Introduction; Test Procedures; Bias Alignment; Bias Adjustment - Adcom GFA-5300 Service Manual

This service manual is intended to assist trained and qualified technical personnel in verifying the
performance of, adjusting, and repairing the ADCOM GFA-5300 amplifier. The procedures described here
are not intended for persons unfamiliar with the appropriate safety and test procedures.
THERE ARE POTENTIALLY LETHAL VOLTAGES WITHIN THE GFA-5300 AMPLIFIER WHICH WILL
BE ACCESSIBLE ONCE ITS TOP COVER IS REMOVED. DO NOT ATTEMPT FAMILIARIZATION,
INSPECTION, OR ANY PROCEDURE WHATSOEVER UNLESS YOU HAVE DISCONNECTED THE
GFA-5300 FROM THE WALL AC OUTLET OR OTHER SOURCE OF AC POWER AND THE
POWER-SUPPLY CAPACITORS ARE COMPLETELY DISCHARGED. THESE INSTRUCTIONS ARE
PROVIDED FOR USE ONLY BY COMPETENT TECHNICAL PERSONNEL. DO NOT UNDERTAKE
ANY SERVICE PROCEDURES IN THE GFA-5300 UNLESS YOU ARE TECHNICALLY QUALIFIED TO
DO SO.
•
All tests are performed with a 115V, low-distortion (less than 2% THD), AC-power source, 8-ohm
resistive load (except slew rate), and a signal source of not more than 600 ohms.
•
An 80kHz low-pass filter is employed during THD distortion measurements.
•
Signal-to-noise measurements are "A" weighted.
•
Damping factor is measured by comparing the 1 watt output voltage with and without an 8 ohm load.
•
Slew rate is measured with an inductive load, and is derived with a dual-time-based oscilloscope
reading the slope of a full power 5kHz square wave. DO NOT OPERATE THE AMPLIFIER AT
FULL-POWER SINE WAVE ABOVE 22kHz OR FULL-POWER SQUARE WAVE ABOVE 5kHz.
BEFORE PROCEEDING WITH ADJUSTMENTS, MAKE SURE AMPLIFIER IS AT ROOM
CORRECT BIAS ADJUSTMENT IS CRITICAL TO THE PERFORMANCE OF THIS AMPLIFIER.
MAXIMUM OUTPUT POWER, MINIMUM THD AND HEAT DISSIPATION ARE AFFECTED BY THE
BIAS SETTING AND MUST BE CORRECT TO MAINTAIN THE SONIC QUALITY AND LONGEVITY OF
THE AMPLIFIER.
Prior to performing BIAS ALIGNMENT turn unit on and allow to idle with rated output (8 ohm) for
approximately 5 MINUTES before attempting adjustments

Bias Adjustment

Connect voltmeter across emitter resister R038 (R138 for right channel) and adjust R043 (R143 for right
channel) to read 20mV +/-2mV
DC Offset
Connect voltmeter across the speaker outputs and adjust R042 (R142 for right channel) to read 0mV +/-
10mV

INTRODUCTION

WARNING

TEST PROCEDURES

IMPORTANT
TEMPERATURE.

BIAS ALIGNMENT

1