8525B/8528 Technical Service Manual Technical Description 4-17
Publication No: 15-02036 Issue 6
4.6.4 External Microphone Audio Clamp
The audio input from the Remote Control connector at pin 11 P502
(Option R) is connected, via C102, R103, R101 and C101 to the Aux
Compressor input of the Audio and IF 1650kHz PCB. FET V101 is
normally biased on via R104 and R105, thus clamping the microphone input
to ground. Operation of the remote PTT switch grounds the junction of
R104 and R105 removing the bias from V101. This allows the microphone
input to be applied to the compressor input.
4.6.5 Additional Audio Facilities—Option RC
Two summing amplifiers, IC401a and IC401b, can be fitted to provide for
special applications. IC401b can accept two inputs from the Audio and IF
1650kHz PCB. Inputs are selected, according to the application, by the
inclusion or omission of R401 and C401 or R402 and C402. R19 is omitted
when the option is fitted and the amplifier output is connected to the Remote
Control connector Option R.
Similarly, by the inclusion or omission of input resistor-capacitor
combinations, IC401a can accept up to three inputs from various sources. Its
output is connected to the pre-volume control input of the IF and Audio
PCB.
4.6.6 Headphone Output—Option PH
A 6.4mm switched jack socket can be fitted to the transceiver front panel
and connected to the loudspeaker circuit to provide for connection of
headphones. The headphone signal is attenuated by a 330Ω resistor in the
line.
4.6.7 CW facility—Option M
A 3.2mm jack socket can be fitted to the transceiver rear panel for
connecting a morse key. When the key is operated, a ground is applied via
the jack socket to the Microprocessor Controller PCB (P2 pin 3). The
microprocessor detects the key function via the I/O port of IC3 and
generates the required PTT and audio tone for transmission.
4.6.8 External Selective Call or RTTY—Option PS
Connector J301 on the rear panel of the transceiver provides an interface for
selective calling or radio teletype equipment. Operation of the external
equipment via this interface is described in Section 4.11.
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