36
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 the 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.
17.1.3 SWR
Protection
adjustment
Adjustment: POT5, marked SWR.
This adjustment require 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 clockwise for DX-1, DX-2, and DX-2SP and
anti-clockwise for DX-3).
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).
17.1.4
EBS ADJUSTMENT (Adjustment: POT4, marked EBS).
Usually this adjustment is performed on 20m (14.200 MHz), but it can be done on any band.
Transmit into a dummy load or antenna.
Apply very low input drive.
With the EBS activated (with the EBS jumper in the lower position, or ON):
Checking the existing adjustment:
increase slowly the input drive, starting from zero, until the EBS system activates the
amplifier. This can be checked by watching the plate current or the RF output power or the current or power taken from the
AC mains. Any one of these can be monitored and a jump in reading happens when EBS switches.
Check that this happens for an input drive between 0.5 – 1 Watt.
Adjusting EBS
: if the adjustment is not correct or you prefer a lower or higher setting, apply the desired amount of power
and then adjust POT6 until you find the limit where the EBS activates.
NOTE 1: a higher threshold is likely to give reports of the beginning of the words being cut off. If this is the case, a lower
threshold (higher sensitivity) can be used.
A low threshold can make the EBS activate erratically, for example due to the ambient noise of in the room, or even due to
the noise generated by the fan. This is not a problem in itself, except that the tube will dissipate extra power during the
times when you hold the key down but do not speak (eg between the words, while speaking). However, these moment are
usually brief and it won’t make much difference.
NOTE 2: for factory adjustment, a negative voltage with the value of 1.6 V is applied to the RFIN pin on the control board
when this board is checked, then the adjustment is only checked with RF when the amplifier is tested.
17.1.5 SCREEN
VOLTAGE
ADJUSTMENT
(Adjustment: POT2, marked SCREEN)
The screen voltage can be measured on the pin marked EG2 on the control board (close to the jumper EBS, this pin has a
blue wire connected to it; in DX-3 it is easier to measure on either side of R18).
The nominal adjustment is 356V unloaded.
NOTE: the screen voltage does not need adjustment unless the value was altered by somebody.
