History 

At the 1998 Dayton hamfest, I found a beat-up Viking II with no tubes, no covers, and a scratched-up front panel. All the transformers and the tank circuit appeared to be there plus the original knobs. 

I was able to find a manual for the transmitter at the hamfest. At home I had the tubes! Fortunately, the Viking II uses common tubes. 

The chassis cleaned up nicely. I plugged in tubes and gave the transmitter a smoke test. As this boatanchor had obviously been out of commission for many years, I decided to use my extra long extension cord for this test!  Surprisingly there weren't any fireworks. The front panel filament light came on and the tubes began to glow. After 30 seconds I turned on the transmit switch. Still no fireworks! I could see a little bit of grid current. I had to use contact cleaner on all the controls to be able to monitor all stages, but after a few minutes the transmitter was producing 100 watts out! 

I quickly plugged in the microphone and discovered the reason that the Viking II had been abandoned: NO MODULATION. 

Fearing the worst, I checked the modulation transformer. It appeared to be OK. I then checked voltages on the 807 modulators. There was no screen voltage. I found that R13, a 50-watt 20Kvariable resistor, was open. 

Regulated Screen Voltage

I didn't have a resistor like that in my junk box. Also I wanted to provide the 807's screens with regulated voltage to reduced audio distortion. I did have some 0A2 gas regulator tubes in my junk box and many fixed power resistors. To repair the modulator, I removed R13. I then fabricated a bracket for two OA2 regulators. I used the two open holes from the old R13 mounting bracket for securing the new mounting bracket (picture one). Two paralleled 20k 10-watt resistors were fed in series between the 600v supply and voltage regulators (drawing one). Fixing the screen voltage problem brought the modulator back to life!

Audio Driver Transformer

The audio fidelity of a stock Viking II is poor. The audio response drops off rapidly below 800 Hz and above 3000 Hz. This is due to the poor frequency response of audio driver transformer T3. I replaced the original transformer with a one from Antique Electronic Supply, the model PT-157. This unit is the same physical size as the original and has a nickel core, which improves the frequency response. (A Stancor A-53C transformer may also be used.) 

The transformer is wired in a step-down configuration. When wired in this manner, there is no center tap available for supplying grid bias to the 807's. In order to supply bias, a voltage divider network using two 10k 1/2 watt resistors is placed across the secondary of the new transformer (drawing two). The value of these resistors was determined experimentally. This value provides the best combination of bandwidth and drive. The audio response is now flat from 150 Hz to 6000 Hz.

Speech Amplifier

While in the process of restoring the Viking II, an excellent article appeared in Electric Radio: "Viking II Modifications - Revisiting a Legend" (June 1998, pages 26-37, 40), written by Thomas Bonomo, K6AD. 

This article analyzes the shortcomings of this transmitter and K6AD provides some great modifications for the Viking II. Following this article, I rebuilt the speech amplifiers V1 and V2 as described by the author. Here is a schematic of the full modulator in MS Word format. 

Click here to download modified Viking II modulator schematic.

Push to Talk

I also borrowed the push-to-talk circuit in Thomas Bonomo K6AD's "Viking II Modifications - Revisiting a Legend, Part 2" article in the July 1998 issue of Electric Radio, page 36 (drawing three). In order to mount the push-to-talk relay, I used one of the open holes from the old R13 mounting bracket for securing a two-lug terminal post. The relay is soldered to the post. (picture one)