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SERVICE MANUAL

5 CHANNEL
POWER AMPLIFIER

GFA-7605
TABLE OF CONTENTS
Introduction . . . . . . . . . . . . . . . . 1
Test Procedures . . . . . . . . . . . . . 1
Parts List . . . . . . . . . . . . . . . . . . 2
Voltage Conversion . . . . . . . . . . . . 5
Specifications . . . . . . . . . . . . . . . 7
PCB Layouts. . . . . . . . . . . . . . . . . . 8
Schematic . . . . . . . . . . . . . . . . . 9

8551 East Anderson Drive,Suite 105
Scottsdale, Arizona 85255
USA
Tel: 480-607-2277
Fax: 480-348-9876
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-7605 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-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.

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-7605 SERVICE PARTS LIST
AMPLIFIER MODULE PCB

SCHEMATIC ADCOM PART NUMBER DESCRIPTION
LOCATION
BR2 BRIDGE RECTIFIER SBU8G
C01 CAPACITOR POLY FILM 2.2uF 100V
C02 CAPACITOR POLY FILM 330pF 100V
C03 CAPACITOR ELEC 100uF 63V
C04 CAPACITOR MYLAR FILM 1uF 100V
C05 CAPACITOR SILVER MICA 33pF 100V
C06 CAPACITOR POLY FILM 4.7nF 100V
C07 CAPACITOR ELEC 10uF 25V
C08 CAPACITOR ELEC 100uF 100V
C09 CAPACITOR ELEC 100uF 100V
C10 CAPACITOR POLY 270pF 100V
C11 CAPACITOR POLY 270pF 100V
C12 CAPACITOR ELEC 100uF 10V
C13 CAPACITOR MYLAR 0.1uF 250V
C15 CAPACITOR ELEC 10uF 25V
C16 CAPACITOR POLY 0.01uF 100V
C17 CAPACITOR ELEC 22uF 35V
C18 CAPACITOR ELEC 10000uF 63WV
C19 CAPACITOR ELEC 10000uF 63WV
C20 CAPACITOR MYLAR 0.22uF
C21 CAPACITOR MYLAR 0.22uF
C22 CAPACITOR ELEC 100uF 16V
C23 CAPACITOR ELEC 220uF 25V
D02-D05 DIODE 1N4148
D08,D09 DIODE 1N4007
D10-D13 DIODE 1N4148
F3 FUSE T6.3A
F4 FUSE T6.3A
J1 RCA INPUT JACK
LED1 DISTORTION LED LTL1254 (YELLOW)
LED2 PROTECT LED LTL1204 (RED)
Q01 TRANSISTOR 2SC3381
Q02 TRANSISTOR MPSA92
Q03 TRANSISTOR MPSA42
Q04 TRANSISTOR MPSA92
Q05 TRANSISTOR MPSA92
Q06 TRANSISTOR MPSA92
Q07 TRANSISTOR 2SA1380
Q08 TRANSISTOR 2SC3502F
Q09 TRANSISTOR 2SC1567
Q10 TRANSISTOR 2SC4793
Q11 TRANSISTOR 2SA1837
Q12 TRANSISTOR 2SC3281
Q13 TRANSISTOR 2SA1302
Q14 TRANSISTOR 2SC3281
Q15 TRANSISTOR 2SA1302
Q16 TRANSISTOR MPSA42
Q17 TRANSISTOR MPSA42
Q18 TRANSISTOR MPSA42

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SCHEMATIC ADCOM PART NUMBER DESCRIPTION
LOCATION
R01 RESISTOR 1K
R02 RESISTOR 4.99K
R03 RESISTOR 4.99K
R04 RESISTOR 49.9K
R05 RESISTOR 1K
R06 RESISTOR 22.1
R07 RESISTOR 249
R08 RESISTOR 347
R09 RESISTOR 22.1
R10 RESISTOR 4.99K
R11 RESISTOR 1.78K
R12 RESISTOR 49.9K
R13 RESISTOR 4.99K
R14 RESISTOR 75K
R15 RESISTOR 249
R16 RESISTOR 5.6K 1/2W
R17 RESISTOR 4.99K
R18 RESISTOR 100
R19 RESISTOR 475
R20 RESISTOR 37.4
R21 RESISTOR 37.4
R22 RESISTOR 75
R23 RESISTOR 2.74K
R24 RESISTOR 1K
R26 RESISTOR 392
R27 RESISTOR 392
R28 RESISTOR 10
R29 RESISTOR 49.9
R30 RESISTOR 49.9
R31 RESISTOR 10
R32 RESISTOR 33.2
R33-R36 RESISTOR 0.33 3W
R37 RESISTOR 2.21K
R38 RESISTOR 22.1K
R39 RESISTOR 33.2
R40 RESISTOR 5.1 2W
R41 RESISTOR 10K
R42 RESISTOR 2.21K
R43 RESISTOR 221K
R44 RESISTOR 2.21K
R45 RESISTOR 4.99K
R46 RESISTOR 22.1K
R47 RESISTOR 6.8K 2W
R48 RESISTOR 6.8K 2W
TB1 THERMAL BREAKER
TH2 THERMISTOR CL150
U1 IC LM555
VR25 BIAS POT 200

