User Guide

PAMS
Technical Documentation
NHD–4
Disassembly & Troubleshooting
Page 37
Original 11/97
•Check the RX_CAL line at R783, It should be logic level high, near 2.75 V.
If not, confirm that the phone is in the AMPS Troubleshooting Mode. Ensure VRXM
is at about 4.15 V at R832. Inspect V1, V708, R6, R765, R783, and R785. Ohm out
the RX_CAL line from pin 114 of the ASIC N704 to R785.
Chart of AMPS RX DC Node Voltages
The following chart summarizes the node voltages of the AMPS RX when
the phone is in the AMPS RX/TX Mode with a 881.52 MHz signal input of
–65 dBm, modulated with 8 kHz FM, 1 kHz audio.
Node Voltage
(V)
D1 – AMPS RX IC, pin 4 4.40
V9 – collector of 1st IF amp, pin 1
3.30
V9 – base of 1st IF amp, pin 3
0.97
V9 – emitter of 1st IF amp, pins 2, 4
1.12
V12 – collector of LNA
3.70
V12 – base of LNA
0.82
RF Gain Switches N701 and N702, pin 1
3.80
RF Gain Switches N701 and N702, pin 8
0.00
SWAGC line at R830
0.00
RX_CAL line at R783
2.75
Demodulation Test
If you have an oscilloscope available you may use it to do a visual check
on the demodulated 1 kHz tone on the ANARX+DA line. Again feed in the
modulated –75 dBm, 881.52 MHz, 8 kHz FM, 1 kHz audio RF signal into
the receiver and initiate the AMPS Troubleshooting Mode from Service
Software. Use a standard scope probe at C1 to look at the 1 kHz tone. It
should appear as a clean sine wave of approximately 380mVpp. Step
down the input RF power. At about –90 dBm the sine wave will begin to
become a bit fuzzy. At –105 dBm the waveform will be fuzzy, but still
distinguishable.
If not, confirm that the phone is in the AMPS Troubleshooting Mode.
Ensure the RF signal is modulated correctly and of the requested signal
level. Inspect R2, C1 and C10.
RF Node Power Checks – AMPS Mode
A high impedance passive RF probe may be used with a spectrum
analyzer to obtain the RF nodal power levels along the receiver chain.
The following levels assume a –65 dBm, 881.52 MHz, unmodulated tone
input into the receiver. Maintain a 1.0 MHz span on the spectrum
analyzer. A CDMA test box with spectrum analyzer capabilities will do.
The RF power levels published below were found a passive probe with the
500 ohm tip. The blank column is reserved for values the user will obtain
with their own probe.