Harris Platinum HT EL 2000LS, Technical Manual

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If the exciter drive level seems normal, try the module in a
different cabinet slot that is known to have proper RF drive. If
the problem doesn’t follow the module, then inspect the cables
leading to the module RF input for that transmitter slot. If the
problem does follow the module, check the RF input cable inside
the module, connected between the black power connector on the
module inside rear panel and the 4-way power divider.
Module Has Full Green LED Illuminated But No RF Output
PA modules: Since an insufficient drive level causes one of the
green LEDs to go out, that cause is ruled out. This condition
would most likely be caused by a failure of the pass FET driving
circuitry on the module control board. The control board logic
has illuminated the green enable LED, but it is not turning on the
pass FETs. This will not allow the quarter modules to receive the
50 Volts DC that they need in order to operate. See the paragraphs
on Pass FET Failure Fault (6 blinks) located on page A-12 in this
section.
Driver modules: The pass FET driving circuitry could also be
the culprit, as in PA modules. In driver modules, however, a more
likely cause is insufficient or no drive.
Try swapping driver modules, if the problem follows the module,
check the module RF path, starting with the RF input cable inside
the module, then moving to the input attenuator (R4, R5, R6) on
the interconnect board, then to the first stage. Also, check the
connections between each stage and the next.
If this doesn’t isolate the problem, check the DC voltage and
current supplied to each quarter module, through the red wire
connected to screw terminal TB1. Measure the applied voltages
and normal idle currents for each quarter module.
If a quarter module indicates 50 Volts present but no current,
check the 15 Volts supplied through J1-1.
If the problem stays in the same transmitter slot, the problem is
within the transmitter (AGC module, phase and gain module if
present, preamp if present, power divider if parallel drivers are
used, or RF cables).
A.3.5
Locating Failed RF FETs
A.3.5.1
DC Resistance Test
The most common symptom of a bad FET is an ISO fault (3
blinks). Using a Simpson 260 (or equal), measure the DC resis-
tance from the gate to ground of each FET. This is done with the
module on the bench with neither RF or DC power applied.
Compare the resistance measured from one FET to the next. The
resistance indicated will vary with the voltage of the multimeter
used. A resistance on one FET significantly lower than the others
indicates a bad FET or leakage in a gate chip capacitor.
If no FET indicates a low gate to ground resistance proceed to
idle current testing.
A.3.5.2 Idle Current Test
First, it is necessary to determine the original bias current per
FET, and to determine on which quarter module the failed FET
lies. For this procedure, no RF drive will be applied; however, a
load resistor should still be placed at the module output to prevent
oscillation.
Starting with the first quarter module (nearest the logic board)
and working toward the front handle, measure the total idle
current of each quarter module in turn. Either insert a current
meter in line with the 50 Volt wire at TB1, or use a clamp-on DC
current meter if available. With no RF drive applied, apply 50
Volts and enable the module. Note the quarter module current,
disable the module, remove the 50 Volts and move the current
meter to the next quarter module.
If no current meter of sufficient range is available, a small
resistance can be placed in series with the 50 V line, and the
voltage drop used to calculate current from Ohm’s Law (I=V/R).
Values from 0.1 to 0.2 ohms should be satisfactory. At 0.1 ohms,
the voltage drop across the resistor will indicate 0.1 Volts for
every 1 amp of current. A sensitive digital meter with a millivolts
range is needed to use this technique.
After taking the current measurements on each quarter module,
determine the correct bias current setting per FET.
The nominal bias current per FET is given in the Table A-2.
Now that the correct bias current is known and the quarter
module with failed FET(s) has been located, one can locate the
failed FET. Move the current meter to the quarter module show-
ing abnormally low current. Again, apply DC power only and
enable the module. While observing idle current, slowly rotate
the bias control for each transistor counterclockwise, one at a
time; this should reduce the current for the corresponding FET.
If the idle current does not drop when the pot is turned fully
counterclockwise, then the RF FET is probably bad. To deter-
mine which pot affects the idle current of each FET, refer to
Figure A-6. Note the difference between high band and low band
quarter modules.
Procedure for setting bias current on a quarter module:
First, determine the correct bias current per FET. Connect a
current meter in series with the 50 Volts to the quarter module.
Next, set the bias pots fully counterclockwise, apply 50 Volts,
and enable the module. The current meter connected to the
quarter module being adjusted should read almost zero current
(less than 20 mA). Slowly turn each bias pot clockwise to set the
current for the corrosponding FET, then adjust the next bias pot
until a total of twice the current per FET is reached, and so on,
until the last FET is adjusted such that the total current is four
times the current per FET.
Example: On a low band class AB stage, after determining that
the correct bias for a given quarter module is 400 mA per FET,
start with all bias pots fully counterclockwise. Slowly turn R25
clockwise until 400 mA is reached, then turn R26 clockwise until
800 mA is reached, then R27 until 1.2 A is reached, and finally
turn R28, stopping at 1.6 A total.
CAUTION: Adjusting the bias pots too far clockwise or too
quickly can destroy an RF FET due to excessive current. Go
slowly.
Appendix A - RF Amplifier Modules
888-2457-001 A-11
WARNING: Disconnect primary power prior to servcing.