8571—Fault finding
NGT Transceiver System Technical Service Manual 355
Checking the slave 8571
If the fault appears to be in the slave 8571, then unless the remote site is suitably
equipped, it is recommended the equipment be returned to the workshop for the
necessary repairs.
To check the slave 8571:
1 Place both the master 8571 and the slave 8571 in close proximity (for ease of
servicing) and connect them together via a 2-wire or 4-wire line. Incorporate a
suitable attenuator to prevent overloading of the signals. Connect a transceiver and
power supply to the master 8571.
1 While pressing the or keys on the handset to change channels, use an
oscilloscope to check for 300 Hz control and FSK signals through the slave 8571
signal path.
A fault in the 300 Hz control circuits can be traced by applying a 300 Hz signal at
100 mV p–p from an audio signal generator to the input of the slave 8571.
1 With an oscilloscope, check for 300 Hz on the Dual Line Equaliser PCB at the
outputs of the hybrid IC101a, the 300 Hz BPF IC106 (the clock frequency should be
30 kHz), and the threshold detector amplifier IC104a.
The measured level at the output of the 300 Hz threshold amplifier IC104a depends
on the DC current applied to the control input pin 1.
1 Set switch S1 on the Microprocessor & TCVR Interface PCB to position D to check
nominal levels shown on the circuit diagram 04-02541.
1 Check that the output of the detector IC105b is high (5 V) when the control tone is
present.
1 Check that the output of combiner inverter IC5 is low (300 Hz detect bus).
1 Check that the 300 Hz detect bus to the microprocessor IC12 pin 18 on the
Microprocessor & TCVR Interface PCB goes high (5 V) while the 300 Hz is
present.
Unless special equipment is available, continuous channel changing from the
handset should be a sufficient signal source to enable the FSK signal to be traced
through the signal path of the slave 8571.
1 Commencing on the Dual Line Equaliser PCB with an oscilloscope, check for a
burst of the FSK signal at the outputs of the hybrid IC101a, the gain-controlled
amplifier IC103a, the equalisation amplifiers IC103b and IC104b, and finally the
output of the 3 kHz LPF IC105a.
Levels depend on the input level to the 8571, and the gain settings of the gain-
controlled amplifier IC103a and equalisation amplifiers IC103b and IC104b.
1 Check the FSK on the Tx audio bus on the Microprocessor & TCVR Interface PCB
(Tx and FSK share the same path at this point).
1 Check that the FSK is applied to the modem input IC6 pin 15, that data appears at
the demodulated output pin 5, and that FSK detect pin 3 goes low during the signal
period.
Drawings 08-03905 and 08-03993
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