Service manual

2000 SERIES RECEIVER DECODER MODULE (RDM)
3-18
March 2000
Part No. 001-3139-502
3.12 900 MHz RECEIVER ALIGNMENT
The receiver frequencies are locked to the
1.250000 MHz reference frequency from the local
oscillator drawer. This frequency is stable to within
±0.1 PPM from -30°C to +60°C as required by the
FCC. To properly align the IF sections of the receiver,
the signal generator must have similar stability. Since
most test equipment is not this stable, a separate refer-
ence oscillator that is stable to ±0.05 or ±0.01 PPM
may be needed to clock the signal generator and fre-
quency counter.
Adjustment of the 1.250000 MHz reference fre-
quency probably should not be attempted for the same
reason. Unless the communications monitor has a sta-
bility of ±0.01 PPM or better, the repeater frequency
may be more correct than that of the monitor. If it is
certain that one or both oscillators in the local oscilla-
tor drawer are off frequency, there is an adjusting
screw on the oscillators that is used to change the fre-
quency slightly. Adjustment should only be per-
formed when the ambient temperature is near the cali-
bration reference of 25°C (77°F).
If power is removed from the local oscillator
drawer, the oscillators require a minimum restabiliza-
tion period of 30 minutes. No frequency-critical
adjustments should be attempted until the oscillator
frequency has stabilized.
Refer to Figure 3-32 for component locations.
Refer to Figure 3-37 for equipment needed and setup
diagram.
3.12.1 PRE-TEST
1. Preset L201, L202, L203, L204, L206, L223 and
L224 tuning screws about 1/4 inch above the top of
the casting.
2. Preset L202 and L205 tuning screws so they are just
barely above their tightening lock nuts.
Figure 3-32 900 MHz RECEIVER ALIGNMENT POINTS
L220
L201
L202
L203L204
L205
L206
L223
L224
TP5
L207
L229
L209
L211
L231
L225
Z213
R284
R261 R264
U206
TP3
TP4
J201
1
2
19
20
TP6
U207
Y201
TP1
U208
U210
U208
1
4
58
1
4
58
1
4
58
1
4
58
1
10
11
20
6