Motorola ASTRO XTSTM 4000 Detailed Service Manual page 85

Troubleshooting Charts: RX RF
RX RF–Page 3
2
Measure RF levels at
first preselector filter and
compute filter loss.
I/P C410, O/P L480
Loss < 3 db?
Measure RF levels at LNA
and compute LNA gain.
VHF: I/P L480, O/P C434
UHF: I/P L480, O/P C436
Gain
about
+12 dB?
Measure RF levels at
second preselector filter and
compute filter loss.
VHF: I/P C450, O/P C455
UHF: I/P C426, O/P C427
Loss
< 3 dB?
Measure RF and IF level at mixer
and compute mixer conversion gain.
VHF: RF I/P C470, IF O/P between
L472 & L490
UHF: RF I/P C470, IF O/P L490
Mixer
gain about
3 dB?
6871620L01-C
No
6
Yes
No
Check
LNA
Yes
No
8
Yes
No Check
mixer
Yes
3
TP551
(Vdc)
Measure
VHF
control voltage
1.10 42.6
at TP551
3.10 47.1
Fvco
too low?
No
VHF: < 42.6 MHz
UHF: < 71.1 MHz
Yes
TP551 No
5
= 5 Vdc?
Yes
Remove SH11
and
inspect second LO VCO
Visual
No
examination
OK?
Yes
Replace
Varactor D550
Measure IF level at XTAL filter
and compute XTAL filter loss.
I/P between L472 & L490,
VHF: O/P C491
UHF: O/P C584
Loss
No
< 4 dB?
Yes
No problem
found
2nd LO
at C560
(MHz)
Fvco
UHF
71.1
75.6
No
TP551
5
= 0 Vdc?
Yes
Repair
defects
7
May 28, 2008
5-23