Harris Platinum HT EL 2000LS, Technical Manual

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then enable the module with a transmitter ON command. If the
fault remains, the problem is likely to be with the module control
board.
If the fault clears when RF drive is removed, check to see that
the module is not being overdriven. If not, then the overdrive
threshold on the control board may be misadjusted. See proce-
dure located on page A-18 in this section to check the Overdrive
Threshold.
ISO Voltage Fault (3 blinks) — The RF input to the quarter
module passes through a two-way divider on the quarter module,
and is then fed to two parallel amplifiers on the quarter module.
The outputs of these two amplifiers are recombined in a two way
combiner on the same board. The combiner contains a 10 Watt
reject load resistor, called an ISO resistor because it is used to
provide isolation between the combiner input ports.
If outputs of the two parallel amplifiers are equal in amplitude
and phase, the voltage across the ISO resistor will be very small.
Should some component fail on one of the amplifiers, its output
would decrease to a level much lower than the other parallel
amplifier, which would cause the voltage across the ISO resistor
to increase significantly. If the ISO voltage of any quarter module
exceeds about 1.9 Volts, the control board shuts the power
amplifier module down and indicates an ISO fault.
An ISO fault will almost always be caused by a component
failure in a quarter module (RF FET, chip cap, ISO resistor, or
open solder connection). The common cause is a damaged RF
FET.
Damaged FETs are sometimes caused by problems in the module
output combiner, examine this area first before trying to re-en-
able the module to avoid further damage. With DC power and
RF drive removed, visually inspect the connections between the
quarter module outputs and combiner inputs, between the com-
biner sections, between the combiner output and the directional
coupler, and between the coupler and the output connector. An
inspection mirror aids the examination greatly. Next, use an
ohmmeter to confirm an open between the output connector
center pin and chassis, and continuity between the center pin and
each quarter module output. Also examine each quarter module,
especially the area near its output.
If no problems are found with the output circuitry, try to confirm
the ISO fault with the module on the test fixture. Put the safety
cover down (applying RF drive), switch on the DC power and
attempt to enable the module. If the ISO fault does not occur
again, there may be a problem in the system rather than with the
module (for example, an open cabinet combiner dump load or a
damaged module RF power input connector).
If the ISO fault is confirmed, check the bias current of each
quarter module, one at a time with no drive applied (lift the safety
cover to remove RF drive). A quarter module with blown FET(s)
will have lower bias current than the others. Check the section
on bias current setting to confirm the correct bias current for each
quarter module. If a quarter module with low bias current is
found, first record its total bias current, then observe the current
while turning off bias to each FET one at a time with the bias
adjustment pots. Record the current after turning each pot off and
look for one or more FETs whose bias current is zero or lower
than the others.
If no quarter modules or FETs indicate low bias current, there
are two possibilities: either a shorted, open or damaged compo-
nent on a quarter module, or a problem with the PCM (logic)
board. Try to rule out a problem with the PCM board first. If a
storage oscilloscope or peak-holding DMM (e.g. Fluke 87) is
available, try to confirm an ISO voltage greater than about 1.9
Volts. Remove DC power, clip a probe onto the ISO voltage line
close the safety cover, connect the probe to the scope or DMM,
apply DC and enable the module. If the ISO voltage does not
appear, look for problems on the module PCM board (check for
0.9-1.0 Volts on U6 pin 10). If no storage scope or peak-holding
DMM is available, proceed to looking for problems on the
quarter modules after checking for 0.9-1.0 Volts on U6 pin 10 of
the PCM board.
To find a problem on a quarter module, first try to locate one
quarter module that is the source of the ISO fault. With DC power
off and RF drive removed, connect a scope or meter to the ISO
voltage line, and disconnect the 50 Volt wires from all but quarter
module #1 (nearest the back of the module). Cover the exposed
ends of the loose 50 Volt lines with electrical tape to prevent them
from shorting within the module. Close the safety cover (apply-
ing RF drive), turn on the DC power and try to enable the module,
observing whether or not an ISO fault occurs. Shut off the DC,
remove the 50 Volt connection from quarter module #1, recon-
nect the 50 Volt line for quarter module #2, and again try to enable
the module. Repeat with each of the remaining quarter modules.
The module should ISO fault during one of these trials (the
quarter module with the problem is the one with its 50 Volts
connected when the fault occurs), and the ISO voltage should
read a low value (several tenths of a Volt or less) during the other
trials.
Once a quarter module with a problem is located, perform a
careful visual inspection, looking for burned or broken compo-
nents, bad solder joints, solder splashes, loose hardware, open
circuit board traces, etc. Check the output ISO resistor (low band
R15; high band R11) by lifting one lead and measuring with an
ohmmeter (should measure 190 to 2102 for low band, or 95 to
1052 for high band).
See the procedure located on page A-18 in this section to check
for the correct ISO Fault Threshold.
Power Supply Voltage Fault (4 blinks) — The RF FET transis-
tors operate on a nominal 50 Volt DC supply. If the power supply
voltage is too high or too low, the devices could be damaged. The
control board monitors the voltage, and reports a power supply
voltage fault if it is not between approximately 44 and 54 Volts.
If several modules exhibit the same fault, check the voltage of
the power supply and look for faulty connections. Remember that
heavy current draw could cause the supply voltage to drop
significantly lower than that measured with only a voltmeter
loading the line. If only one module exhibits the fault, check the
DC supply voltage and connections, plus the module power
Appendix A - RF Amplifier Modules
888-2457-001 A-9
WARNING: Disconnect primary power prior to servcing.