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-5200 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 with rated output (8 ohm) for
approximately
5 MINUTES
before attempting adjustments
Step
Coupling
Adjust
Adjust For
Plus Lead Minus Lead
1
R035
R035
R038
DC Voltmeter
2
R135
R135
R138
reads 20mV
3
L SPKR +
L SPKR -
R039
DC Voltmeter
4
R SPKR +
R SPKR -
R139
reads 0mV
GFA-5300 SERVICE PARTS LIST
WARNING
THERE ARE POTENTIALLY LETHAL VOLTAGES WITHIN THE GFA-5200 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-5200 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-5200 UNLESS YOU ARE TECHNICALLY QUALIFIED
TO DO SO.
IMPORTANT
BEFORE PROCEEDING WITH ADJUSTMENTS, MAKE SURE AMPLIFIER IS AT ROOM
TEMPERATURE.