Vco Failure - Motorola APX 1000 Detailed Service Manual

5-46
5.16 VCO Failure – Page 1 (VHF, UHF1, UHF2, 700/800 MHz, 900 MHz)
Check parts around
No
U702. If OK, replace
U702.
Check if VCO is lock at
No
desired frequency using
field probe.
VCO locked?
Yes
Check lock_det line
circuitry in controller
section. Check R2811 and
R2812 for continuity.

VCO Failure

Sniff: Using an inductive field probe
as an antenna to measure
frequency. Place the probe
* Sniff * frequency
approximately ½ inch away from
near VCO shield
components to be sniffed
(SH11).
Frequency No
detected?
Yes
Remove FGU shield
(SH5).
Check control
voltage at R705.
< 0.3Vdc or
> 10.6Vdc or
voltage drifting?
Yes
Check power on
prescaler buffer output,
C823 using field probe.
No
Is power at
C823 >-15dBm?
Yes
Check loop filter R701,
R705, R708, C703,
C707, C713 for
continuity. Fault found?
Yes
Replace suspected
components.
Troubleshooting Charts: VCO Failure
Remove VCO shield
(SH11).
UHF1 Radio?
No
7/800 MHz
Radio?
No
UHF2 Radio?
No
VHF Radio?
No
No
900 MHz Radio?
No
No fault found.
Yes
1
Yes
2
Yes
3
Yes
4
Yes
5