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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-7807 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 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.
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. Initial bias of individual modules shuld be adjusted to 3mv across the 0.2 ohm emitter resistors.
Recheck module bias when amp modules are in the chassis and warm.
- Insert a 0.1 ohm resistor in the top fuse location F2 ( Note: I did this by taking 2 each 0.2 ohm
emitter resistors in parallel and soldered to a burnt fuse. )
- Measure voltage across the 0.1 ohm resistor -¦ check and adjust ( VR1) bias to 10 mV or 100
mA at this point.
NOTE: The bias adjustment must be performed with the heatsink attached and the amplifier module
installed in the chassis.
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-7807 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-7807 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-7807 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-7807 SERVICE PARTS LIST
AMPLIFIER MODULE PCB
SCHEMATIC ADCOM PART
NUMBER DESCRIPTION
LOCATION
BR1, BR2 16007000 BRIDGE RECTIFIER KBU8J
C01 ,C04 ,C13 12005090 CAPACITOR ELEC 10uF 25V
C02 ,C03 ,C14 ,C15 12007800 CAPACITOR ELEC 5600uF 100V
C05 ,C09 12006030 CAPACITOR ELEC 0.1uF 50V
C06 ,C17 ,C23 12005220 CAPACITOR ELEC 100uF 10V
C07 12007801 CAPACITOR POLYPROPYLENE 6800pF 63V
C08 12005380 CAPACITOR ELEC 47uF 25V
C10 12001555 CAPACITOR POLY 100nF 250V
C11 12001700 CAPACITOR SIVLER MICA 7pF 500V
C12 12001365 CAPACITOR ELEC 47pF 500V
C16 ,C20 12001085 CAPACITOR POLY 0.22uF 50V
C18 ,C19 12005225 CAPACITOR ELEC 100uF 100V
C21 ,C22 12007802 CAPACITOR POLYPROPYLENE 330pF 100V
D01 ,D07 16007800 DIODE ZENER 15V 1/2W 1N965B
D02 ,D04 ,D08 ,D09 ,D10 16004148 DIODE 1N4148
D03-D06 16004004 DIODE 1N4007
U01 ,U04 21004250 IC 4N25N
U02 21005550 IC NE555N
U03 IC H11AA814
U05 21000071 IC TL071
F1 ,F2 19009800 FUSE T6.3A
K1 28007400 RELAY G4A-1A-PE
Q01 33001380 TRANSISTOR 2SA1380
Q02 ,Q09 ,Q23 ,Q24 Q26 ,Q28 33003904 TRANSISTOR 2N3904
Q03 33004793 TRANSISTOR 2SC4793
Q04 ,Q12 ,Q13 33005401 TRANSISTOR 2N5401
Q05 ,Q06 ,Q07 ,Q08 ,Q11 33005200 TRANSISTOR KSC5200
Q10 33001567 TRANSISTOR 2SC1567A
Q14 ,Q15 ,Q16 ,Q17 ,Q22 33001943 TRANSISTOR KSA1943
Q18 33001873 TRANSISTOR 2SA1837
Q19 33003502 TRANSISTOR 2SC3502
Q20 33000600 TRANSISTOR MPSA06
Q21 33000042 TRANSISTOR MPSA42
Q27 TRANSISTOR 2N3906
R01 ,R43 27004050 RESISTOR 1K
R02 ,R45 ,R56 ,R57 27003055 RESISTOR 10K 2W 5%
R03 ,R19 27004070 RESISTOR 2K21
R04 27004120 RESISTOR 22K1
R05 27004670 RESISTOR 100
R06 ,R35 ,R38 27001565 RESISTOR 221
R07 ,R14 ,R24 ,R36 ,R39 ,R40 ,R63 27002410 RESISTOR 4.75K
SCHEMATIC ADCOM PART
NUMBER DESCRIPTION
R08 ,R13 ,R22 ,R37 27004210 RESISTOR 100K
