Service manual

BASIC THEORY OF REPEATER
TRUNKING
Repeater trunking was designed to permit multiple users
to
equally and automatically share all channels. If a
repeater
is busy and the system has two or more repeat-
ers, the incoming
signal may be "handed off" to another
repeater. This provides more rapid communication than
with single repeater systems, where each call must be
completed before another can be handled. Trunking
systems are more convenient for the mobile/portable
user who
is no longer required to monitor the channel or
adjust
squelch. The user simply depresses the radio
PTT switch to access a channel. Since the user cannot
monitor a
channel, trunking systems permit higher
security in business communications.
The
MRS series repeaters are completely compatible
with
all Uniden trunking radios. In these radios, each
"system" setting is normally a different repeater site. This
provides a wide area
of coverage. The repeater retrans-
mits the Group (digital)
I. D. codes that trunking radios
generate to provide private conversation.
DESIGN CONCEPT
The design of the MRS 904T repeater system employs
the module concept. The following table describes the
modules and their location as a part of the system:
Module
Descriotion Location
Receiver Full Shield~d
Plugs into Motherboard
Plug-in
Local Full Shielded Plugs into Motherboard
Oscillator Plug-in
Mother board PCB Sub
Mounts horizontally
Chassis over Logic Board.
You
may raise Motherboard
to vertical on hinged
mounting for ease of
service.
Logic
PCB
Mounted in cabinet
Board
base under Motherboard
DC relay PCB
Mounted on back inside
panel
TRUNKING SYSTEM OPERATION
Mobile/portable radios are programmed to monitor one
specific "home" channel in the trunking system. They
monitor this
channel at all times in the idle state . The
mobile/portable radio receives subaudible data from the
"home" channel. this subaudible data tells the radio that a
call is coming in and steers the radio to the correct
channel to receive the call. The mobile/portable radio
also completes a "handshake" with the "home" channel
every time the user depresses the radio PTT switch. If
the "home" channel is not busy handling another call, the
mobile/portable radio repeats
on the "home" channel. If
the "home" channel was busy when the user depressed
the
PTT switch, the mobile/portable radio "handshakes"
with the next available repeater in the system.
All mobiles in the same group must be "homed" on the
same channel. There are
250 (001-250) group !.D.'s per
channel and up to 20 (01-20) channels per system.
Therefore a
20 channel system has the capability of, SOOO
unique !.D .' s.
Each repeater uses the Repeater Network Data Link
(RNDL) bus to communicate with the other repeaters
in
the system. The RNDL bus is a high speed serial data
bus that communicates information regarding the re-
peater state (idle, busy) and the
mobile I. D. A block
diagram of the RNDL bus is included on page xx. A 100
Ohm pull-up
resistor must be located at the first and last
device
on the RNDL. To provide these pull-up resistors in
an MRS 904 repeater, install a jumper from CN2-1 to
CN2-4. The pull-up resistor is already installed in the
ARX
850 Air Time Billing Unit and ARX 820 Validator.
The
MRS 904T repeater uses broad band technology to
cover the
following frequency ranges without retuning:
• Transmit- 935 to 941 MHz
• Receive - 896 to 901 MHz
There are
400 channels available to the repeater.These
channels are spaced 12.5 kHz apart.
Page 15 MRS 904
1/1/90
I
SERVICING INFORMATION
DISASSEMBLY
1. Place the MRS 904 unit on a flat static-free work surface.
2. Remove the eight screws that secure the top cover. There are two on each side, two on the top, and two on the
rear.
3. Loosen the two captive slotted screws on the sub-chassis. You can now lift the sub-chassis on hinges to a
verticle position. This
allows access to the Logic/Controller Board and the Mother Board.
4. To acheive bottom access, remove the eight screws on the bottom panel. Secure the sub-chassis before you
turn the unit over to access the bottom panel.
SUGGESTED TEST EQUIPMENT
Table 3 shows the test equipment needed to repair this unit. A service monitor will replace many of the individual
items.
1 a01e 3. ~uaaestea 1 est E IUioment
TEST INSTRUMENT
REQUIRED SPECIFICATIONS
SUGGESTED
TYPE or Eauivalent
DC Power Supply
Voltage: 13.6 VDC
Ratelco PS-9
Current: 8 Amoeres Continuous
12
VDC Batterv
i RF Wattmeter
Frequency: 935-941 MHz Bird Model 43 with
Power: 0-1 Watts 50E Element
RF Dummy Load
Impedance:
50 Ohms
Bird Model 8085
Power: 50
Watts
Bird Model8135
DC Voltmeter
Range:
0-20 VDC Simpson 260
DC Resistance: 10 Meg Ohm
Fluke D802 or D804
AC Voltmeter
Ranae: 3
mV-20V AC Leader LMV 181 A
Audio Generator
Frequency: 10Hz-10kHz
Leader LGA 120 A
Level:
0-1V
Deviation Meter
Ranae:
0-5 kHz
Marconi
TF 2304
Distortion Meter
Range:
30Hz-10kHz
Leader LDM 170
or Level: 1 mV-300 V
Sinadder
lnout Level:
1-5 VAC
Heloer's Sinadder 3
RF Frequency Counter Range: 1.3 GHz
HP-5325B OPT 010, 030
Accuracy: +.001 ppm/vr.
Oscilloscope
Bandwidth: 50-100 MHz
Tektronix 2213
Triggered
Sweep
Tektronix 465B
Hameo HM705
RF Signal Generator
Range:
800-910 MHz
HP 8640
Level: 0.1-1000 uV
HP
8642
Modulation: lnt/Ext
Deviation:
0-5 kHz
Attenuator
Attenuation:
20 dB
Bird 8341-200
Power: 50
Watts Minimum
Bird
8343-200
FM Linear Detector
Service Monitor Cushman CE-50 or
Cushman CE-4000
IFR 1200S
or IFR 1500
Motorola RS2001
Wavetek 3000
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