Page 1
1 INTRODUCTION
This service manual is intended to assist trained and qualified technical personnel in verifying the
performance of, adjusting, and repairing the ADCOM GFA-5500 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
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 (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 WARNING THERE ARE POTENTIALLY LETHAL VOLTAGES WITHIN THE GFA-5500 AMPLIFIER WHICH WILL BEACCESSIBLE ONCE ITS TOP COVER IS REMOVED. DO NOT ATTEMPT FAMILIARIZATION,INSPECTION, OR ANY PROCEDURE WHATSOEVER UNLESS YOU HAVE DISCONNECTED THE GFA-5500 FROM THE WALL AC OUTLET OR OTHER SOURCE OF AC POWER AND THE POWER-SUPPLYCAPACITORS ARE COMPLETELY DISCHARGED. THESE INSTRUCTIONS ARE PROVIDED FOR USEONLY BY COMPETENT TECHNICAL PERSONNEL. DO NOT UNDERTAKE ANY SERVICEPROCEDURES IN THE GFA-5500 UNLESS YOU ARE TECHNICALLY QUALIFIED TO DO SO.IMPORTANTBEFORE PROCEEDING WITH ADJUSTMENTS, MAKE SURE AMPLIFIER IS AT ROOMTEMPERATURE.
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2 GFA-5500 SERVICE PARTS LIST
AMPLIFIER MODULE PCB
SCHEMATICADCOM PART NUMBERDESCRIPTION
LOCATION
C0112001535CAPACITOR MICA390pF 100V
C0212001080CAPACITOR MYLAR2.2uF 100V
C0312005280CAPACITOR ELEC47Uf 50V
C0412001085CAPACITOR MYLAR0.22uF 100V
C0512001480CAPACITOR SILVER MICA15pF 500V
C0612001470CAPACITOR SILVER MICA20pF 500V
C0712005325CAPACITOR ELEC4.7uF 50V
C0912005380CAPACITOR ELEC47uF 25V
LED116001204THERMAL LEDLTL1204 (RED)
LED216001254DISTORTION LEDLTL1254 (YELLOW)
P0135001550DC OFFSET POT5K MULTI TURN
P0235001550BIAS POT5K MULTI TURN
Q01,Q0233002100TRANSISTORIRFD210
Q03,Q0433000092TRANSISTORMPSA92
Q0533000092TRANSISTORMPSA92
Q0633009210TRANSISTORIRFD9210
Q0733009610TRANSISTORIRF9610
Q0833002100TRANSISTORIRFD210
Q0933000042TRANSISTORMPSA42
Q10,Q1133000610TRANSISTORIRF610
Q1233000042TRANSISTORMPSA42
Q1333000042TRANSISTORMPSA42
Q1433000092TRANSISTORMPSA92
Q1533002400TRANSISTORIRFP240
Q1633000924TRANSISTORIRFP9240
Q1733002400TRANSISTORIRFP240
Q1833000924TRANSISTORIRFP9240
Q1933002400TRANSISTORIRFP240
Q2033000924TRANSISTORIRFP9240
Q2133002400TRANSISTORIRFP240
Q2233000924TRANSISTORIRFP9240
Q2333002400TRANSISTORIRFP240
Q2433000924TRANSISTORIRFP9240
R0127001565RESISTOR ROED221R (1/4W 1%)
R0227002480RESISTOR ROED1.82K (1/4W 1%)
R0327002365RESISTOR ROED49.9K (1/4W 1%)
R0427002480RESISTOR ROED1.82K (1/4W 1%)
R0527002030RESISTOR ROED10K (1/4W 1%)
R0627002280RESISTOR ROED15K (1/4W 1%)
R0727002170RESISTOR ROED475R (1/4W 1%)
R0827001565RESISTOR ROED221R (1/4W 1%)
R0927001565RESISTOR ROED221R (1/4W 1%)
R1027002010RESISTOR ROED100R (1/4W 1%)
R1127002030RESISTOR ROED10K (1/4W 1%)
R1227002365RESISTOR ROED49.9K (1/4W 1%)
R1327002240RESISTOR ROED7.5K (1/4W 1%)
R1427002025RESISTOR ROED1.5K (1/4W 1%)
SCHEMATICADCOM PART NUMBERDESCRIPTION
