Clear-Com PS-704 - Manual (Page 13)

P S - 7 0 4 F O U R - C H A N N E L P O W E R S U P P L Y
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FRONT PANEL INDICATORS
2 OVERLOAD INDICATORS: If a short-circuit condition appears on any
of the four intercom lines, the sense circuitry for that line will disconnect
the DC power to that line only and the red LED for that channel will be
on. If a red LED is illuminated, the fault on that channel must be located
and removed. The overload LED will go out and the PS-704’s automatic
short reset circuitry will automatically bring the power back up on that
channel when the fault is removed. If there are no shorts, but the LED
stays on, an overload exists, possibly due to too many intercom stations
connected on that intercom line.
If the failure continues, unplug the intercom line connected to the affected
channel. Its red short LED should go out. This indicates a short in that
line. Follow that intercom line connecting and disconnecting cables along
the line until the fault is isolated.
TO CHECK BEFORE POWERING SYSTEM
Before you turn on the power:
CHECK TERMINATION
There should be one and only one termination for each channel in the system.
This termination is usually set to on at the main station or power supply. To
ascertain that only one termination is present on the channel, perform the
following test:
1. Using a multimeter, measure the resistance between pins #1 and #3 on
one of the channel A XLR connectors at the rear of the unit.
2. If the channel is terminated properly, then the resistance should measure
approximately 4 K Ω. A very high channel resistance means the channel is
not terminated. Channel resistance of 2 K Ω indicates a
double-termination. If a double-termination is indicated, locate the other
power supply or main station and set its termination to off.
Note: The location of the termination switch varies with model. On
some products, the termination switches are on the rear panel;
on others, they are inside the unit. Consult the unit’s manual
for the location.
3. Repeat for the other channels.
4. Check resistance between chassis ground and pin #1. Using an
Ohmmeter, measure the resistance from pin #1 on the main station or
power supply to chassis ground. The measurement should read 10 Ω. A
high reading (over 100 Ω) indicates that the 10Ω resistor in the unit has
failed and requires replacement. Failure to perform the replacement will
result in an audible “buzz” in the system. A reading of less than 10 Ω (or a
short) typically indicates that the shell and pin #1 of one of the
interconnect cables are shorted together. Test the individual cables until
the culprit is located and repair or replace the cable.