Adcom GFA 5802 Service Manual

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SERVICE MANUAL

ADCOM ®
deio/is you can hear

POWER AMPLIFIER

GFA-5802

TABLE OF CONTENTS
Introduction ................ 1
Test Procedures ............. 1
Parts List .................. 2
Specifications ............... 5
Schematic ................. 6
Board View ................ 9

EM
10 Timber Lane
Marlboro, NJ 07746
USA

Tel: 732-683-2356
Fax: 732-683-9790

Web: http://www.adcom.com

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INTRODUCTION

This service manual is intended to assist trained and qualified technical personnel in verifying the
performance of. adjusting. and repairing the ADCOM GFA-5802 amplifier. The procedures described here
are not intended for persons unfamiliar with the appropriate safety and test procedures.

A WARNING rib

THERE ARE POTENTIALLY LETHAL VOLTAGES WITHIN THE GFA-5802 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-5802 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-5802 UNLESS YOU ARE TECHNICALLY QUALIFIED
TO DO SO.

TEST PROCEDURES
. 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 BOkHz 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.

IMPORTANT
BEFORE PROCEEDING WITH ADJUSTMENTS, MAKE SURE AMPLIFIER IS AT ROOM
TEMPERATURE.

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 both channels. Operate the amplifier without load or input
connection for these adjustments.

1. Turn the amplifier on and allow to idle for 5 minutes.

2. Connect a voltmeter across source resistor R88 on the amplifier board and set bias pot R61 for a
33mV reading.

3. As the amp warms, the idle current will vary. Continue to monitor the voltage across R88 and adjust
R61 until the reading stabilizes at 33mV.

4. Measure the voltage across each of the eight source resistors R88 through R114. (Measurement
across this bank of resistors is strongly suggested, as bank R8 through R25 idles near -90V).
The readings across these resistors should average 33mV and all should be between 23mV and
43mv.

Note: a properly biased amplifier will take approximately 15 minutes from initial power up to
reach stable idling current and temperature. The heat sinks will be warm to the touch.

DC OFFSET ALIGNMENT
1. Connect a millivoltmeter across the speaker output terminals and adjust DC offset pot R179 on the

input board for OmV (+/- 5mV). I!