Broadcast Electronics Marti Electronics ME-40, Manual

Page: 33 / 133

23
©2011 Broadcast Electronics
With S3 in the positive logic input position, U3B will output a HlGH to the base of Q2
when a LOW from a transmitter is applied to the inverting input of UJB through Q3 and
Q4. This HlGH biases Q2 ON which applies a LOW to voltage regulator U4 compensation
input to disable the RF. A HlGH from the AFC circuit (unlocked condition) applied to U3B
non-inverting input will also inhibit the RF.
2) FORWARD/REFLECTED AMPLIFIER CIRCUITS. The forward/reflected amplifier circuits
provide information from the directional couplers to the power control circuit and the
metering circuit board. The forward amplifier circuit consists of meter amplifier U1A,
FWD CAL control R5, diode D1, and AUTO/MAN switch S1. The reflected amplifier circuit
consists of meter amplifier . U1B, RFL CAL control R9, diodes Dl and D2, and VSWR
indicator driver U2A.
Forward Amplifier. Output from the forward directional coupler is applied to the non-
inverting input of U1A which operates as a voltage follower with the gain determined by
potentiometer R5. The output of U1A is routed to: 1) the metering circuit board for
display, 2) a rear-panel barrier strip for remote metering, 3) diode D1, and 4) the
inverting input of voltage regulator U4 through S1.
Reflected Amplifier. Output from the reflected directional coupler is applied to the non-
inverting input of U1B which operates as a voltage follower with the gain determined by
potentiometer R9. The output of U1B is routed to: 1) diodes D1 and D2, 2) the metering
circuit board for display, and 3) the rear-panel barrier strip for remote metering.
Output from U1B is also routed to the inverting input of voltage regulator U4 through S1
and D1, and the non-inverting input of U2A which operates as a comparator circuit. If
the reflected power level at U2A non-inverting input exceeds the reference potential at
the inverting input, U2A will output a HIGH to illuminate VSWR indicator DS7.
3) TEMPERATURE SENSE CIRCUIT. The temperature sense circuit provides automatic RF
power reduction if the RF amplifier assembly temperature exceeds a preset level. This
circuit consists of temperature sensor U401, TEMP CAL control R25, over temperature
comparator U3A, TEMP TRIP control R27, diode D3, temperature indicator driver U2B,
and TEMP indicator DS8.
The output of U401 on the RF amplifier regulator assembly is calibrated by R25 and
applied to the inverting input of U3A. As the temperature increases, the output level of
U1 increases. If this potential exceeds a threshold level established by R27, the output of
U3A will be reduced and applied to the non-inverting input of U4 through D3. U4 will
reduce the RF power output to stabilize the temperature.
The output of U3A is also routed to the inverting input of U2B which operates as a
comparator circuit. If this level decreases below the reference potential at U2B, U2B will
output a HIGH to illuminate indicator DS8. This HIGH is also routed to the rear-panel
barrier strip.
4) OPEN FUSE DETECTOR CIRCUIT. This circuit provides a visual indication of an RF amplifier
malfunction. If the PA transistor current is excessive, fuse F1 will open to bias transistor
switch Q5 ON which outputs a HIGH to illuminate RF indicator DS6. In addition, Q5
applies a HIGH to mute switch Q2 to enable the mute circuit.
5) POWER CONTROL CIRCUIT. The power control circuit provides automatic. power control,
over voltage protection, and short circuit protection for the RF power transistor. This