User's Manual

351804-1 Rev. 100

COM5000-8 TECHNICAL MANUAL

5.4.1.4 Driver/Splitter Maintenance

To gain access to the Driver/Splitter, remove the 1.25kW PA Module bottom cover.

The location of the Driver/Splitter is shown in figure 5.4.1C.

Adjustments:

The following adjustments should only be performed when a PCB assembly has been repaired in the field.

All PCB's are factory aligned prior to shipment.

Bias current adjustment:

a) Operate the PA Module as a “stand alone” Amplifier – while still part of the Transmitter - as

described in section 5.4.1 above.

Initially turn the 5-kW amplifier system OFF and make sure that the T-4180 RF Output connector

is terminated into a 50 Ohm Dummy Load to prevent RF from being applied while performing the

adjustment.

b) Remove fuse F1 on the Driver/Splitter PCB and insert an ampere meter in place of the fuse to

measure the current drawn by Q1.

Pre-adjust the bias adjust potentiometers R34, R35, R36 and R37 to their counter clockwise

position for minimum bias in Q1 and Q4.

c) Q1 bias is turned ON by shorting R30 to ground (do not key the system to apply bias). Note the

"residual" current. It should be 70-90 mA. Slowly turn R36 clockwise for a current reading of 230 mA

above the residual current.

Then turn R37 slowly clockwise until the total current reads 460 mA above the residual current.

e) Q4 bias is always ON.

With the DVM, measure and record the DC voltage across C20. This “start” voltage should be 47 –

50V.

f) Slowly turn R34 clockwise until the “start” voltage drops by 2.0 volt. Then turn R35 slowly clockwise

until the “start” voltage has dropped by 4.0 volt.

g) Repeat f) a number of times until the voltage measured across C20 has dropped to 22-23 V. To

improve the adjustment accuracy it may be necessary to drop the voltage by 1.0 V as the voltage

across C20 approaches its final value.

h) Verify that the voltage measured on the gates of Q4 (@ R31 and @ R29) are within 0.2 volt of each

other.

i) After adjusting bias current, turn OFF the 5-kW Amplifier system, remove the ampere meter and

reinstall the fuse.