Service manual

June 21, 2004 1-35
NP88 FLEX GENERAL
Troubleshooting
3. Refer to the applicable schematic waveform and voltage information in the
appropriate supplement.
4. Ensure the pager is in self-diagnostic mode 5 (refer to "Self-Diagnostic Fea-
ture").
5. Turn off the RF output of the signal generator.
6. Monitorthecollector oftheFirstMixer Transistorandnote the maximumand
minimum dc voltage levels as the oscillator is enabled and disabled. The
maximum dc voltage occurs when the oscillat or is inactive (that is, the base of
the Oscillator Transistor is grounded). The difference between these two dc
voltage levels must be between 5 and 40 mV.
Anotherway of expressingthemixerinjection voltage measurementis mixerdelta
current (
ΔI). Mixer ΔI is a formulated percentage measurement representing the
change in mixer dc bias current when the oscillator is off (maximum dc voltage),
then on (minimum dc voltage). Mixer
ΔI is defined by the following equation:
Mixer
Δ
I=
Vmax - Vmin
x100%
Vreg - Vmax
where:
• Vmax is when the oscillator is inactive.
• Vmin is when the oscillator is active.
• Vreg is the regulated voltage at TP3.
With a properly functioning mixer/tripler circuit, mixer
ΔI is between 2% and 15%,
corresponding to the 5 mV and 40 mV difference reading from the previous
measurement.
If the mixer injection voltage measurement i s incorrect or cannot be obtained, it
indicates thateitherthemixerortheoscillatorisnotworkingproperly.Inthiscase,
troubleshoot the oscillator and mixer circuits by checking the dc voltages given in
the schematic diagram and then isolating the defective component.
Iftheinjectionmeasurementisnormal,proceedwithtakeover/noiselevelmeasure-
ments.
Takeover/Noise Level Measurements
TakeovermeasurementsarerecommendedforcheckingtheRFAmplifierandmixer
circuit.TheRF takeoverisdefined asthedropinnoiselevel(measuredat M1)when
theRFAmplifierisdisabledfromt hereceiver;forexample,baseoftheRFAmplifier
isgrounded.The mixertakeoveristhedropinthenoiselevelmeasuredatM1when
the mixer stage and the RF Amplifier are disabled.
The theory behind system noise figure and receiver sensitivity shows a receiver
with sufficient takeover has a noise figure that is dominated by the noise figure of
theRFAmplifier.Inotherwords,withsufficientRFtakeover,thenoiseofthestages
following the RF amplifier is “taken over” and the receiver noise level is as good as
the noise figure of the RF amplifier.
Receiver Noise Level
1. Ensure a battery with a minimum output of 1.3 volts is installed in the pager.
2. Refer to Tables 1-9 and 1-10 for lists of recommended test equipment.
3. Refer to the applicable schematic waveform and voltage information in the
appropriate supplement.