14
The plate is connected to the RF network and to the choke supplying the 3400VDC voltage to the plate of the
GU78B tube.
8.2 Mains transformer
The transformer is the heart of the amplifier, a high performance type, designed to reduce the size and weight.
It is built with a generous reserve of power, handling easily the DX-3 requirements in continuous operation.
The primary can be wired for 200VAC, 220VAC, 230VAC or 240VAC operation.
See Figure
3. There are
several secondary windings, as follows:
•
Filament supply
•
Low voltage supply to the controller board
•
QSK module supply
•
Bias supply
•
Screen grid supply
•
High voltage supply
8.3
H. V. Power Supply
A full wave bridge rectifier and filter converts the high voltage AC into high voltage DC with a huge reserve
capacity of 116 microfarads. Two standard Emtron HV modules are paralleled in DX-3.
8.4
Soft start module
At power up, the mains power is gradually applied to the transformer by the soft start module. The power is
gradually applied over a time of about 5 seconds, avoiding the high inrush current typical for such big loads.
8.5
Sensor module
A small metal box positioned at the output of the DX-3. It detects the forward and reverse power, for
measurement and display.
8.6
Antenna relay / QSK module
Fitted as standard in DX-3, the antenna relay is a QSK module, controlled by the transceiver, via the control
board. It switches the antenna between the transceiver, during reception, and amplifier, during transmission. It
is also used to bypass the RF from the transceiver, directly into the antenna, when the amplifier is in standby
mode, or during special conditions like overdrive, bad SWR or other error conditions.
The QSK module uses two very fast, high power vacuum relays, and control circuitry. The purpose of QSK
is to allow very fast switching between transmit and receive, during digital communications.
See Appendix 2
showing oscilloscope displays of fast switching times and contact bounce-free operation of the QSK module
.
8.7
Display board
This board has a number of inputs where various voltages are connected. The board converts these voltages
into a "moving LED" type of display. Two of these displays have flashing light indication (Ig2 and plate
current).
8.8
Controller board
The brain of the amplifier is the Controller Board. The following circuitry is part of this board:
•
Bias voltage regulator, also incorporating a sensitive grid current detector for overdrive detection .
Adjustable pre-bias, removes the distortion at the beginning of the word, when EBS is activated.
