Adcom GFA-7400 Service manual

SERVICE MANUAL

5 CHANNEL

POWER AMPLIFIER

GFA-7400

TABLE OF CONTENTS

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

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

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

Specifications . . . . . . . . . . . . . . . 5

Schematic . . . . . . . . . . . . . . . . .

8551 E. Anderson dr.

Suite 105

Scottsdale,AZ 85525

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-7500 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-7400 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-7400 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-7400 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

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.

1

GFA-7400 SERVICE PARTS LIST

AMPLIFIER MODULE PCB

SCHEMATIC ADCOM PART NUMBER DESCRIPTION
LOCATION

BR2 16007000 BRIDGE RECTIFIER SBU8G
C01 12001075 CAPACITOR POLY FILM 2.2uF 100V
C02 12001390 CAPACITOR POLY FILM 330pF 100V
C03 12005295 CAPACITOR ELEC 100uF 63V
C04 12001400 CAPACITOR MYLAR FILM 1uF 100V
C05 12001490 CAPACITOR SILVER MICA 33pF 100V
C06 12001580 CAPACITOR POLY FILM 4.7nF 100V
C07 12005090 CAPACITOR ELEC 10uF 25V
C08 12005225 CAPACITOR ELEC 100uF 100V
C09 12005225 CAPACITOR ELEC 100uF 100V
C10 12001585 CAPACITOR POLY 270pF 100V
C11 12001585 CAPACITOR POLY 270pF 100V
C12 12005220 CAPACITOR ELEC 100uF 10V
C13 12001555 CAPACITOR MYLAR 0.1uF 250V
C15 12005090 CAPACITOR ELEC 10uF 25V
C16 12002075 CAPACITOR POLY 0.01uF 100V
C17 12005165 CAPACITOR ELEC 22uF 35V
C18 12005455 CAPACITOR ELEC 10000uF 63WV
C19 12005455 CAPACITOR ELEC 10000uF 63WV
C20 12001085 CAPACITOR MYLAR 0.22uF
C21 12001085 CAPACITOR MYLAR 0.22uF
D01-D04 16004148 DIODE 1N4148
D08,D09 16004004 DIODE 1N4004
D10-D12 16004148 DIODE 1N4148
D13 16003600 DIODE ZENER 3.6V
F3 19009800 FUSE T6.3A
F4 19009800 FUSE T6.3A
J1 22006000 RCA INPUT JACK
LED1 16001254 DISTORTION LED LTL1254 (YELLOW)
LED2 16001204 PROTECT LED LTL1204 (RED)
Q01 33003381 TRANSISTOR 2SC3381
Q02 33000092 TRANSISTOR MPSA92
Q03 33000042 TRANSISTOR MPSA42
Q04 33000092 TRANSISTOR MPSA92
Q05 33000092 TRANSISTOR MPSA92
Q06 33000092 TRANSISTOR MPSA92
Q07 33001380 TRANSISTOR 2SA1380
Q08 33003502 TRANSISTOR 2SC3502F
Q09 33001567 TRANSISTOR 2SC1567
Q10 33004793 TRANSISTOR 2SC4793
Q11 33001837 TRANSISTOR 2SA1837
Q12 33003281 TRANSISTOR 2SC3281
Q13 33001302 TRANSISTOR 2SA1302
Q14 33003281 TRANSISTOR 2SC3281
Q15 33001302 TRANSISTOR 2SA1302
Q16 33000042 TRANSISTOR MPSA42
Q17 33000042 TRANSISTOR MPSA42

2

Q18 33000042

TRANSISTOR MPSA42

SCHEMATIC ADCOM PART NUMBER DESCRIPTION
LOCATION

R01 27004050 RESISTOR 1K
R02 27004135 RESISTOR 4.99K
R03 27004135 RESISTOR 4.99K
R04 27004500 RESISTOR 49.9K
R05 27004050 RESISTOR 1K
R06 27004610 RESISTOR 22.1
R07 27003420 RESISTOR 249
R08 27003425 RESISTOR 347
R09 27004610 RESISTOR 22.1
R10 27004135 RESISTOR 4.99K
R11 27004310 RESISTOR 1.78K
R12 27004500 RESISTOR 49.9K
R13 27004135 RESISTOR 4.99K
R14 27004091 RESISTOR 75K
R15 27003420 RESISTOR 249
R16 27003190 RESISTOR 5.6K 1/2W
R17 27004135 RESISTOR 4.99K
R18 27004670 RESISTOR 100
R19 27004225 RESISTOR 475
R20 27004610 RESISTOR 37.4
R21 27004610 RESISTOR 37.4
R22 27003415 RESISTOR 75
R23 27004260 RESISTOR 2.74K
R24 27004050 RESISTOR 1K
R26 27004145 RESISTOR 392
R27 27004145 RESISTOR 392
R28 27004170 RESISTOR 10
R29 27004085 RESISTOR 49.9
R30 27004085 RESISTOR 49.9
R31 27004170 RESISTOR 10
R32 27004420 RESISTOR 33.2
R33-R36 37006055 RESISTOR .33 3W
R37 27004070 RESISTOR 2.21K
R38 27004120 RESISTOR 22.1K
R39 27004420 RESISTOR 33.2
R40 27003200 RESISTOR 5.1 2W
R41 27004200 RESISTOR 10K
R42 27004070 RESISTOR 2.21K
R43 27004630 RESISTOR 221K
R44 27004070 RESISTOR 2.21K
R45 27004135 RESISTOR 4.99K
R46 27004120 RESISTOR 22.1K
R47 27003280 RESISTOR 8.2K 2W
R48 27003280 RESISTOR 8.2K 2W
TB1 32007000 THERMAL BREAKER
TH2 31007400 THERMISTOR CL150
U1 21005550 IC LM555
VR29 35001500 BIAS POT 200

