Introduction; Test Procedures - Adcom GFA-7605 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-7605 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-7605 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-7605 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-7605 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.
BIAS ALIGNMENT
The component references are the same for all channels. Operate the amplifier without load or input
connection for this adjustment.
1. Turn the amplifier on and allow to idle for 5 minutes.
2. Connect millivoltmeter across emitter resistor R36. Adjust bias pot R25 for a 10mV +/- 1mV reading.
NOTE: The bias adjustment must be performed with the heatsink attached and the amplifier module
installed in the chassis. The best way to perform the bias adjustment is by connecting one probe of
millivoltmeter to emitter of Q15 (lowest pin) and connecting the other probe to the positive (red) speaker
output.
DC OFFSET CHECK
1. Connect a millivoltmeter across the speaker output terminals and confirm a reading of 0mV +/- 10mV.

INTRODUCTION

WARNING

TEST PROCEDURES

IMPORTANT
TEMPERATURE.
1