900 Mhz Receive Rf Troubleshooting Chart - Motorola XTS 2500 Service Manual

6.5-16

6.5.17900 MHz Receive RF Troubleshooting Chart

NOTE: INJECT MODULATED ON CARRIER
FREQUENCY SIGNAL AS REQUIRED.
If present
No
Check
at U401,
FL3
pin 47?
Yes
Check tank
Is
components L402,
2nd IF on
No
L403, C419, C420.
U401, pins 4
Check for 3Vdc
and 5?
at pin 48
Yes
Before replacing U401,
check U401 voltages.
Check for no RX audio
VOCON flowchart
February 14, 2012
900 MHz Troubleshooting Charts: 900 MHz Receive RF Troubleshooting Chart
Bad SINAD
No Recovered
Audio
Spray or Inject 1st IF
into XTAL Filter FL3
IF FREQS(700/800):
IF = 109.65MHz
2nd IF = 2.25MHz
2nd LO = 107.4MHz
Audio
Heard?
Yes
Inject RF
into J101
Trace IF signal
Yes
IF Signal
from L10 to
at L10?
U401, pin 47
No
RF Signal
at L7?
No
RF
Check filter FL2.
Yes
Signal at FL2
Check voltage
pin 1?
at C7
No
Check bias
Yes
RF Signal
voltages on Q1
at L4?
No or
Weak RF
Check FL1 filter.
RF
Program filter to test
Yes
Signal at FL1
freqs. and check
pin 1?
varactor voltages
on FL1, FL2
No
Check harmonic
filter C108-C120
and antenna switch
NOTE: The presence or absense of RF signals is generally not enough
to diagnose sensitivity degradations. It is best to compare signal levels
at each decision node, to the same circuit in working radio.
Inject -47 dBm into J101 and
No
toggle signal continuously
(on/off) while monitoring
TP403 data pulse
Does
the pulse
frequency
vary?
Remove SH401.
Check
Check 2nd LO
FGU Chart
Control Voltage at
U401, pin 38
No
1st LO
Yes
O/P OK?
Yes
Locked?
VCO locked?
Yes
Check Mixer
16.8MHz
(U1) circuit
check at pin 35
on U401?
18MHz
at ABACUS
(U401) pin
20?
Before replacing U401, check 2nd
VCO Q1. Check VCO O/P level;
Disconnect R403 and verify that
a 0.5V to 5V control voltage VCO
will sweep about the IF frequency
Check IF
Yes
spectrum at
U401, pin 47
No
Check IF filter
Is it
No
FL3. Make sure
symmetric
LO is locked and
centered?
on frequency
Yes
See VOCON
chart, RX
audio problems
No
Check
No
FGU
Chart
Yes
Check U401 voltages,
No
programming, and
10MHz clock VCO
components
Yes
MAEPF-27342-O
6816985H01-F