Service manual

Page 18
The dot paddle and dash paddle inputs are filtered and fed to two edge-driven interrupt lines
(port D, bits 2 and 3). The manual key input is fed to bit 0, where it is sampled continuously in
the 1ms interrupt routine. Outputs from the Keyer are the PTT_Keyer line, which is routed to
the controller to allow the Keyer to control the PTT line, and the Key line, which is routed to
the transmitter.
The Keyer processor also has its own SPI bus for programming and a power-on reset chip. This
processor is not programmable by the user. Any changes to its internal firmware must be done
either by the factory or in the field through the use of an inexpensive Atmel programmer board
(STK500).
Display board
The VFD board is a Noritake GU256x64C-3900 model that is programmed with ASCII control
command sequences. It connects to port C as just another I/O device. DispRDY and /DispCk
signals provide the handshaking. Jumper JP1 on the Keyer processor tells the main controller if
a front panel is present. One of the effects of this is to cause the display routines to be ignored if
no front panel is present.
Receiver: BPF, Receiver and IF Filter Boards
Refer to the block diagram on page xxx and to the schematics on pages xxx-xxx.
Antenna to bandpass filters
RF from the main antenna ports (A and B) passes through low pass filters and a transmit/
receive (T/R) PIN diode switch on the transmitter or 100W amplifier boards. From there, the
signal enters the antenna input (J2) on the RXBPF board (See RXBPF schematic, Sheet 2). Re-
lay K6 selects either this input or one from the Receive Antenna input (J3) after the latter is
passed through a 35MHz low pass filter. The Receive Antenna does not pass through band-
specific low pass filters or a T/R switch as does the main antenna, which reduces the loss, but
also exposes it to potential intermodulation distortion (intermod) from strong shortwave sta-
tions. If you experience intermod when using the receive antenna, we recommend use of an ex-
ternal bandpass filter such as those made by Array Solutions. Back to back zener diodes on the
Receive Antenna input protect against extremely strong signals.RFC1 is a 1mH choke that
routes low frequencies to ground, reducing hum from power lines.
High-pass and bandpass filters, attenuator1, PSB
TM
The selected signal is then fed through a high-pass filter located between GaAsFET switches
U5 and U6. This filter reduces the chance of intermod from strong AM broadcast stations and is
engaged automatically when the receive frequency is above 1.6MHz. The signal then passes
through a 10dB attenuator which can also be switched out. Pressing the front panel Pre2/Atten
button activates this attenuator unless preamp1 is on. The output, “A” on the schematic, is then
passed through eleven bandpass filters, each controlled by a pair of GaAsFET switches, with
each filter handling a different segment of the HF spectrum. In addition, one pair of GaAsFETs