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POWER SUPPLY PCB

SCHEMATIC ADCOM PART NUMBER DESCRIPTION
LOCATION
C01 CAPACITOR ELEC 1000uF 25V
C18 SPARK KILLER
C02 CAPACITOR ELEC 220uF 16V
D1-D6 DIODE 1N4004
F1 MAIN FUSE 12A
F2 STANDBY FUSE 1A
K1 TURN-ON RELAY
LED1 POWER LED LTL1204 (RED)
Q1 TRANSISTOR MPSA13
R1 RESISTOR 1K 1W
R2 RESISTOR 3.32K
R3 RESISTOR 10K
R4 RESISTOR 3.32K
T1 STANDBY TRANSFORMER
TH1 THERMISTOR CL100
U2 REGULATOR IC 7812
POWER SWITCH

OTHERS

SCHEMATIC ADCOM PART NUMBER DESCRIPTION
LOCATION
T2 MAIN TRANSFORMER Multi Voltage Models
T2 MAIN TRANSFORMER 115VAC only Models
FOOT
FRONT PANEL
HEATSINK
POWER BUTTON
POWER BUTTON FRAME
TOP COVER

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GFA-7605 Voltage Conversion

The GFA-7605 amplifier is produced in both a 120VAC only version and a multivoltage (120VAC or
230VAC convertible) version. The version can usually be distinguished by external inspection. The
120VAC only version has a fixed AC line cord. The convertible version has a detachable line cord.
Internally, their are two differences. The toroid transformer in the convertible version has 2 primaries; the
first primary has a black and brown lead, the second primary has a black and brown lead with white
stripe. The toroid transformer in the 120VAC only version has only the brown and black lead (1 primary
winding). Additionally, the connecting block mounted next to the power switch is different in both
versions. In the multivoltage unit, this connecting block has 5 pair of mounting leads (as is shown in the
figures below). In the 120VAC only version, the connecting block has only 3 pairs of mounts.

The conversion procedure for the multivoltage GFA-7605 unit is listed below.

Conversion of Multivoltage Units

120V to 230V
1. Unplug the amplifier and remove the top cover.
2. Locate the connecting block mounted next to the power switch.
3. Move the transformer leads so they are configured as shown in figure 1.
4. Remove heavy gauge wire connections between screw down portion of terminals.
4. Change the rear panel AC fuse from 12A to 6A, AGC. Place a sticker on the rear panel at the fuse
holder to show the new value fuse (6A) and new value operating voltage (230VAC).
5. Test the unit with a 230VAC source.

Figure 1: Transformer lead configuration for 230VAC operation (multivoltage units)

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230V to 120V
1. Unplug the amplifier and remove the top cover.
2. Locate the connecting block mounted next to the power switch.
3. Move the transformer leads so they are configured as shown in figure 2.
4. Add heavy gauge wire between screw down portion of terminals as shown.
4. Change the rear panel AC fuse from 6A to 12A, AGC. Place a sticker on the rear panel at the fuse
holder to show the new value fuse (12A) and new value operating voltage (120VAC).
5. Test the unit with a 120VAC source.

Figure 2: Transformer lead configuration for 120VAC operation (multivoltage units)

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GFA-7605 SPECIFICATIONS
Power Rating (To EIA/CEA-490-A Requirements)
125 watts continuous average power per channel into 8 ohms at any frequency between 20Hz to 20kHz with
all channels driven at less than 1.00% 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 1.00% 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 ................................................................................................................................................................... 50k

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)

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