The plan: Develop a solidly built HF transmitter for experiments with Amplitude Modulation. I like the Johnson VIking II construction but the thing is very heavy to move about, especially for an old guy wanting to make internal changes often. I decided to strip out the Viking II iron and build the power supplies and modulator on a separate platform.
POWER SUPPLY:
1. Found a
bread board large enough for the project (22" x 22" x 3/4").
2. Added two variacs for the high voltage plate supply and the low voltage
driver supply.
3. Added in the Push to Talk control relays to
switch 6146 plate B+, screen and 6AQ5 B+ and 120 VAC for the external antenna
relay.
RF DECK:
1. Removed all the iron and the modulator leaving just the RF chain.
2. Removed the 6AQ5 clamp tube circuitry and built a 75 volt bias supply by wiring a backward connected
filament transformer across the filament line.
3. Replaced the microphone
connector with a mike connector which has other contacts for push-to-talk.
4.
Removed the damaged 25K drive pot and replaced it with an available 30K pot.
The 30K pot was physically larger than the original and wouldn't fit the
original space, so I moved it across the
chassis in place of the audio gain pot.
5. Removed the oscillator tube and
put a disc ceramic capacitor between the grid and plate so I could drive the buffer stage
of Viking II
from my remote VFO, which is an IC-718.
MODULATOR:
1. Wired in the large
Hammond 1642SE mod transformer, 75 watts, 20-20kHz
between the 6146's and B+.
2. Connected up the 8 ohm audio power amplifier to the mod transformer
input.
3. Ran fidelity tests with dummy load, signal generator, oscilloscope, CD player and
offline receiver. The
transmitter produces hi-fi audio at all power levels from 1 watt
to 110 watts
of carrier output.
4.
This modulator has been tested on a Viking Ranger, a DX-60 and the
Viking II.
It should work on any class "C" tube
final from 1 to 110 watts output.
Note (right most photo): The Viking II with the Hammond transformer is flat from 20 Hz to 5 kHz; therefore there is no need to use digital processing from the DEQ2496. I now use just the compressor-limiting function.
SCHEMATIC:
PROJECT COMPLETE:
I ran all the tests into the dummy load and the transmitter makes hi-fi audio. Block diagram shows audio chain. If I need to make mods, I can now lift the "Viking II" with one hand ! The variacs are immediately adjacent to the operating position should an output power adjustment be necessary to feed my linear amplifiers.
