
3/6/08
888-2463-001
5-23
WARNING: Disconnect primary power prior to servicing.
Section 5 Maintenance and Alignments
Diamond Series™
5.7
Driver Set-up
Complete set-up of the DD-1 Driver consists of several short procedures. Each of the
procedures should be read and understood before starting.
5.7.1
Setting Bias for the Preamp
The following procedure should be used when replacing the Driver Preamp Board or to
simply re-bias the preamp as part of a complete DD-1 Driver tune up.
a.
Shut off AC power to the driver with the AC switch on the rear of the driver.
b.
Connect 50-ohm terminations to the SMA input and output connectors on the rear
of the DD-1 Driver.
c.
Turn R64 Preamp Bias Adjust on the Driver Control board fully CCW (ten-turn
pot). See Figure 5-9 for location of R64.
d.
Turn on the AC supply and enable the Control board by pressing S1 on the Driver
Control Board.
e.
Turn Preamp Bias Adjust R64 CW until the voltage across R77 (on the Driver
Control Board) is 50mVdc. This corresponds to a Preamp current of 1 amp. See
Figure 5-9 for location of R77.
f.
Turn OFF the Driver by depressing S2 on the Driver Control Board.
5.7.2
Setting Bias for the Driver Pallet
The following procedure should be used when replacing the Driver Pallet or to simply
re-bias the pallet as part of a complete DD-1 Driver tune up.
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.
NOTE:
If the 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.