Motorola APX 3000 Detailed Service Manual page 143

Troubleshooting Charts: PA Failure
PA Failure – Page 3 (VHF PA)
3
Measure TX current
at VHF test
frequencies
Is current less
than 350mA?
No
Is current
Yes
approximately
500mA?
No
Is current greater
than or close to
2A?
Yes
Use a spectrum
analyzer and high-
impedance RF
probe. Measure
RF_IN_1 at C901.
Yes
No
Is the frequency
correct?
Yes
Is the power No
level about 3 dBm?
Yes
Measure power at
C925
Is the power
No
approximately
37 dBm?
Yes
Measure power at
C2704
Is the power
No
approximately
37 dBm?
Measure power at
C1222
Is the power
approximately
37 dBm?
Yes
Check power out at
C1122 and any
process issue
Yes
Measure DC voltage
at pin 4 of U901
Is voltage
Yes
approximately 6V
at DAC=4095?
Replace Q901
No
Measure voltage at
pin 5 of U901
Is voltage
Yes
approximately
7V?
No
Measure voltage at
pin 3 of Q1101
See FGU
troubleshooting
Is voltage Yes
charts
approximately
0V?
No
See FGU
Measure voltage at
troubleshooting
pin 1 of Q1101
charts
Yes
Is voltage greater
than 1.5V?
No
4
Replace Q901
Check D2703
positive node
voltage
No
Check pin 3 and
Is the voltage
pin 2 of Q2701
near 7.0V?
Yes
Replace D2703
Is pin 3 of
No
Check continuity of
Q2701=Raw B+
R2706 and L2702
and pin 2=3.0V
Yes
Replace Q2701
Measure DC voltage
at pin 5 of U901
No
Measure voltage at
Was voltage
pin 1 of Q1101
near 7.5V?
Yes
With the radio in RX
mode, measure the
Replace U901
DC voltage on both
pins of L903
Was voltage No
near 7.5V on
both sides?
Yes
With the radio in RX
solderability issues and
Replace Q1103 mode, measure the
replace as necessary
DC voltage on the
drain of Q901
Check L906 under shield
No
SH1 for solderability
Was voltage
issues and replace as
near 7.5V?
Yes
Replace Q1101
With the radio in RX
mode, measure the
resistance on the
node shared by
C916 and R905
Check resistors R901,
No
R902, R903, R904,
Is the resistance
and R905 for resistance
near 9.4 kOhms?
and solderability issues
and replace as necessary
Yes
In TX mode,
measure the voltage
on pin 4 of U901
Yes Is TX current just
Is the voltage
near 6V?
Is the TX current
between 300 and
Is the TX current
above 900mA?
Yes
Is the voltage
4
below 4V?
No
If the voltage is between 4 and 5.5V then
the control loop should be working and
power should be available. Check D2703
positive node voltage.
No
Check pin 3 and
Is the voltage
pin 2 of Q2701
near 7.0V?
Yes
Replace D2703
Is pin 3 of
Q2701=Raw B+
and pin 2=3.0V
Replace Q2701
No Measure the logic
Was voltage
inputs of TXH_1
near 3V?
from R1111 and
TX_INH from U1138
Yes
Trace voltages and check for solderability
issues on R1102, Q1101, and Q1103.
Replace parts as necessary.
No Check E1101 for
Is there 7.5V
solderability issues and
on L908?
replace as necessary
Yes
Check L903 for
necessary
If resistance is very low, in the
100's of ohms, and there are no
issues with the resistors then
there is a gate short on the PA.
Clear short or replace Q901.
The PA and Driver are
Yes
likely damaged. Replace
above standby
Q901 and U902.
current?
The PA is likely
Yes
damaged. Replace
Q901.
900mA?
If no issues found, lift shield
Lift shield SH1 and check
SH8 and check for solderability
Yes for solderability or part
issues on all 900 series
series parts. Replace parts as
parts. Replace parts as
necessary.
No
Check continuity of
R2705 and L2702
Yes
5-53
or part issues on all 1200
necessary.