Adcom GFA-5503 Service manual

SERVICE MANUAL

3 CHANNEL

POWER AMPLIFIER

GFA-5503

TABLE OF CONTENTS

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

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

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

Voltage Conversion . . . . . . . . . . . . 5

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

Schematic . . . . . . . . . . . . . . . . . 8

8551 East Anderson Drive, Suite 105

Scottsdale, Arizona 85260

USA

Tel: [phone]

Fax: [phone]

Web: http://www.adcom.com

INTRODUCTION

This service manual is intended to assist trained and qualified technical personnel in verifying the
performance of, adjusting, and repairing the ADCOM GFA-5503 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-5503 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-5503 FROM THE WALL AC OUTLET OR OTHER SOURCE OF AC POWER AND THE POWER­SUPPLY CAPACITORS ARE COMPLETELY DISCHARGED. THESE INSTRUCTIONS ARE
PROVIDED FOR USE ONLY BY COMPETENT TECHNICAL PERSONNEL. DO NOT UNDERTAKE
ANY SERVICE PROCEDURES IN THE GFA-5503 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 for approximately 5 MINUTES before
attempting adjustments.

1. Connect millivoltmeter across emitter resistor R52 for left channel (R52 for center channel, R51 for

right channel)

2. Adjust bias pot P2 until meter reads 50mV +/- 5mV

DC OFFSET ADJUSTMENT

1. Connect millivoltmeter across speaker output

2. Adjust DC offset pot P1 until meter reads 0mV +/- 10mV

1

a GFA-5503 SERVICE PARTS LIST

AMPLIFIER MODULE PCB

SCHEMATIC ADCOM PART NUMBER DESCRIPTION
LOCATION

C01 XXXXX CAPACITOR 390pF 100V
C02 12001075 CAPACITOR 2.2uF 100V
C03 12005280 CAPACITOR ELEC 47uF 50V
C04 12001085 CAPACITOR 0.22uF 100V
C05 12001480 CAPACITOR SILVER MICA 15pF 100V
C06 12001470 CAPACITOR SILVER MICA 20pF 100V
C07 12005325 CAPACITOR ELEC 4.7uF 50V
C09 12005380 CAPACITOR ELEC 47uF 25V
F1,F2 19000800 RAIL FUSE 8A 250V
LED1 16001204 THERMAL LED LTL1204 (RED)
LED2 16001254 DISTORTION LED LTL1254 (YELLOW)
P01 35001550 DC OFFSET POT 5K
P02 35001550 BIAS POT 5K
Q01,Q02 33002100 TRANSISTOR IRFD210
Q03,Q04 33000092 TRANSISTOR MPSA92
Q05 33007570 TRANSISTOR ZTX757
Q06 33009210 TRANSISTOR IRFD9210
Q07 33009610 TRANSISTOR IRF9610
Q08 33002100 TRANSISTOR IRFD210
Q09 33006570 TRANSISTOR ZTX657
Q10,Q11 33000610 TRANSISTOR IRF610
Q12 33000042 TRANSISTOR MPSA42
Q13 33006570 TRANSISTOR ZTX657
Q14 33007570 TRANSISTOR ZTX757
Q14 33000092 TRANSISTOR MPSA92
Q15 33002400 TRANSISTOR IRFP240
Q16 33000924 TRANSISTOR IRFP9240
Q17 33002400 TRANSISTOR IRFP240
Q18 33000924 TRANSISTOR IRFP9240
Q19 33002400 TRANSISTOR IRFP240
Q20 33000924 TRANSISTOR IRFP9240
Q21 33002400 TRANSISTOR IRFP240
Q22 33000924 TRANSISTOR IRFP9240
Q23 33002400 TRANSISTOR IRFP240
Q24 33000924 TRANSISTOR IRFP9240
R01 27001565 RESISTOR ROED 221R
R02 27002480 RESISTOR ROED 1.82K
R03 27002365 RESISTOR ROED 49.9K
R04 27002480 RESISTOR ROED 1.82K
R05 27002030 RESISTOR ROED 10K
R06 27002280 RESISTOR ROED 15K
R07 27002170 RESISTOR ROED 475R
R08 27001565 RESISTOR ROED 221R
R09 27001565 RESISTOR ROED 221R
R10 27002010 RESISTOR ROED 100R
R11 27002030 RESISTOR ROED 10K
R12 27002365 RESISTOR ROED 49.9K
R13 27002240 RESISTOR ROED 7.5K

2