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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 forward 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 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 moments
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 (X6.4) 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. The actual EBS threshold could be slightly different between bands.
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). Since the pin is isolated, a more easily accessible point can be used: FB3 (top of the ferrite
bead) or V1 (the MOV pin) or either end of the zero-ohm resistor marked D23.
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 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 7 kohm the reading will be
about 50 mA and the voltage starts dropping. The reading should be 345 – 350 V.
Continue reducing the resistance until the current reaches about 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
