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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-7605 amplifier. The procedures described
here are not intended for persons unfamiliar with the appropriate safety and test procedures.
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 -śA-ť 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.
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.
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.
IMPORTANT
BEFORE PROCEEDING WITH ADJUSTMENTS, MAKE SURE AMPLIFIER IS AT ROOM
TEMPERATURE.
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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
