The “Wheat Box” Revealed
Allow me to explain the mess you see
before you. The board visible in the upper right is the receiver
board. It is mounted with four screws, component side down, over
the center partition of the main chassis. It can be swung to
one side for servicing, as shown in the picture. The chassis slides
into the main cabinet from the rear along grooves cut into the side panels.
Of course, this rig is built using
the “Ugly Construction” method for
the RF circuitry.
Top Partition (L-R):
The receiver board contains the RX
mixer, post mixer amplifier, two Collins mechanical filters, the 455 KHz
IF amplifier, envelope detector, AGC, and T/R control circuitry.
When the receiver board is mounted,
all of its adjustments are accessible from the edges. These adjustments
are S-meter zero and full scale, AGC bias set point, and the 455KHz IF
Center Partition (L-R):
Pilot lamp for S/RF meter
Behind the lamp is a small Vector board
with the RIT control circuitry. This board also contains the end
point frequency adjustments for the VTO, which are accessible from the
left side of the transceiver.
20V, 12V and 5V volt regulators
AF power amplifier
The rear panel has a small cooling
fan to bring air into the cabinet.
Bottom Partition (L-R):
The frequency display unit.
The transmitter board containing the
crystal controlled carrier oscillator, VTO, transmit mixer, buffers, and
filters. The last stage is a “high level” drain modulated FET providing
about 50MW of real AM drive to the linear output amplifier located in the
Behind the TX board on the rear panel
are connectors for a speaker, microphone, and amplifier control.
There are two parallel boards in front.
The bottom one contains the three resonator
filter and receiver RF amplifier stage.
The upper one is the seven resonator
elliptical filter. Yup, I know there are eight cans on this board.
I needed two to get the desired inductance value for one of the resonators.
The speech amplifier and modulator.
linear power amplifier.
Antenna changeover & external amplifier
Low pass harmonic filter.
Coaxial antenna connector
Small vent just above PA for pressurized
warm air to escape
The control cable carries two shielded
cables connected to the AF GAIN pot.
All other connections to the front
panel are DC voltages.
The main tuning control is a ten-turn
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