18
NOTE
: If the output is low (say, less than about 1500W), 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 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 overdrive protection is activated at low power – this could be due to incorrect loading. Turn the
LOAD control clockwise and tune PLATE. Increase the drive and repeat in small steps. A lack of screen
current (red Ig2 display) is an indication that the loading is incorrect.
•
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-3 is adjustable, but it is far preferable for you to
fix your antenna.
10.2
ALC ADJUSTMENT
The DX-3 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-3 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-3 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-3.
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-3'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.
