40
There are 3 bias adjustment potentiometers: POT3 (BIAS), affecting both tubes and POT10 (BS1) and POT9 (BS2) only
adjusting the bias to the tube connected to the pins BS1 and BS2 respectively.
Adjust initially POT10 and POT9 fully clockwise, then rotate back about 20 degrees (they are likely to be already in the
required position).
Turn the screen switch (on the sub-chassis) say – to the left. Key the amplifier with NO RF DRIVE and adjust POT3 for ½
of the total standing current, or about 0.25 A.
Key off and switch to the other (right) tube.
Key ON again and adjust POT9 (BS2) for the same current. (This is correct if the white bias wire is connected to the BS1
pin and white/black to the BS2 pin).
Key off and put the switch in the middle position.
Key ON and adjust POT3 for the total standing current desired (0.47 A).
Then the operations above can be repeated to ensure the 2 tubes have approximately the same standing current.
Switch again to the left tube only, then right tube only and check the current.
If there are differences more than about 5 mA, adjust only POT9 or POT10 to make them equal. Once the 2 tubes are
balanced, POT3 can be adjusted again to fine tune the final value.
The recommended bias currents is 470 mA (460 to 480), or -470 mV nominal on the voltmeter at TP2.
NOTE
: it is important to have POT9 and POT10 close to the clockwise end and use them only for small adjustments to
balance the tubes, while the main adjustment is done for both tubes simultaneously, using POT3.
CAUTION:
With a new tube, start at a low plate current. Rotate POT3 (BIAS) for minimum current (counter-clockwise)
before keying the amplifier.
With the adjustment in the wrong position, the plate current could reach very high values.
During this adjustment there is no output power. All the power taken from the high voltage supply is dissipated by the tube.
Be brief and quick. Put PTT on for a few seconds, then off. Repeat several times until the desired standing current is
achieved.
NOTE
: After adjustment, move the EBS link back in the ON position (default) if EBS is desired.
The above 2 adjustments are required when changing a tube or when changing the control board.
The remaining following adjustments are pre-adjusted with a new control board and normally there is no need to change
them. A possible exception could be the
SWR
protection which might require some fine tuning after changing the control
board or the RF sensor module, but usually the factory pre-adjustment is close enough. Also
EBS
might need adjustment
depending on the personal preference of the operator (See 17.1.4)
17.1.3 SWR PROTECTION ADJUSTMENT
Adjustment: POT5, marked SWR.
This adjustment requires a special jig with a load made of 2 non-inductive, high power resistors (at least 300W).
They can be switched between 130 ohm and 140 ohm. With a 50 ohm output these 2 loads give an SWR of 2.6:1 and
2.8:1 respectively. The adjustment is nominally for a ratio of 2.7:1 to trigger the protection in the form of antenna by-pass.
Usually this adjustment is performed on 20m (14.200 MHz) at full power.
With a 50 ohm dummy load, transmit and adjust for full power (or close to full power).
Stop transmitting and replace the 50 ohm dummy load with the special dummy load described above.
Switch the dummy load to 140 ohm and key the amplifier for just one second.
If the protection wasn’t activated immediately, increase the sensitivity (rotate anticlockwise).
Repeat applying power for a short time only, until the protection is triggered.
Switch the special dummy load to 130 ohm and repeat. This time the protection should not trigger. If it triggers, reduce the
sensitivity. Try several times using 130 and 140 ohm load. The protection threshold should be between these 2 values
(135 ohm, corresponding to 2.7:1 SWR).
