Tx Or Rx Vco Unlock (Fail 001) (136-174 Mhz)-Part 1 Of 2 - Motorola ASTRO XTL 1500 Detailed Service Manual

5-20
5.6.9 TX or RX VCO Unlock (Fail 001) (136–174 MHz)—Part 1 of 2
TX or RX VCO
Unlock (Fail 001)
"Sniff" Frequency
near TX or RX
VCO shield or
injection string
Frequency
Detected?
Yes
Check VCO
Control Voltage
at TP6756
<.6VDC or
>11.5VDC or
drifting?
Yes
12VDC
at Charge Pump
Capacitor
C3822?
Yes
Go to Transmitter
or receiver
Injection string
troubleshooting
charts
Figure 5-11. TX or RX VCO Unlock (Fail 001) (136–174 MHz)—Part 1 of 2
June 15, 2005
Place an Inductive field probe
connected to a spectrum analyzer
near area to be sniffed.
8.2VDC
No
at Q3756-C
Collector?
Yes
3.0VDC
on pins 5, 36,
and 13 of
U3751?
No
Check Voltage
No
ultiplier
Circuitry
Troubleshooting Charts: Flowcharts
Check 9.3 Volt
No
supply path from
regulator U0950 in
Controller section
No
Check if VCO
is locked using
spectrum analyzer
Locked?
Yes
3VDC
at pin 4 of
U3751?
Yes
Go to
Controller
troubleshooting
charts
A
Go to Transmitter
or receiver
No
Injection string
troubleshooting
charts
Check for short
No
on pin 4 to
controller or
replace U3751
AEPF-27812-O_VHF
6815854H01-A