Harris DX 25U, Техническое руководство

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b. Set the oscilloscope sync to “external” and connect the
input to J5 on the Oscillator.
c. Adjust the horizontal vernier on the scope so that one full
RF cycle occupies 9 divisions on the screen. Each division
now equals 40 degrees of phase shift.
d. Using the Horizontal positioning and triggering level on
the scope, place the zero crossing of the waveform on the
center crossing of the vertical and horizontal lines of the
scope.
e. Increase the vertical sensitivity of the scope to expand the
waveform. Switch the scope to the X10 position and
readjust the horizontal position so that the RF transition
again crosses the center line of the scope. This will be the
reference for the phase measurements. If another RF
amplifier transition occurs at the first large division on the
left, this amplifier is operating at 4 degrees lagging from
the reference. See Figure 6-6.
f. Now that a reference phase has been established, without
changing any of the scope settings, move the scope probe
to the desired RF amplifier to be measured. It is usually a
good idea to first measure the drive phase of the Steps
RF33-RF38. Then set your reference phase to the module
that is typical of the six. Some modules may be operating
at the same phase and the others may be a few degrees off.
There will be some phase difference between the A side
and B side of the same module, but typically the A sides
of the RF amplifiers should all line up as well as all the B
sides should be within specifications. Typically, there
may be two to four degrees difference between A and B
sides and there should not be more than +/-4 degrees
difference between all the A sides when referenced to an
A side. The same maximum difference should also be
seen between B sides when referenced to a B side.
6.31.7.1 Excessive Drive Phase Difference
If a module is out of specifications on drive phasing, substitute
in a new module. If the problem is on the module, it is most
likely caused by a defective drive transformer, T1 or T2, a
defective MOSFET, or associated circuitry. If a module has just
been repaired, check the control components, including transis-
tors and diodes. A poor solder connection can cause a drive
phase problem on an RF amplifier. Substitution is the only way
to troubleshoot this problem. If by changing the module the
drive phase is still not correct, the problem may be the drive
cable. The drive cable can be swapped with another temporarily
to determine if it is the cable.
6.31.8
RF Amplifier Drain Phasing
Even though the drive phasing to a particular amp may be within
limits, it is possible for the output phasing to be out of specifi-
cation and cause problems such as module overheating and
failure. Measurement of the drain phasing is only necessary
when isolating a specific module problem. The drain phasing of
the Binary Amplifiers can be adjusted. Refer to SECTION V,
Maintenance, for this procedure.
RF amplifier drain phasing should be /-4 degrees of
each other. Typical phasing is usually /-2 degrees.
Measure the Drain phasing as follows.
WARNING
ENSURE ALL AC POWER IS REMOVED FROM TRANSMITTER
AND THAT THE GROUNDING STICK HAS BEEN USED TO DIS-
CHARGE ANY RESIDUAL POTENTIAL WHERE POWER HAS
BEEN APPLIED ANY TIME THE INNER FRONT DOOR IS
OPENED TO ACCESS THE RF AMPLIFIER MODULES.
a. Use a 10x scope probe with an extended tip, Harris part
#610-1131-000, and connect it to TP1 (Q3 drain) on
RF33. The probe can be attached through the interlocked
inner door. Make sure the scope case is properly grounded
and ground the probe to the door.
b. Set the scope on AC coupled, 50V per division, and the
trace to center of the screen.
c. Connect the external sync of the scope to J5 on the
Oscillator and make sure the scope sync is set to External.
d. Operate the transmitter at 25 kW with no modulation.
Adjust the horizontal vernier on the scope so that a full
RF cycle occupies 9 divisions on the screen. Each division
now equals 40 degrees of phase shift.
e. Using the Horizontal positioning and triggering level on
the scope, place the zero crossing of the waveform on the
crossing between the center vertical and horizontal lines
on the scope. Increase the vertical sensitivity of the scope
to expand the waveform.
f. Switch the scope to the X10 position and readjust the
horizontal position so that the RF transition again crosses
the center line of the scope. This will be the reference for
the phase measurements.
g. If another RF amplifier transition occurs at the first large
division on the left, this amplifier is operating at 4 degrees
lagging from the reference. See Figure 6-7.
h. Now that a reference phase has been established, without
changing any of the scope settings, move the scope probe
to the desired RF amplifier to be measured. It is usually a
good idea to first measure the drain phase of steps RF33-
RF38 then set your reference phase to the module that is
typical of the six. Some modules may be operating at near
the same phase and the others may a few degrees off. Set
the reference to the most common phase. Note that there
will be some phase difference between the A side and B
side of the same module, but typically the A sides of the
RF amplifiers should all line up as well as all the B sides
should be within specifications. Typically there may be 2
to 4 degrees difference between A and B sides and there
should not be more than +/-4 degrees difference between
all the A sides when referenced to an A side and all the B
sides when referenced to a B side.
6.31.8.1
Excessive Drive Phase Difference
If a module drain phasing is out of specifications, substitute a
different module in that position.
DX-25U
6-20 888-2297-002 Rev. R: 11-11-96
WARNING: Disconnect primary power prior to servicing.