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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 when the amplifier is tested the adjustment is only checked with RF, but not changed.
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 amplifier must be READY and on
OPR for the screen voltage to be present.
The nominal adjustment is 356V unloaded.
NOTE: the screen voltage does not need adjustment unless the value was altered by somebody.
17.1.6
SCREEN CURRENT LIMIT ADJUSTMENT (Adjustment: POT1, IG2LIMIT)
Do not adjust this potentiometer, it has been factory adjusted and there is no need to change.
With the amplifier switched off or on STANDBY, remove the blue wire on pin EG2.
Connect an analogue mA-meter with a 100 mA full scale range (+ to the control board) and then (-) to an adjustable
resistive load with a range from 3-5 kohm to about 20 kohm (this jig is also required for the display board adjustment, it is
convenient to adjust the ig2 indicator at the same time). The other end of the resistor goes to the chassis.
The power rating must be 20 – 25 W (short time loading only).
Connect also a voltmeter to measure the screen voltage.
Put the front panel switch on OPR.
Start with a higher value (20 kohm) of the resistor and reduce the resistance gradually. At 10 kohm the reading will be
about 35 mA.
Continue reducing the resistance until the current reaches about 60 – 70 mA. The screen voltage must drop to about 300
V or less under this load.
If not, adjust POT1.
NOTE 1: While adjusting ig2 limit, the ig2 indicator on the display board can also be checked / adjusted at the same time.
NOTE 2: IG2 LIMIT can be checked / adjusted also by simply putting a mA-meter in series with the blue wire connected to
the EG2 pin on the control board. While operating the amplifier at full load, by rotating the LOAD knob, IG2 can be varied
and the screen voltage can be measured. This way the current limit of the screen regulator and the ig2 indicator on the
front panel can be checked quickly, without the need for a special load.
However, the amplifier has to be opened and wires used to extend the screen voltage connection to the blue wire to an
external current meter and volt meter.
17.1.7
IPTRIP ADJUSTMENT (Adjustment: POT6, marking: IP TRIP)
This is the sensitivity of the high plate current protection (cut-off).
When the plate current exceeds the adjusted limit, even for a very short time, the start-up timer is reset. The screen
voltage is cut off and the FAULT light turns on. To make FAULT light go off, the amplifier must be put on STANDBY. If left
on OPR, the operation will resume automatically at the end of the (about) 2 minutes warm-up time. During this time, any
RF power applied to the input is bypassed into antenna.
Connect a DC supply to pin TP2 on the control board (in the top corner).
With the amplifier powered and the READY light on, put the front panel switch on OPR.
Apply a current gradually increasing from zero and check that the protection is triggered at about 1.85 A of current. When
this happens, the FAULT light will turn on.
If the current value found is much different from 1.85 A, adjust POT6.
To repeat the adjustment or checking, you need to wait every time for the READY light to turn on again (about 2 minutes).
To speed up, the resistor R20 (5.6 Mohm) can be paralleled briefly with a lower value resistor (1k – 100k). Touch R20
with, say 10 kohm in parallel and the Ready light will turn on immediately.