Motorola APX 3000

5-38 Troubleshooting Charts: VCO Failure
5.12 VCO Failure – Page 1(VHF, UHF1, UHF2, 7/800MHz)
VCO Failure
Remove FGU
shield (SH6)
“Sniff” frequency near
VCO shield (SH15)
Check control voltage
at R705
Check if VCO is lock at desired
frequency using field probe.
Frequency Detected?
Check lock_det line
circuitry in controller section.
Check R2811 and R2812
for continuity
Check power on
prescaler buffer output.
C823 using field probe.
Remove VCO shield
(SH15)
< 0.3Vdc or
> 10.6Vdc or
voltage drifting?
Is power at
C823 >
- 15dBm?
VCO locked?
Check loop filter R701,
R705, C703, C707,
C713 for continuity fault.
Replace suspected components.
Check parts around U702. If OK,
replace U702.
Note: “Sniff”: Using an inductive field probe as
an antenna to measure frequency. Place
the probe approximately 1/2 inch away
from component to be sniffed.
No
No
No
No
No
Yes
Yes
Yes
Yes
Yes
UHF1
radio?
7/800 MHz
radio?
No fault found
No
No
Yes
Yes
1
2
UHF2
radio?
VHF
radio?
No
No
Yes
Yes
3
4