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25A1090 (Rev.3)
TROUBLESHOOTING
Servicing will be more efficient when the history of the unit is known and can be taken into ac-
count. The service strategy should be different when a unit fails on the production line than when
it fails in the field, because if it fails on the line there is a possibility of incorrect or missing parts.
If the unit has failed in the field, check for signs of tampering or hand soldering that could indicate
that the customer has modified the unit or has attempted to repair it.
PRELIMINARY TESTS
1.
Install two fresh "AA" batteries and turn the unit on. Verify normal display operation. Set
the unit to the lowest frequency in the group.
2.
Observe the radiated output on a spectrum analyzer by holding it near the analyzer's
antenna. Measure the frequency and RF power output level and verify that they are within
expected limits.
3.
Speak into the microphone. Observe the modulation on the spectrum analyzer display and
listen to the audio output on a matching SLX4 receiver tuned to the same channel; prefer-
ably the customer's unit, if this is a field return. Check for normal audio level. Listen for dis-
tortion, noise, or any unusual sounds.
4.
Change the frequency to the highest frequency in the group. Repeat steps 2 and 3 above
and verify that operation is normal.
RF FREQUENCY OR SIGNAL PRESENCE PROBLEMS
If there is no carrier present at the transmitter antenna port or measured RF power is unusually
low, check the earlier stages to verify decent continuity of the signal path. An amplifier stage or
filter may have a part placement error or other defect. Usually the first step is to visually examine
the board for skewed / missing parts before going in with a probe. If there is no signal out of the
VCO output then check the control voltage at TP_PLL_TV when the transmitter is set at the low-
est channel in the group. If the voltage is not tunable into the 1.1-1.4 V window but can be tuned
to a lower or higher voltage, this means the VCO might be built for the wrong band or has a wrong
part.
Check the values of the caps C522, C523, C528, C525, C534, and inductor L505 one by one
and try to tune into the window. (Note that these caps are Low-ESR series!) If the voltage at
TP_PLL_TV is 0 V and does not respond to tuning CV500, the loop is unlocked.
Check Y500 for a 16 MHz oscillation by setting the analyzer to CENTER FREQ=16 MHz, REF.
LEVEL=-40 dBm, SPAN=1 MHz. Hold the probe right above the crystal and look for a spike-like
signal. If one is present, check the parts in the VCO, including parts other than the capacitors
mentioned above to see if anything is not soldered correctly.
Check bias voltages on the transistors to verify that they are powered and biased correctly. If
an oscillation exists at the output and is close to the nominal center frequency (within 100 kHz)
but cannot be tuned to the +/- 3 kHz window via CV501, check the value of the caps C536 and
C537.
Check that the trimmer CV501 is soldered correctly.
LOW RF OUTPUT POWER
If RF signal is present at the right frequency with low power, compare the RF power measure-
ments with a known good board at various points moving from the antenna backwards towards
the VCO. Look for signal discontinuities in the path (sudden large drop in measured power by
more than a few dB).
Check the board visually for missing/skewed parts. Try to confine the problem to a specific cir-
cuit segment, and then check solder connections and part values or DC voltages for error. If the
power out of the VCO is very low, check the bias voltages on the transistors and the values of
L502, C551, C543, C530.
EXCESSIVE CURRENT DRAIN
Try isolating different sections of the transmitter, such as the RF, Audio, and Digital circuits.
Look for reversed polarity capacitors, wrong resistor values, poorly soldered components, and
shorted traces.
