5.19 Fgu Failure - Motorola APX 5000 Service Manual

5-46

5.19 FGU Failure

VCO TX/RX
unlock
"Sniff "
Frequency near
VCO shield
No
Frequency
detected?
Yes
Remove
VCO shield.
Check control
Voltage at R764.
<1Vdc
No
>10Vdc
or drifting?
Yes
Remove
TRIDENT shield.
Check power
on prescalar buffer
output ,C779 using
field probe.
Is power at
No
C779
> -15dBm?
Yes
Check parts
No
around VCOs and
loop filter. If OK,
replace VCO.
See Note 2
Sniff: Using an inductive field probe
as an antenna to measure
frequency. Place the probe
approximately ½ inch away from
components to be sniffed
Remove
4.6V at
VCO shield.
TP853?
Check parts
around U703.
If OK, replace
U703.
Check if VCO
Is locked using
spectrum analyzer.
VCO
Locked?
Check
VOCON board.
Is power at
Yes
1.5Vdc at
U709 output
L701?
> -15dBm?
No
Check parts around
Is power at
Q774. If OK,
VCO output
replace Q774.
> -15dBm?
Yes
Check parts
2.5V at
No
around U703.
logic pins
If OK, replace
on U709?
U703.
See Note 3
Yes
Replace U709.
No
Remove
TRIDENT shield.
Yes
2.5V at
No
logic pins
on U703?
Yes
Replace VCO.
No
Remove
TRIDENT shield.
Yes
Check parts
around U702.
If OK, replace
U702.
No
Check
1.5V DC supply.
Yes
Troubleshooting Charts: FGU Failure
No
5V at
C742?
Yes
See Note 1
See Note 2
Note 1:
Pin Freq Band
4 VHF RX
8 VHF TX
10 700 RX / 800 TX
9
700 TX / 800 RX
6 7/800 TX
8 UHF1 RX (380-450) / UHF2 RX (450-520)
15 UHF1 RX (450-470)
12
UHF1 TX (380-445)
13 UHF1 TX (445-470) / UHF2 TX (450-520)
Note 2:
Ref Des
Y704 VHF TX / UHF1 RX / UHF2 RX
Y705 UHF1 TX / UHF2 TX
Y706 Y705 700 RX/TX 800 RX/TX
Y707 7/800 TX / VHF RX
Note 3:
Pin Pin Freq Band Freq Band
VHF TX/ UHF1 RX / UHF2 RX
3
6 700 RX/TX / 800 RX/TX /UHF1 TX/UHF2 TX
U709
7 VHF RX / 7/800 TX
Check
5V DC supply.
Check parts
around U702.
If OK, replace
U702.
VCO