Service manual

68P81076C25-C July 1, 2002
Troubleshooting Procedures: Power Amplifier Procedures 4-55
Control-Voltage-Limit Circuitry Test
Disconnect J5901 (transmitter injection) from the PA input. With all other connections in normal
condition, key the transmitter and monitor the control voltage at J1 pin 2. If the voltage exceeds
10.0 V, troubleshoot the control voltage limit circuitry.
Current-Limiting Circuitry Test
When ready to adjust current limit, decrease the relative current limit value with the keyboard per
instructions. After several decrements, the current limit should begin to reduce power in 0.5 to 1.0
Watt increments. After this test, reset the current limit to its original value. If the circuitry does not
perform as indicated, troubleshoot the current limit circuitry.
Directional Coupler and Power-Leveling Test
The directional coupler combined with the RPCIC form a closed-loop power leveling circuit. This
circuit keeps forward power essentially constant under variations of line voltage, frequency, and
VSWR.
The directional coupler samples a small amount of forward power during transmit. This power is
rectified by a detector diode CR5906. This rectified DC voltage is fed back to the RPCIC where it is
compared to a reference voltage. An error voltage is generated which is ultimately translated into the
control voltage via RPCIC circuitry and amplifiers Q503 and Q504 on the command board. Control
voltage is routed to the LLA stage, thereby completing the feedback loop. In operation, the control
loop tends to maintain the forward detected voltage constant versus frequency and line voltage
variations. Proper operation can be observed by monitoring the forward detected voltage while
varying the supply voltage from 13.4 to 16.1 V. Forward-detected voltage should not change more
than a few hundreths of a volt. Note that the forward power may not necessarily be level if one of the
other protection circuits such as temp-sense or current limit are engaged.
PA Voltage Protection Circuit
Some versions of the PA board may include a voltage protection circuit. This circuit is intended to
prevent premature failure of a transmitter operated in extreme conditions. An example of an extreme
condition would be operation at above normal battery voltages (greater than 15 V) combined with
high temperatures (greater than 500°C or 122°F).
The circuit monitors the A+ voltage from the battery, and it is activated if the A+ voltage exceeds
approximately 15 V. R5825 and R5823 form a voltage divider connected to A+. The divided A+
voltage is connected to the base of Q5805. The emitter of Q5805 is connected to Zener diode Z1.
This 5-V Zener diode, combined with the voltage divider action of R5825 and R5823, sets the
voltage “trip point" at which 05805 turns on (A+ near 15 V). When Q5805 turns on, this provides a
path for current to flow through the base-emitter junction of Q5802. Q5802 then acts as a switch to
connect the K9.4 voltage supply to R5826 and the directional coupler circuit composed of C5924,
R5916, R5905, and R5904. A fixed DC bias voltage is applied to the forward power detector.
This fixed DC bias voltage is summed with the rectified RF signal that is coupled from the output of
the transmitter. Since the PA power control requires that the detected voltage is a constant value, the
output power of the power amplifier must be reduced by an amount proportional to the applied DC
bias. The values of R5916, R5905, and R5904 are chosen such that power is cut in half. The
reduced output power decreases the current drain of the transmitter, and therefore reduces the
internal temperature of the amplifier devices which increases their lifetime. The circuit disengages
and full rated power is restored if the over-voltage condition is corrected.