Adcom GFA-5002 Service manual

SERVICE MANUAL

2 CHANNEL

POWER AMPLIFIER

GFA-5002

TABLE OF CONTENTS

Introduction . . . . . . . . . . . . . . . . . . 1

Test Procedures . . . . . . . . . . . . . . 1

Parts List . . . . . . . . . . . . . . . . . . . . 2

Voltage Conversion . . . . . . . . . . . . 6

Specifications . . . . . . . . . . . . . . . . 7

PCB Layout . . . . . . . . . . . . . . . . . . 8

Wiring Diagram . . . . . . . . . . . . . . . 9

Schematic . . . . . . . . . . . . . . . . . . .10

10 Timber Lane

Marlboro, NJ 07746

USA

Tel: [phone]

Fax: [phone]

Web: http://www.adcom.com

INTRODUCTION

Step

Coupling

Adjust

Adjust For

Plus Lead

Minus Lead

1

TP3

TP1

VR603

DC Voltmeter

2

TP4

TP2

VR604

reads 10mV

This service manual is intended to assist trained and qualified technical personnel in verifying the
performance of, adjusting, and repairing the ADCOM GFA-5002 amplifier. The procedures described here
are not intended for persons unfamiliar with the appropriate safety and test procedures.

WARNING

THERE ARE POTENTIALLY LETHAL VOLTAGES WITHIN THE GFA-5002 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-5002 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-5002 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 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.

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

Prior to performing BIAS ALIGNMENT turn unit on and allow to idle with rated output (8 ohm) for
approximately 5 MINUTES before attempting adjustments

1

a GFA-5002 SERVICE PARTS LIST

Amplifier PCB
AF705514 and AF705524

Schematic Part Number Description
Location

C603, C604 12001260 Capacitor
C607, C608 12001400 Capacitor
C609, C610 12005470 Capacitor
C611, C612 12001420 Capacitor 22pF 125V
C613, C614 12003070 Capacitor 39pF 125V
C615, C616 12005325 Capacitor
C617-C620 12003080 Capacitor 68pF 125V
C621, C622 12001450 Capacitor
C623, C624 12001170 Capacitor
C625,C626 12001450 Capacitor
C901-C904 12005450 Capacitor
D001,D002 16001254 Distortion LED (Yellow) LTL-1254
D003,D004 16001204 Thermal LED (Red) LTL-1204
D601-D608 16004148 Diode 1N4148
D610-D613 16003001 Diode DSC30TC
D801,D802 16004148 Diode 1N4148
D901,D902 16008030 Bridge Rectifier PBPC903
F601-F604 19009400 Fuse 4A 250V
Q601-Q606 33002362 NPN Transistor 2SC2362K
Q607,Q608 33001016 PNP Transistor 2SA1016F-T
Q609,Q610 33001376 PNP Transistor 2SA1376L-T
Q611,Q612 33009700 PNP Transistor 2SA970B-T
Q613,Q614 33001376 PNP Transistor 2SA1376L-T
Q615,Q616 33003478 NPN Transistor 2SC3478L-T
Q617,Q618 33006000 NPN Transistor 2SD600KE
Q619,Q620 33003478 NPN Transistor 2SC3478L-T
Q621,Q622 33001376 PNP Transistor 2SA1376L-T
Q623,Q624 33003902 NPN Transistor 2SC3902
Q625,Q626 33001507 PNP Transistor 2SA1507
Q627,Q628 33001047 NPN Transistor 2SD1047
Q629,Q630 33008170 PNP Transistor 2SB817
Q631,Q632 33001047 NPN Transistor 2SD1047
Q633,Q634 33008170 PNP Transistor 2SB817
Q801,Q802 33001016 PNP Transistor 2SA1016F-T
R603,R604 27004260 Resistor
R605,R606 27004135 Resistor
R607,R608 27004050 Resistor
R609,R610 27004120 Resistor
R611,R612 27004420 Resistor
R613,R614 27004350 Resistor
R615,R616 27004225 Resistor
R617, R618 27004140 Resistor
R619,R620 27004160 Resistor
R621,R622 27004120 Resistor
R623,R624 27001550 Resistor

2.2µF 100V
1µF 100V
47µF 100V Electrolytic

4.7µF 50V Electrolytic

0.22µF 100V

0.1µF 100V

0.22µF 100V
6800µF 63V Electrolytic

2.74KΩ 1%MF

4.99KΩ 1%MF
1KΩ 1%MF

22.1KΩ 1%MF

33.2Ω 1%MF
825Ω 1%MF
475Ω 1%MF
332Ω 1%MF

47.5KΩ 1%MF

22.1KΩ 1%MF
121Ω 1%MF

2