16
8.6 Antenna relay / QSK module
The QSK module acts as an antenna relay 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 is a module with two very fast, high power vacuum relays, and the 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
Control board
The brain of the amplifier is the Control 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.
•
Screen voltage regulator, with adjustable voltage and adjustable current limiting, for tube protection.
•
Overdrive protection and timer - based on detecting grid current, it initially gives visual warning and,
at higher level of overdrive, it cuts off the amplifier for 2 seconds. It ensures a clean signal, making
it virtually impossible to overdrive the DX-3SP.
•
SWR detector and timer - switches the amplifier to bypass mode for about 3 seconds when high SWR
is detected. Adjustable SWR level. Factory adjusted to about 2.6 : 1
•
EBS - the Electronic Bias Switch - senses the presence of RF input power and turns on the bias to start
transmission. Factory adjusted for about 0.5W. When the input power exceeds this limit, the plate
current starts flowing. When there is no speech, even for a very short time, the tube dissipation is cut
to a low value, as set by the pre-bias adjustment.
•
Interlocking, timer for filament warm-up
•
Relay control: ready relay, SWR relay, over-drive relay, antenna relay
•
QSK switching control
•
Display control: Ig2 measurement and display, SWR cut-off display, overdrive warning display,
READY display, FAULT display
•
Logic circuitry, two low voltage supplies to the various electronic circuits
