Wednesday, April 25, 2012

Johnson Viking II finally on the air

It’s been a long time since I’ve updated my experience with getting my Johnson Viking II on the air. It is working now and I have been enjoying it on the crystal controlled frequencies of 3.775 and 3.885 with just a little over 100 watts output on AM. After several years of having only a 25 watt low-level AM signal from either my Yaesu FT-857 or FT-450 I was quite excited to have a “proper” plate modulated rig on the air. But after a few nights on AM I discovered nearly everyone else (at least the ones with an armchair copy signal) were running full legal limit on AM – which is about 375 watt! Just to clarify, that equates to a 1,500 PEP linear amplifier. So, again, I have the desire to further improve my signal but for now, I must be content with the 100 watt Viking II.

So here’s how it went.

Even before I left the Orlando Hamfest with my $50 Viking II transmitter I went over to the table of one of the vacuum tube dealers seeking two 6146s to fill the empty PA tube sockets.

http://www.esrcvacuumtubes.com/index.html

My new friend Jeff Crovelli was nearby to remind me there is a difference between 6146 and 6146A tubes and the later 6146B tubes. You would think you’d want the most recent model but in the case of these 1950s era transmitters it is my understanding the plain 6146 or 6146A tube is much preferred to the newer B model.

There is a comprehensive discussion of the reasons here.

www.n6wk.com/kenwood/The_6146_Family_of_Tubes-1.doc

The only difference between the 6146 and the 6146A is the type of filament. The 6146A was developed by RCA and is called a "dark heater". The two types can be mixed in use. 8298 is the same as the 6146A. Cannot be interchanged with 6146B as it has different bias requirements, inter electrode capacitances, etc.

Friend Jeff also informed me I had a kit built radio instead of a factory built radio. He knew because the tube sockets and all of the hardware were mounted with tiny nuts and bolts instead of rivets. I think I like that because if I have to remove something from the chassis it will be much easier than drilling out tube socket rivets

Anytime you bring an old radio into the shack there is a powerful urge to immediately plug it in and turn it on. And for the days and weeks this Viking II sat inverted on the bench I admit to coming so close to doing that – just to see if it would play. I even read several discussions on the AM bulletin boards that suggesting performing a wholesale capacitor replacement before powering up an old radio was overkill. Then I saw pictures of how a ruptured electrolytic capacitor can just ruin the inside of an otherwise clean chassis.

So, I waited.

I made a list of all the electrolytic capacitors in the radio and emailed it to the people at Just Radios.

http://www.justradios.com/

Remember, I have never repaired a non-working radio in my life. So what would have taken a qualified technician a few hours to perform, I took days to read and research. I described what I was intending to do and let the people at Just Radios fill the order with their recommendations. There were a few instances where the replacement capacitor was a higher voltage rating than the original. That’s OK – and even preferred.

In a couple of days a pouch of capacitors arrived. It was time to go to work. The first thing I discovered was that my 55 year old eyes were not nearly as good as I remember as a teenager so a handheld magnifying glass has become an added tool in my bag.

The Viking II transmitter chassis is spacious by comparison to any modern rig but for me, as a first timer, there were places that were not easy for me to access with a soldering iron and needle nose pliers. Trying to get to a terminal strip with multiple component leads and remove only a single capacitor lead was hard for me. Soon I decided to just cut the capacitor out of the circuit close to the component, leaving most of the old wire leads still nicely soldered to the terminal strip. Then I only needed to twist a loop in the old lead and attach the new capacitor to the old leads.

This is probably not what the purist would do it but it worked for me.

Pretty soon I had replaced all but the most hard to access capacitors. In fact, there are three I still have not replaced.

I spent a lot of time staring into the chassis looking for anything that didn’t look right. It was then I found a wire that had broken from a terminal strip and was laying, shorted to the chassis, under the big HV oil-filled electrolytic. I’m really glad to have found that because there would have been, most certainly, some fireworks.

Once I was satisfied I’d done all that I could it was time to light it up with 120 volts, 60 cycles, AC.

One of the tools I am glad to own is a big, heavy Variac that I can use to slowly increase the line voltage from zero to 120 volts.

I had read that it was desirable to slowly heat up a radio that had not been energized for years – maybe decades. But I never knew if that meant hours or days. So I plugged the radio in to the Variac and slowly turned to knob to about 20 volts.

In some of my research I read about a simple test bench where the author had a milliamp meter inline between the Variac and the radio. With knowledge of what the particular radio’s line fuse was rated you could watch the increasing current as you ramped up the voltage. If the radio began to draw current approaching the fuse rating you could back it down and search for the problem before you actually burned something up.

