User Manual

5-1
June 1997
Part No. 001-3412-002
SECTION 5 SERVICING
5.1 GENERAL
5.1.1 PERIODIC CHECKS
This transceiver should be put on a regular main-
tenance schedule and an accurate performance record
maintained. Important checks are receiver sensitivity
and transmitter frequency, modulation, and power out-
put. A procedure for these and other tests is located in
Section 6. It is recommended that transceiver perfor-
mance be checked annually even though periodic
checks are not required by the FCC. During the first
year, make an additional check or two to ensure no
TCXO frequency drifting has occurred.
5.1.2 SURFACE-MOUNTED COMPONENTS
A large number of the components used on the
transceiver board are the surface-mounted type. Since
these components are relatively small in size and are
soldered directly to the PC board, care must be used
when they are replaced to prevent damage to the com-
ponent or PC board. Surface-mounted components
should not be reused because they may be damaged by
the unsoldering process.
5.1.3 SCHEMATIC DIAGRAMS AND COMPO-
NENT LAYOUTS
Schematic diagrams and component layouts of
the PC boards used in this transceiver are located in
Section 8. A component locator guide is also pro-
vided to aid in component location.
5.1.4 REPLACEMENT PARTS LIST
A replacement parts list with all the parts used in
this transceiver is located in Section 7. Parts are listed
alphanumerically according to designator. For infor-
mation on ordering parts, refer to Section 1.8.
5.1.5 TCXO MODULE NOT SERVICEABLE
The ±1.5 PPM TCXO module is not field ser-
viceable. Part changes require a factory recalibration
to ensure that the oscillator stays within its ±1.5 PPM
tolerance.
5.2 SYNTHESIZER SERVICING
5.2.1 INTRODUCTION
When there is a synthesizer malfunction, the
VCO is not locked on frequency. When an unlocked
VCO is detected by the lock detector circuit, U801,
pin 18 goes low (0V).
NOTE: The user-supplied circuitry must disable the
transmitter and receiver when an out-of-lock condi-
tion is indicated.
When the VCO is unlocked, the fR and fV inputs
to the phase detector are not in phase (see Section
4.1.2). The phase detector in U801 then causes the
VCO control voltage to go to the high or low end of
its operating range. This in turn causes the VCO to
oscillate at the high or low end of its frequency range.
As shown in Figure 4-1, a loop is formed by
VCO Q850, amplifier Q851/Q852, and the RF IN of
U801. Therefore, if any of these components begin to
malfunction, improper signals appear throughout the
loop. However, correct operation of the counters can
still be verified by measuring the input and output fre-
quencies to check the divide number.
Proceed as follows to check the synthesizer I/O
signals to determine if it is operating properly.
5.2.2 REFERENCE OSCILLATOR
Check the signal at U811, pin 8. It should be
17.5 MHz at a level of approximately 1V P-P. If the
TCXO module is defective, it is not serviceable and
must be replaced with a new module as described in
Section 5.1.5.
5.2.3 VCO
Output Level
The output level of Q853 can be measured with
an RF voltmeter or some other type of high impedance
meter. The minimum level after a power splitter at
R893 should be -10 dBm.