JBL HTPS-400, Service Manual, Page: 15 / 32

HTPS-400 ADJUST BIAS PROCEDURE
(Mandatory when any output MOSFET transistors Q305,310,320,321,306,307,311,312 are replaced)
1. Amplifier should be unplugged and OFF.
2. Remove Amp assembly from cabinet.
3. Identify the Linear board assembly (PCB with the output transistors)
4. Adjust RP301 and RP302 fully Counter Clockwise. See diagram below.
5. Apply 120 VAC power to unit.
6. Verify LED is ON.
7. Connect voltmeter set to DC millivolt range to twin pins on terminal J303, on Linear board.
8. Verify initial voltage is less then 0.1 mV.
9. Adjust RP301 Clockwise until voltmeter reads 0.3 mV + the initial current from step #8.
10. Adjust RP302 Clockwise until voltmeter now reads 0.6 mV + the initial current from step #8.
11. Turn amplifier OFF. Disconnect AC power to unit.
12. Remove voltmeter from terminal J303.
13. Replace amp assembly back into cabinet.