Harris Gates One 994 9202 002, Technical Manual

Page: 79 / 110

SECTION E
OUTPUT MONITOR (A18)
E.1.
Principles of Operation
The Output Monitor board provides the
VSWR detection, the dc voltage sample for
the Power Output meter, and an adjustable
RF Output sample for the six power levels.
The VSWR detector consists of a circuit
to detect any difference between an RF
voltage sample and an RF current sample
taken from the Output Network at the back
end of L2.
The two RF samples are matched up in
amplitude and phase on the Output Monitor
board by C13 and C15. With these samples
being equal in all respects, there is no dif-
ferential applied to the primary of T1.
However, a change in impedance in the
Output Network will alter the amplitude
and/or phase relationship of these two sig-
nals. This will create a difference signal
which T1 will couple to the diode circuit for
rectification.
The diode circuit creates a dc voltage
which is read on the VSWR Detector Null
reading on the Multimeter. This same dc
voltage is used by the VSWR trip circuit for
transmitter protection.
For the Power Output Meter, the RF cur-
rent sample is rectified, filtered, and sent to
the Controller board.
The Modulation Monitor sample is pro-
vided by a toroidal pickup in the Output
Network. Relays K1 through K5 switch
rheostats in circuit to affect the sample volt-
age so that the monitor sample may be the
same for all six power levels.
E.2.
Replacement/Alignment
If the Output Monitor needs to be re-
aligned for any reason or replaced, the fol-
lowing procedure applies.
Before aligning the VSWR detector cir-
cuit, it is important that the Output Network
be properly tuned for 50 ohms j0 at the
bracket at the back end of L2. Refer to the
description of Output Network tuning in
Section D.
With the Tuning of the Output Network
having been verified as good, disconnect
plug P1 from the Output Monitor, and con-
nect an RF generator tuned to the carrier
frequency across R18. Connect a scope at
the junction of L4 and C11.
Adjust trimmer capacitor, C14, to mini-
mize the signal as seen on the scope. This
resonates the primary circuit of the VSWR
Detector Null transformer, T1.
Plug P1 back in. Turn the transmitter back
on, and alternately adjust C13 and C15 to
minimize the VSWR Detector Null reading
on the multimeter. This matches the VSWR
Detector circuit to the 50 ohm tuning of the
Output Network.
The monitor sample levels should be set
according to the needs of your particular
monitor. If you are not connecting a modu-
lation monitor to J3, then leave rheostats
R8, R11, R23, R24, R27, and R30 fully
counterclockwise. This will result in no
power being lost in the rheostats.
E.3.
Troubleshooting
E.3.1.
Symptom: Detector Null Read-
ing Is High and Cannot Be Ad-
justed To Zero.
Possible causes:
E.3.1.1.
Bad Load Impedance
Verify that the load impedance is normal.
There should be 50 ohms j0 at the bracket
end of L2.
E.3.1.2.
Sample Signal Missing
Given that the impedance at the L2
bracket is normal, the most likely cause for
the problem is that one of the RF samples is
missing. Check for presence of RF at both
inputs of the detector circuit (J1-5 and J1-7).
It takes both inputs, equal in amplitude and
phase, to have zero output from this circuit.
If the RF current sample is missing, the
Power Meter should also read zero. This
might be caused by a broken wire leading
up to the toroid, or possibly the protection
diode A18CR11 on the toroid assembly
A18T1 is shorted. This assembly is located
on the bracket end of L2.
Rev.AE: 03-05-2002 888-2314-001 E-1
WARNING: Disconnect primary power prior to servicing.