystals and a VFO would be nice. Johnson did make one for the Viking Mobile. A little hunt around the auction sites produced a candidate pretty quickly. It was in fair shape, and arrived in a few days.

While it didn’t work as received, a little component testing and replacement, and wiring cleanup soon had it functional. It plugs directly into the back of the Viking Mobile, and gets its power through an octal (8-pin) plug. A second coax cable carries its output to the transmitter.

. . . And a classy mic

Finally, there was the matter of a microphone. There are lots of choices on eBay, but I wanted one that looked the part. I ultimately settled on a New, Old Stock (NOS) Turner 9D Hi-Z mike. It was mounted on an old Realistic Desktop Microphone stand.

I haven’t spent too much time on the air with it yet. But when working in the shop this winter, I plan to have my Hammarlund HQ-170 tuned to the 10M AM calling frequency . I also need to get a 10-meter dipole up over the shop. Should be a lot of fun as the Sunspot Cycle nears its peak!