3/6/08
888-2463-001
6-5
WARNING: Disconnect primary power prior to servicing.
Section 6 Troubleshooting
Diamond Series™
6.2.2.3
Driver Power Supply Over Current
Too much current drawn from +28volt supply. The current draw for the preamp and
amplifier pallet in the Driver are summed together for this fault. Most likely fault is a
shorted FET on the preamp or amplifier pallet but could also be caused by
misadjustment of the over-current threshold. R13 (on the Driver Control board) should
be adjusted for 4.75 Vdc at TP2 to set the proper overcurrent threshold. Driver will turn
off output. Latching.
6.2.2.4
FET Fail
FET failure has been detected on the driver amplifier pallet. Most likely, the PA Pallet
needs re-biasing, see "5.7.2 Setting Bias for the Driver Pallet" on page 5-23.
The amplifier in the DD-1 driver pallet is the same as the amplifier pallet used in the PA
modules. Each pallet contains 2 separately biased push-pull amplifiers. The pallet is
biased for class A operation with each half of the pallet drawing 2.5 amps for a 5 amp
pallet total (idle current with no RF input applied). The Driver control board contains a
bias control circuit which when adjusted properly will reduce the total pallet bias to 4
amps and keep it there. As LDMOS FETs age, their idle current tends to drop. The Bias
control circuit then increases the bias to compensate.
If the actual pallet bias drops below 4 amps, the bias control circuit will no longer be
able to increase the bias and the Driver will be shut off with the indication being “FET
FAIL”. An FET has most likely not failed, but the bias control circuit is wide open and
the logic assumes that the current draw is low due to FET failures. If this happens the
Driver pallet biasing procedure should be performed to re-bias the pallet. If the proper
bias currents cannot be achieved, then the pallet should be replaced.
If the pallet is replaced, the new pallet must be biased for Driver operation, see "5.7.2
Setting Bias for the Driver Pallet" on page 5-23.
6.2.2.5
Over-Temp (Heatsink)
A thermistor near the PA pallet heatsink is monitoring the temperature. This could be
caused by extremely high ambient air temperature, loss of fans, dirty fan filters (located
inside the dirver chassis) or misadjustment of the over-temperature threshold. R52 (on
the Driver Control board) should be adjusted for 0.975 Vdc at TP10 to set the proper
overtemp threshold. Driver output will turn off. Latching.