Motorola ASTRO MT 1500, Service Manual, Page: 276 / 817

August 16, 2007 6816985H01-B
6.5-16 900 MHz Troubleshooting Charts: 900 MHz Receive RF Troubleshooting Chart
6.5.17900 MHz Receive RF Troubleshooting Chart
MAEPF-27342-O
NOTE: INJECT MODULATED ON CARRIER
FREQUENCY SIGNAL AS REQUIRED.
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.
Bad SINAD
No Recovered
Audio
Check
FGU Chart
Check Mixer
(U1) circuit
VCO locked?
16.8MHz
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
Remove SH401.
Check 2nd LO
Control Voltage at
U401, pin 38
Inject RF
into J101
Is it
symmetric
centered?
Yes
No
Yes
Yes
Yes
Yes
No
Does
the pulse
frequency
vary?
See VOCON
chart, RX
audio problems
Yes
Yes
No
1st LO
O/P OK?
Locked?
Inject -47 dBm into J101 and
toggle signal continuously
(on/off) while monitoring
TP403 data pulse
RF Signal
at L7?
Check filter FL2.
Check voltage
at C7
RF
Signal at FL2
pin 1?
RF Signal
at L4?
RF
Signal at FL1
pin 1?
Check harmonic
filter C108-C120
and antenna switch
Check FL1 filter.
Program filter to test
freqs. and check
varactor voltages
on FL1, FL2
Spray or Inject 1st IF
into XTAL Filter FL3
No
No
No
No
No or
Weak RF
No
Yes
Yes
Check bias
voltages on Q1
Yes
Yes
If present
at U401,
pin 47?
Is
2nd IF on
U401, pins 4
and 5?
Before replacing U401,
check U401 voltages.
Check for no RX audio
VOCON flowchart
Check tank
components L402,
L403, C419, C420.
Check for 3Vdc
at pin 48
Yes
Yes
Check
FL3
No
No
IF Signal
at L10?
Yes
Audio
Heard?
Trace IF signal
from L10 to
U401, pin 47
Check IF
spectrum at
U401, pin 47
No
Check
FGU
Chart
No
Check U401 voltages,
programming, and
10MHz clock VCO
components
No
Check IF filter
FL3. Make sure
LO is locked and
on frequency
IF FREQS(700/800):
IF = 109.65MHz
2nd IF = 2.25MHz
2nd LO = 107.4MHz