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17 APPENDIX 3: ADJUSTMENTS
ADJUSTMENTS TO EMTRON “DX” 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 (+) to the test point TP2 (top corner). (-) to the chassis.
Make sure the EBS link is in the lower (vertical board) or rear position (horizontal board) - (EBS ON).
See also Pictures in Fig. 6 and 7.
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.
Adjust POT7 (PRE-BIAS) to obtain the desired pre-bias current.
Recommended pre-bias currents (POT 7, PRE_BIAS):
DX1b, DX-1d 10 to 30 mA
10 to 30 mV on the voltmeter Clockwise to reduce current
Vertical board
DX2:
15 to 40 mA
15 to 40 mV on the voltmeter Clockwise to reduce current
Vertical board
DX2SP:
15 to 40 mA
15 to 40 mV on the voltmeter Clockwise to increase current Horizontal board
DX3:
20 to 60 mA
10 to 30 mV on the voltmeter Clockwise to increase current Horizontal board
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:
Remove the cover and connect a voltmeter to the test point TP2 (top corner).
Put the EBS link is in the upper (vertical board) or forward position (horizontal board) - (EBS Off).
