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10.1.3 Final tuning
Once the preliminary tuning has been done for each band / antenna combination, the values found for PLATE and LOAD can be used
for final tuning, with no need to repeat the preliminary tuning every time.
Tuning for full power
• Start with low drive level and the PLATE and LOAD knobs in the known initial positions
• Put PTT ON and increase the power (fairly quickly), to the onset of the overdrive indication. If the screen current (red Ig2)
goes too high, adjust LOAD, by turning the knob to the left (increasing the load).
• Adjust the drive level and LOAD until the overdrive indicator is just about to turn ON, and at the same time Ig2 shows two or
three red LED's ON.
• Fine tune PLATE for maximum output and maximum Ig2.
• Repeat the last two steps above if required.
Tuning for less than full power
• Start at low power, with the initial values for PLATE and LOAD
• Put PTT ON and apply drive until the output is about 1/2 of the desired power.
• Adjust LOAD for maximum output. Most likely, the knob will have to be rotated to the right, towards higher numbers on the
dial.
• Adjust PLATE for maximum output
• Apply more power
• Repeat the 3 steps above until the required power is achieved. If screen current is indicated by Ig2, is should, display two or
three red LED's. Maximise this Ig2 indication (and the output power at the same time), by tuning PLATE
• If the output goes higher then desired, reduce the drive and re-tune. Never reduce the output by de-tuning the amplifier!
NOTE: If the output is low (say, less than about 700W), Ig2 will never show any screen current at all. Ig2 might even indicate a
negative current, in green - this is normal and is no cause for concern).
However, if there (green) negative current indication, usually this will go away with better tuning of the PLATE control.
If there is no Ig2 red indication., simply rotate LOAD to the right in small steps, and each time maximise the output by turning PLATE,
until the highest possible maximum in the output power is achieved.
10.1.4 Potential problems during tuning
The tuning operation. should be completed fairly quickly. If the tuning process takes a longer time, it should be done by switching the
transmission on and off several times, to allow the tube time to cool.
• If the fan changes to high speed, the operation can continue, as there is ample reserve of cooling power. However, it is the best
to make a pause (about 1 minute will be sufficient) and wait for the tube to cool.
• If the FAULT light comes on, it is possible that the plate current has reached too high values due to incorrect plate tuning. In
this case, reduce the excitation power, wait for the FAULT light to turn off (about 2 minutes) and resume tuning.
• If the overdrive protection is activated, by cutting off the amplifier for 2 seconds. It is likely in this case that you are applying
too much power at the input. Reduce the power and watch the "overdrive" LED. Increase the power only until this light starts
flashing.
• If the SWR protection cuts in - it is likely that your antenna is not a good 50 ohm match at the operating frequency. In
principle, the SWR protection level in DX-2 is adjustable, but it is far preferable for you to fix your antenna.
10.2 ALC ADJUSTMENT
The DX-2 can be operated without any ALC connection. However, if you wish to use it, and your transceiver has an ALC input, then
the potentiometer on the rear panel must be adjusted. This adjustment is not critical, since its voltage output is zero under normal
operation and it increases sharply (towards a negative value - up to -11 V), when the overdrive limit is approached. The ALC in DX-2
amplifier is not a conventional design, this circuit operates for only one reason - to prevent overdriving and to keep the signal clean.
To adjust the ALC potentiometer:
• Turn the potentiometer initially all the way to the right (minimum ALC voltage)
• In CW mode, after tuning, apply increasingly higher drive, until the "overdrive" LED just starts to turn on
• Increase the drive slightly more, to have the light fully on, but make sure the DX-2 overdrive protection does not cut you off.
At this power level, adjust the ALC potentiometer to the left, until the transceiver drops its power output.
Alternatively, in voice mode, increase the RF gain or the microphone gain until the overdrive led is flashing at peak of modulation.
Then, increase ALC (rotate potentiometer to the left) until the transceiver begins to limit the drive to DX-2.
NOTE: Some types of transceivers (e.g. YAESU FTI00MP, FT920 and others) are not compatible with this type of ALC. They require
an ALC signal proportional with the output power. DX-2' s ALC is zero until the drive reaches the over-drive limit, when the negative
voltage increases sharply. YAESU operating manual indicates that this type of ALC might not work well with their transceivers, since
grid current could be generated by amplifier mis-tuning not related to excessive drive conditions. If this is the case, simply do not
connect it.
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