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This service manual is intended to assist trained and qualified technical personnel in verifying the
performance of, adjusting, and repairing the ADCOM GPA-5400 amplifier. The procedures described
here are not intended for persons unfamiliar with the appropriate safety and test procedures.
, -7 WARNING v
THERE ARE POTENTIALLY LETHAL VOLTAGES WITHIN THE GFAÂ»54OO 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-5400 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-5400 UNLESS YOU ARE TECHNICALLY QUALIFIED
TO DO so.
VERSION 1 vs. VERSION 2
There are 2 versions of the GFA-5400 amplifier. The primary difference between the original version 1
and the updated version 2 is the addition of the ADCOM output protection circuit in version 2. This circuit
can be noted by comparing the schematics of both versions in this manual. The protection circuit in
version 2 includes trigger le |C711 and |C712 and a second power supply of standby transformer
TI'OO2, rectifier D701 and regulator |C701 that is mounted to a small circuit board at the right front hand
side of the unit. The power transformer, speaker binding posts and input jacks are also unique to each
Besides these distinctions, the amplifier circuit, parts reference numbers and values are nearly identical in
both units. Even so, a schematic diagram for both versions is included in this manual. The parts list is for
version 2 but contains an addendum section to list parts exclusive to version 1. The board and chassis
layout diagrams are for version 2 only. The alignment procedures are the same for both versions.
. All tests are performed with a 115V, low-distortion (less than 2% THD), AC-power source, 8Â»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.
BEFORE PROCEEDING WITH ADJUSTMENTS, MAKE SURE AMPLIFIER IS AT ROOM
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 and DC OFFSET CHECK
Prior to performing BIAS ALIGNMENT and DC OFFSET CHECK turn unit on and allow to run with rated
output for approximately 5 MINUTES before attempting adjustments