TN105 Dual Subrack Interconnection
MT-3/4 Radio Systems
TECHNICAL NOTES
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TECHNICAL NOTE: TN105, REV 6-0-0, © November 2018
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The SR-39-1 subrack is designed to hold and interconnect the MT-3 and MT-4 series of receiver, transmitter and
control modules on one universal motherboard. This motherboard distributes audio, control and power signals
between the separate modules plugged into the subrack. The two main power signals used by the MT-3 and MT-4
modules are the +13.8 Vdc (power input) and +9.5 Vdc (regulated) power.
The +13.8 Vdc power input (+13.8 Vdc nominal, +10 to +17 Vdc range) is connected to the motherboard by a
terminal strip mounted on the back of the subrack. The +13.8 Vdc signal line is routed through the System Regulator
(SM-3) module and then distributed across the motherboard to the other modules. This allows the user to un-plug
the SM-3 module and turn off all power supplied to the other modules. This feature is not used very often as all
Codan modules are capable of being hot swapped.
The +9.5 Vdc regulated power is generated in the System Regulator module by the internal +9.5 Vdc voltage
regulator built into the SM-3. This +9.5 Vdc regulated voltage is then distributed across the motherboard to the
other modules. The voltage regulator is designed to source enough current to operate all modules in the subrack.
If a Codan radio system is designed so that a second subrack is required, the confi guration is dependent on the
modules in the second subrack:
Receivers, Transmitters or Control Cards In Second Subrack
If the second subrack requires receiver, transmitter or control cards, a system regulator is required in the second
subrack to source enough current for the +9.5 Vdc regulated voltage to these modules.
AMP-4 30 Watt Power Amplifi ers
If the second subrack contains Codan AMP-4 series 30 Watt power amplifi er modules, the AMP-4 series amplifi ers
do NOT use +9.5 Vdc power, so ONLY the +13.8 Vdc connection is required on the second subrack.
AMP-2 30 Watt Power Amplifi ers
If the second subrack contains Codan AMP-2 series 30 Watt power amplifi er modules (see Figure 1), the +9.5 Vdc
power is required, however, the regulated voltage current draw for these modules is very low, and the SM-3 in the
fi rst subrack can source enough current for the power amplifi ers in the second subrack. In this case, a second SM-3
module is not required, and the +9.5 Vdc regulated voltage is jumpered from the fi rst subrack with the SM-3 to the
second subrack with no SM-3 module, using the +9.5 Vdc input / output connector on the motherboards.
Jumper JU1 required on all second subracks with no SM-3 (AMP-2 and AMP-4 series)
If the second subrack has no SM-3 module installed, jumper JU1 is required to be installed in the second subrack
to allow +13.8 Vdc power to be connected to the 30 Watt power amplifi ers (the +13.8 Vdc is typically routed through
the SM-3). Jumper JU1 is typically a 16 AWG wire soldered across the jumper points (see Figure 2 for jumper wires
and JU1 placement). On older motherboards (Serial # 123125 and earlier) the jumper is in the same location, but
is labelled as JU40.
If a radio system is ordered from the factory with these confi gurations, the jumper wires are included in the shipment
and jumper JU1 is installed at the factory.
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