User's Manual

351804-1 Rev. 100

COM5000-8 TECHNICAL MANUAL

5.4.1.2 350 W Amp Assembly Maintenance

To gain access to the bottom two 350 W Amp's remove the bottom cover. To gain access to the top two 350

W Amp's proceed as follows:

1. Loosen the two "captive" screws that secure the Filter assembly to the 1.25kW PA Module front

panel.

2. Carefully move the filter module up to past its vertical position and make sure that it "leans back"

onto its back stops.

3. For location of the 350 W Amp's see figure 5.4.1B and 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.

Initial potentiometer setting: R42 midrange

R44, 47, 60 and 64 fully counter clockwise

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.

Insert an ammeter in the 50 VDC connection to the 350 W Amp module to be adjusted. If

necessary, temporarily remove the auxiliary 50VDC connection to eliminate its effect on the current

reading. This is the thin red wire going to the Fan Interface Board.

Pre-adjust the bias adjust potentiometer R44, 47, 60 and 64 to their fully counter clockwise position.

Note: R42 is not used and should be left in its approximate midpoint position.

b) Turn ON the 5-kW Amplifier system. Key the system and observe the residual current. Slowly turn

R44 clockwise for a current increase of 400 mA. Then turn R47 for an additional current increase of

400 mA (800 mA above residual). Now turn R60 clockwise for a current increase of 400 mA (1200

mA above residual). Finally turn R64 clockwise for a current increase of 400 mA (1600 mA above

residual). This provides a bias current of 400 mA through each of the four MOS FET transistors.

Perform this bias adjustment quickly to avoid the transistor heating effects from influencing the

adjustment. Be sure that the heatsink is getting adequate cooling during alignment to allow stable

settings.

Note: If it is not desirable to key the 5-kW Amplifier system, a particular 350 W Amp can be biased

independently by applying 5-8 VDC to J1-5 after removing the plug connection to J1-5.

c) After adjusting bias current, turn off the 5-kW amplifier system, remove the ampere meter and

reconnect 50 VDC.