With a keen sense of smell I watched and waited for any signs of smoke. So far, so good. I probably could have safely just ramped the voltage on up but I remembered all those times when I had waited. A few more hours would not hurt.

I began to make some safety checks, measuring the AC voltage on the cabinet of the transmitter. There was nothing but a Hammerlund HQ129X receiver I had been working had almost 40 volts on the chassis! Likely the result of a leaky bypass capacitor. With a non polarized two wire plug I knew there was the potential for a dangerous situation. I’ll change out that power cord very soon but I always have a good earth ground to all of my radios.

Finally, after a slow all day warm-up I had the transmitter up to 110 volts. I work for an electric utility company and know the standard AC line voltage at the meter socket today is about 120 volts and could be a few volts higher. But in the 1950s the AC voltage standard was closer to 110 or 115 volts. We all know that a conventional incandescent light bulb will last a whole lot longer if you dim it slightly (run it at 115 volts instead of 120). Knowing the scarcity of certain vacuum tubes I have chosen to keep the Variac inline all the time and run the radios at 110 volts. This reduces the filament voltage by a slight amount and, hopefully, will prolong their life.

Using my Fluke 87 digital voltmeter I measured every voltage I could find under the chassis and didn’t see anything out of order.

Now, for the big moment. It was time to flip on the high voltage. On the Viking II the high voltage power supply produces about 865 volts. If there was a bad capacitor we were about to find it. I connected my air cooled dummy load, reached for the safety glasses and quickly flipped the switch on and back off.

No smoke. No burned fuses. So far, so good.

By now I had already studied the operator manual from cover to cover so the process of loading the transmitter on 3.885 MHz was pretty much a non event.

I am grateful to have been introduced to amateur radio in 1973 when every ham had to learn how to peak the exciter, set grid drive then dip and load the PA to the specified plate current.

I was, by now, feeling pretty proud of my old $50 transmitter. The next step was to connect a microphone and see if it would transmit audio.

During all of my research on Johnson transmitter restorations I learned of people who had to spend much time and effort undoing modifications of a previous owner. The most damaging were those that involved extra holes in the chassis or changing plugs. That said, I determined that I would keep the original microphone jack on the Viking II and order a mating plug. But in the mean time, a couple of clip leads and I had a D-104 connected and verified my radio would transmit audio. Pretty good sounding audio actually!

Up until this point I had resolved to do as little as possible, both mechanically and cosmetically, until I could get the transmitter on the air. If it worked and stood up to a few weeks of frequent use, then I would get serious about other elements of the restoration.

It was finally time to bring the radio into the house. This monster weighs about 85 pounds and it is easy to see why the radios are called boatanchors.

I placed the Viking II atop my HQ 129X and with my two 80 meter crystals I was finally on the air with a “proper” plate modulated rig. Cosmetically, this rig looks a little rough but electrically it appears to be operating within its specifications. I’ve not made any modifications and at the moment I plan to keep it is its stock condition unless I decide to make some audio improvements.

I have come to really like this transmitter. People frequently use the term “built like a tank” but that seems to best describe the construction of this radio. I’ve been told that Johnson purchased a lot of surplus WWII components such as modulation transformers etc. and some of those parts became the basis for their transmitters of that era.

Now that I know it works, I have removed it from the ham shack and will begin a more in depth restoration. I plan to check all of the resistor values then do a thorough cleaning of the chassis. I don’t feel comfortable removing the transformers and other components. I will mask and repaint the transformers. I’ll also clean and polish the knobs. The front panel has what looks like very fine pock marks on the paint. As much as I would love to have a new silk screened panel I am going to experiment with cleaning with some 0000 steel wool followed by a rubbing compound and then a paste wax.

Don’t hold me to that. I will try it in a small area first. I am reminded of the physician’s creed, “First, do no harm.”

Here are some of the areas of concern that I will address.

There are two 807 modulator tubes. During my initial bench testing I observed one of the two modulators not lit. I found this to be due to a loose pin on the tube base (not the socket). There is probably no way to fix that other than a replacement tube. I touched it with a soldering iron for a temporary fix.

There is a front panel control that switches in 160 meters. The front panel control operates the switch with a loop of dial cord. The switch is stiff and frozen and it appears the dial cord might break if I force it. Inasmuch as I may never operate 160 meters I have left it alone for now. Remember. “First do no harm.”

The next most obvious need is an external VFO. These radios were crystal controlled and at the current price for FT243 crystals I could buy a Johnson VFO for the price of a full rack of crystals.

That’s about it. My $50 radio has taught me a lot. With two new 6146s and a hand full of capacitors and two new crystals I’ve been able to get on the air on 80 meters AM and have really gained a high degree of confidence that I can troubleshoot and repair simple radio transmitters.

It’s a good feeling!



1 comment:

James said...

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