Motorola APX 7000 Detailed Service Manual page 148

5-44
RX RF Failure – Page 12
16
Remove shield, SH4.
Measure RF levels at first
preselector filter and
compute filter loss.
UHF2: Input side of C1935
& output side of C1939.
Loss < 4
dB?
Measure RF levels at LNA
and compute LNA gain.
UHF2: Input side of C1931
and output side of C1936.
Gain about
18 dB?
Measure RF levels at second
preselector filter and compute
filter loss.
UHF2: input side of C1959 &
output side of C1961
Loss < 4
dB?
Remove shield, SH7. Measure
RF levels at the filter and
compute filter loss. Output
side of C1961 and output side
of C510.
Filter Loss
< 3 dB?
Measure RF levels of the RF
switch, U501, at the input to the
mixer and compute loss.
UHF2: output side of C510 and
input side of C511.
U501 loss
< 1 dB?
Check components
No
and replace defective
Autotune the radio.
Yes
No
Yes
Check components
No
and replace defective
Autotune the radio.
Yes
Visually inspect
No
and replace
components if
necessary.
Yes
Check digital logic
to the RF switch
No
and replace U501 if
needed.
Yes
8
Note: RF Test frequency used:
VHF: 154.275 MHz
UHF1: 424.975 MHz
UHF2: 485.075 MHz
700: 769.0625 MHz
800: 860.0625 MHz
Measured with a high impedance probe for
relative comparisons and troubleshooting only.
Actual S21 gain or loss may differ if the
test point is not 50 ohms.
parts.
Check DC bias voltages of
U1932 at the base (pin 1) &
collector (pin 3).
Is base voltage =
0.787 V? Collector
voltage = 1.56 V?
parts.
Check DC bias voltages of
Q1922 at the base (pin 2 or
5), collector (pin 3) & emitter
Is base voltage = 1.022 V?"
Collector voltage =
0.787 V? Emitter
voltage = 1.561 V?
Check DC input line voltages
and visually inspect and
replace defective
components if needed.
Troubleshooting Charts: RX RF Failure
Check LNA:
No
Yes
(pin 4).
No
Yes
Replace
U1932
Replace
Q1922