3

POWER SUPPLY PCB

SCHEMATIC ADCOM PART NUMBER DESCRIPTION
LOCATION

C01 12005010 CAPACITOR ELEC 1000uF 25V
C18 12001510 SPARK KILLER
C02 12005120 CAPACITOR ELEC 220uF 16V
D1-D6 16004004 DIODE 1N4004
F1 19001200 MAIN FUSE 12A
F2 19000100 STANDBY FUSE 1A
K1 28007400 TURN-ON RELAY
LED1 16001245 POWER LED LTL1204 (RED)
Q1 33001300 TRANSISTOR MPSA13
R1 27003110 RESISTOR 1K 1W
R2 27004270 RESISTOR 3.32K
R3 27004200 RESISTOR 10K
R4 27004270 RESISTOR 3.32K
T1 24007402 STANDBY TRANSFORMER
TH1 31005800 THERMISTOR CL100
U2 21007812 REGULATOR IC 7812
37007400 POWER SWITCH

OTHERS

SCHEMATIC ADCOM PART NUMBER DESCRIPTION
LOCATION

T2 24007401 MAIN TRANSFORMER
13005805 FOOT
13007401 FRONT PANEL
13007402 HEATSINK
11001155 POWER BUTTON
11001165 POWER BUTTON FRAME
13007400 TOP COVER

4

GFA-7400 SPECIFICATIONS

Power Rating (To FTC Requirements)

100 watts continuous average power per channel into 8 ohms at any frequency between 20Hz to 20kHz with
all channels driven at less than 0.075% THD
150 watts continuous average power per channel into 4 ohms at any frequency between 20Hz to 20kHz with
all channels driven at less than 0.075% THD

IM Distortion (SMPTE)

1 watt to 100 watts into 8 ohms........................................................................................................................... ≤ 0.075%

1 watt to 150 watts into 4 ohms........................................................................................................................... ≤ 0.075%

IM Distortion (CCIF, Any Combination from 4kHz to 20kHz)

100 watts into 8 ohms.......................................................................................................................................... ≤ 0.025%

150 watts into 4 ohms.......................................................................................................................................... ≤ 0.025%

THD + Noise at 100 watts into 8 ohms (Typical)

20Hz..................................................................................................................................................................... 0.0012%

1kHz..................................................................................................................................................................... 0.0012%

10kHz....................................................................................................................................................................... 0.02%

20kHz..................................................................................................................................................................... 0.025%

THD + Noise at 150 watts into 4 ohms (Typical)

20Hz....................................................................................................................................................................... 0.025%

1kHz....................................................................................................................................................................... 0.025%

10kHz..................................................................................................................................................................... 0.035%

20kHz..................................................................................................................................................................... 0.045%

Frequency Response @ 1 Watt into 8 ohms (10Hz to 20kHz)................................................................................. +0, -0.25dB

Power Bandwidth (-3dB) ...................................................................................................................................... 1.5Hz to 65kHz

Dynamic Headroom into 4 ohms....................................................................................................................................... 2.5 dB

Signal to Noise Ratio, “A” Weighted (100 watts into 8 ohms) ..................................................................................... ≥ 110dB

Gain........................................................................................................................................................................................ 29dB

Input Sensitivity

for 1 Watt............................................................................................................................................................... 0.1 volts

for 100 Watts......................................................................................................................................................... 1.0 volts

Input Impedance................................................................................................................................................................... 50kΩ

Damping Factor (20Hz to 20kHz)........................................................................................................................................ ≥ 400

Rise Time (5kHz, 90V, peak-to-peak square wave, 20% to 80%)...................................................................................... 4.5µS

Power Consumption (Continuous, All Channels Driven)

Quiescent.................................................................................................................................................................. 96VA

Maximum............................................................................................................................................................... 1440VA

100 watts into 8 ohms............................................................................................................................................ 1165VA

Power (Available in 230VAC on special order)................................................................................................. 115VAC - 50/60Hz

Chassis Dimensions............................................................................................... 5” (127mm) x 17” (432mm) x 14” (355.6mm)

Maximum Dimensions......................................................................................... 5 3/4” (146mm) x 17” (432mm) x 15” (381mm)

Weight.................................................................................................................................................................... 42 lb. (19.1 kg)

Weight, Packed ..................................................................................................................................................... 48 lb. (22.8 kg)

5