www.mentorradio.com
Phone: 216-65-2315 Fax: 216-267-2915
MB-R-OM-03/11
INTRODUCTION
The Mentor Radio Model MB-R receives and transmits on up to six discrete channels in
the VHF aviation band between 118 and 136 Mhz. It is a base transceiver for air
carriers, airports, fixed-base operators, corporate flight departments, hospital
helipads, etc.
The unit is a separate M15 radio and a PA-25 power amplifier, which produces 10
watts of transmitter RF power. There is also a connection for connecting an aircraft
standard microphone using a 3/16” microphone plug.
Remote operation is an optional feature when you use a DB-15 plug. (See page 5 for
pin out)
The MB-R uses a switching power supply that allows the unit to operate from any
input voltage from 88-264 volts AC, 50/60 Hz power line. Power cords are available
for nearly any country in the world. A built in fuse is included inside a tray at the
power plug module on the rear panel. (Spare fuse included)
CIRCUIT DESCRIPTION
The receiver is a single-conversion super-heterodyne with four varactor-tracked RF
tuned circuits and dual-gate MOSFET transistors in the RF amplifier and mixer stages.
The local oscillator uses 3rd overtone crystals, and is followed by a MOSFET frequency
tripler stage. Receiver selectivity is primarily determined by a six-pole 10.7 MHz
crystal filter connected between the mixer and I.F. amplifier. The latter consists of
two cascade integrated circuit amplifier stages. Automatic gain control is applied to
the RF amplifier and the first I.F. amplifier stages.
A conventional diode detector is followed by a noise limiter, audio amplifier, squelch
“gate” and an integrated circuit audio power amplifier capable of delivering 4.5 watts
of audio power into a 4-ohm speaker. The transmitter oscillator also uses the 3rd
overtone crystals, followed by varactor tracked MOSFET tripler and buffer amplifiers.
Three un-tuned broadband stages boost the transmitter carrier power to
approximately 1 watt. Amplitude modulation is applied to the last two stages, the
"driver" and “final” amplifiers. The output from the M15 radio is applied to the PA-25
power amplifier that boosts the RF power up to 10 watts at the antenna connector.
The transmitter signal passes through a 7-element Chebychef low-pass harmonic
filter, which also contains two PIN diodes functioning as a T-R switch. The
transmitter modulator is separate from the receiver audio amplifier, and includes an
audio AGC amplifier that automatically adjusts for variations in operator “microphone
technique”. This circuit also prevents over modulation.
