Motorola ASTRO XTSTM 4000, Detailed Service Manual

Page: 85 / 210

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