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A more effective solution
is
accomplished
in
newer Digital Logic boards by the inclusion of integrated
circuit
US.
The newer type Digital Logic board
is
a direct replacement for any older versions.
Interlock Switches
The interlock switches have two poles, one that
is
in
the coil circuit ofthe contactors and HV shorting
solenoids, and one that
is
used to operate the led indicator circuit.
Sometimes the interlock switch contacts that are
in
the coil circuit may open, but the contacts that are
in
the indicator circuit may not close. This creates a situation
in
which you have symptoms
of
an
open interlock, but
no
indication to that effect.
In
an FM-25K,
an
open interlock
in
the HV cabinet will
disable the contactors
so
that the screen and plate supplies do not energize.
In
the Main cabinet, an
open interlock will keep the blower motor, bias supply, and HV Power Supply from running.
If you experience these symptoms, test the AC side of the interlock switches for closure. This can be
done with the power off, using
an
ohmmeter. Exercising of the switches may clear the problem, so
be
conscious of this possibility. Otherwise, you may defer the problem to another time by not
knowing which switch was the problem.
Power Cutback
Several customers have had a need for a power level cutback function. Typically the reason for this
is
operation
on
a standby AC power generator that
is
unable to handle the normal full power load of
the transmitter.
There are various ways to do this, each with their own advantages. The best
in
terms of electrical
performance and functionality
is
to switch variacs
in
the screen power supply. This gives you the
ability to easily and independently set the two power levels, and yields the most stable operation for
the low power mode.
At the other end of the spectrum in terms of cost
is
switching another potentiometer
in
the IPA power
control circuit.
In
either case, steps will need to
be
taken to either disable or accommodate the automatic power
control ci
rcu
it.
If you have a need for power cutback, you may contact us to discuss your situation and alternatives.
Remote/Local Switching
Depending on whether the remote local switch breaks before it makes, your transmitter might
sometimes trip off when you switch between one mode and the other.
To solve the problem, add a 1
uF
capacitor, Harris part number 526-0050-000, across R99 on the
Analog board.
VSWR Foldback Kit (994-9006-001)
Automatic VSWR Foldback circuit
is
available to retrofit older transmitters. The VSWR Foldback kit
provides the means of automatically lowering the screen voltage
as
the VSWR gradually becomes
too high, then automatically returns it to the normal level as the VSWR problem clears. This is a very
useful function
in
climates and installations affected by the accumulation of ice on the antenna.
Also please refer to the discussion of the Full Range Screen Control.
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