Service manual

8000 SERIES RECEIVER DECODER MODULE (RDM)
2-5
March 2000
Part No. 001-3139-502
2.8 900 MHz RECEIVER DRAWER ALIGNMENT
2.8.1 INTRODUCTION
This alignment procedure should be performed
with the drawer installed in the repeater because a ref-
erence signal from the local oscillator drawer is
required for proper operation of the receiver drawer.
2.8.2 TEST EQUIPMENT FREQUENCY STABIL-
ITY
The receiver frequency is locked to the 1.250000
MHz reference frequency from the local oscillator
drawer. This frequency is stable to within ±0.1 PPM
from -30° to +60° C as required by the FCC. To prop-
erly align the IF sections of the receiver, the signal
generator must have similar stability. Since most test
equipment is not this stable, a separate reference oscil-
lator that is stable to ±0.05 or ±0.01 PPM may be
needed to clock the signal generator.
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. If it is certain that one
or both oscillators in the local oscillator drawer are off
frequency, there is an adjusting screw on the oscilla-
tors that is used to change the frequency slightly.
Adjustment should only be performed when the ambi-
ent temperature is near the calibration reference of 25°
C (77° F).
If power is removed from the local oscillator
drawer, the oscillators require a restabilization period
that is related to the length of time power is removed.
No frequency-critical adjustments should be
attempted until the oscillator frequency has stabi-
lized. Sample restabilization times are listed below.
Do not place a repeater in service until the oscillators
have had time to stabilize. Battery backup is recom-
mended on at least the repeater or repeaters containing
the local oscillator drawer.
Length of Time Power Restabilization Time
Was Removed
24 hours 24 hours
1 month 48 hours
3 months 4 weeks
2.8.3 SYNTHESIZER ADJUSTMENT
Refer to the alignment points and test setup dia-
gram in Figure 2-5 and proceed as follows:
(Refer to Appendix B for frequencies.)
1. Connect a DC voltmeter to TP201 on the synthe-
sizer board. While depressing S204, adjust L201 for
a meter reading of 3.50V. If L201 adjusts properly
and out-of-lock LED CR205 is off, proceed to Sec-
tion 2.8.4.
2. If CR205 is on or if L201 does not adjust properly,
the multiplier stages which provide feedback to the
synthesizer may not be tuned properly. Perform the
next step and Section 2.8.4 and then start again with
Step 1.
3. Connect a DC voltmeter to TP4 in the Receiver and
adjust L201 on the Synthesizer board for a maxi-
mum meter reading.
2.8.4 MULTIPLIER TUNE-UP
1. Connect a DC voltmeter to TP4 and adjust L25 for
maximum (typically 0.14V).
2. Connect a DC voltmeter to TP5 and adjust L18 and
L19 for maximum (typically 1.1V).
3. Connect a DC voltmeter to TP6 and adjust L20, L21
and L22 for maximum. Then adjust L23 for mini-
mum (typically 1.0V).
4. Momentarily press S201 on the Synthesizer board
and then press and hold S204 until LED CR205
turns off. Check the DC voltage at TP201 and read-
just if necessary as described in Section 2.8.3,
Step 1. Then repeat steps 1-3 of this Section.
2.8.5 SECOND OSCILLATOR ADJUSTMENT
1. Using a pickup loop connected to a communications
monitor, monitor the second oscillator signal by
inserting the loop through the opening in the shield
over the second oscillator stage.
2. Adjust C55 for 34.3000 MHz.