Service manual

Page 17
Note that a similar switch is not needed for the RXVFO, because it operates at a much higher
frequency range of 70-100MHz. Any harmonics of 70MHz fall well outside the upper cutoff of
the Butterworth bandpass filter (105MHz).
LED Backlight
Sheet 7 shows backlighting circuitry for the meters. Q13, Q14 and Q15, along with the associ-
ated resistors, form a 3-bit binary control, allowing up to 8 brightness levels. The control lines
for these MOSFETs come from the Keyer processor. High intensity LEDs D1-D4 provide
plenty of light for the two meters.
MIC Bias
A 9V bias for electret mics can be turned on and off via Q16 (Sheet 7). The MICBIAS control
bit from U13, pin 19 on Sheet 5 is used for this.
Keyer microprocessor
Sheet 3 is a diagram of the Keyer processor. The keyer speed, dot weight, dash weight and pitch
pots from the Front Panel board are fed into the first four A/D inputs on port A (ADC0-3), simi-
lar to those of the main processor. A/D input 4 (the 5th input) comes from the transmitter
(routed through the receiver board) and is a buffered, uncompressed version of the microphone
audio, level shifted so that it idles at 2.5V with a peak-to-peak AC voltage of up to 5V. For
VOX detection, the processor takes a running average of the AC voltage and computes an RMS
value that is tested against a trip threshold set by a menu option. (During FM transmit, all A/D
inputs are disabled except the mic, so that rapid sampling of the audio can occur, with this in-
formation passed to the main microprocessor in order to re-program the DDS chip controlling
the BFO in real-time.) Similarly, the buffered and level-shifted speaker output is fed into A/D
input 7 to form the Antivox input. Another RMS calculation is done on this signal and used
along with a menu item to adjust the trip point of the VOX input up or down. A/D input 5 is the
external keypad input. Yaesu designed the FH-1 keypad as a set of 12 buttons with resistors in
series with them such that 12 different DC voltages from 0 to 5V are produced. The firmware
tests these voltages to determine which button has been pushed and sends this coded informa-
tion over the internal inter-processor bus (port C on both processors, with KeyerRDY,
/KeyerRd and /KeyerWr). The Keypad input from the back panel is routed to A/D input 5,
which reads the 0V to 5V signal to determine which of 12 buttons was pushed.
A/D input 6 is grounded. This bit allows the processor to determine if the controller board is rev
A or B. Rev A boards left this line floating (pulled high by an internal pullup inside the proces-
sor). Different code is executed depending upon whether the board uses rev A or B hardware.
Port B, bit 3 is used as a Pulse Width Modulated output signal whose frequency is determined
by the Pitch pot. The output is filtered by an RLC network and routed to the Front Panel board
where it goes through the Sidetone pot and back to J11 which then goes to the Receiver board’s
audio output stage. If no front panel is present, jumper JP2 (Sheet 7), assures that the Sidetone
(at full volume) will make it to the receiver board.