Tait T850 II Series, Service Manual

Page: 131 / 370

M850-00
T856/857 General Information C1.7
Copyright TEL 31/09/98
1.1 Introduction
The T856 is a synthesised, microprocessor controlled FM base station transmitter
designed for single or multichannel operation in the 400 to 520MHz frequency range
1
with a standard power output of 25W. The RF section of the transmitter comprises a fre-
quency synthesiser which provides 100mW of frequency modulated RF drive to a two
stage, wide band output driver followed by a 25W power amplifier. A thermal shut-
down feature is provided in the T856 in case operating temperatures exceed acceptable
levels.
The T857 is a synthesised, microprocessor controlled FM base station exciter designed
for single or multichannel operation in the 400 to 520MHz frequency range
1
. With a
standard power output of only 1W, the exciter is designed for use with the T858 50W or
T859 100W power amplifiers. The RF section of the exciter comprises a frequency syn-
thesiser which provides 100mW of frequency modulated RF drive to a two stage, wide
band output amplifier.
A wide selection of audio characteristics may be obtained from the audio processor.
Optional circuit blocks are an audio compressor and a pre-emphasis stage. They can be
bypassed or linked to one or both audio inputs, and then back into the remaining audio
circuitry in almost any combination. All audio processor options are link selectable.
The synthesiser frequency is programmed via the serial communications port. Eight
channel select lines are accessible via an optional D-range connector (D-range 2 -
T800-03-0000) at the rear of the set.
All components except those of the VCO are mounted on a single PCB. This is secured
to a die-cast chassis which is divided into compartments to individually shield each sec-
tion of circuitry. Access to both sides of the main circuit board is obtained by removing
each of the chassis lids. There is provision within the chassis to mount small option
PCBs.
The front panel controls include line sensitivity, microphone socket and carrier switch.
This switch turns on the carrier (unmodulated) as an aid to servicing.
The T856 and T857 are both 60mm wide and each occupies a single space in a Tait rack
frame, which has the ability to accommodate up to seven standard modules.
1. Although capable of operating over the 400-520MHz frequency range, the T856 and
T857 have an 8MHz switching range (see Section 1.2.3 and Section 3.1).