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17 APPENDIX 3: ADJUSTMENTS
ADJUSTMENTS TO EMTRON DX-3SP AMPLIFIERS
CAUTION
Most of the following adjustments require the amplifier to be open and powered up.
This also implies defeating the mains interlock safety switch, which is extremely
dangerous since high voltage / high power DC and AC and RF voltages are exposed.
We do not recommend anyone to do this
, since extreme precaution and safety
measures are required.
If however, this is attempted, it should be only by technical people qualified and
experienced in working with high voltage and high power circuits, including RF.
17.1 Control Board
This procedure refers to adjustments made to the board already installed in the amplifier.
This would be required, for example, after replacing the tube or after certain repairs.
After a tube replacement only one or two adjustments are required: BIAS and (if EBS used): PRE-BIAS
17.1.1 PRE-BIAS ADJUSTMENT
Adjustment: POT7, marked PRE-BIAS
The pre-bias is required when the Electronic Bias Switching (EBS) is used. If the EBS link is kept in OFF position, the pre-
bias does not need to be adjusted. PRE-BIAS must be adjusted before BIAS.
When EBS is activated (the default situation), keying the amplifier causes the plate current to stay cut off (as if the PTT
was off), until a small amount of drive is applied. The “Pre-bias” establishes a small plate current as soon as PTT is on.
The transition to full ON is then less abrupt, while the tube dissipation is still reduced by using EBS.
PTT Off:
Ip = 0
PTT On (no drive) and EBS on:
Ip = Ipb (pre-bias current)
PTT On (no drive) and EBS off:
Ip = Ip0 (standing current)
PROCEDURE:
Remove the cover and connect a voltmeter between the test point TP2 (top corner) and the chassis.
Make sure EBS is ON. (The EBS jumper is in the position towards the rear of amplifier).
See also Pictures in Fig. 18 and 19.
With the amplifier running, wait the warm-up time.
When READY, put OPR switch on and key the amplifier with NO INPUT DRIVE.
Read the voltage at TP2.
NOTE
: the voltage at TP2 is NEGATIVE with respect to ground.
Adjust POT7 (PRE-BIAS) to obtain the desired pre-bias current.
The recommended pre-bias current is 30 to 60 mA, or a reading of -30 to -60 mV on the voltmeter at TP2.
NOTE:
the pre-bias current is not critical.
17.1.2 BIAS ADJUSTMENT
Adjustment: POT3, marked BIAS
Adjust Pre-Bias before adjusting Bias.
This adjusts the plate standing current, with the EBS Off and the amplifier keyed with NO INPUT DRIVE.
PROCEDURE:
There is a switch on the sub-chassis which allows switching off the screen voltage at any of the 2 tubes. When turned to
the left, the left hand side tube is operational and the other tube is disabled. In the middle position, both tubes are
operational. When turned to the right, the right hand side tube is operational and the left tube is disabled.
