Adcom GFA 5800 Service Manual

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r SERVICE MANUAL

ADCOM®

dam/75 you can hear

POWER AMPLIFIER

GFA-5800

TABLE OF CONTENTS
Introduction ....................................... 1
Test procedures ................................ 1
Parts fist ............................................ 2
= Specifications ................................... 5
PCB layouts ...................................... 6
Schematics ....................................... 8
m

TECHNICAL SERVICE DEPARTMENT
11 ELKINS ROAD
EAST BRUNSWICK, NJ 08816
PHONE: (908) 390-1130
FAX: (908) 390-9152

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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-SBOO amplifier. The procedures described here
are not intended for persons unfamiliar with the appropriate safety and test procedures.

J, WARNING «a ,
THERE ARE POTENTIALLY LETHAL VOLTAGES WITHIN THE GFA-5800 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-5800 FROM THE WALL Ac OUTLET OR OTHER SOURCE CF AC POWER AND THE POWER-
SUPPLY CAPACITORS ARE COMPLETELY DISCHARGED, PLEASE TAKE NOTE THAT THE
POWER SUPPLY CAPACITORS CAN TAKE AS LONG AS 15 MINUTES TO DISCHARGE. THESE
INSTRUCTIONS ARE PROVIDED FOR USE ONLY BY COMPETENT TECHNICAL PERSONNEL. DO
NOT UNDERTAKE ANY SERVICE PROCEDURES IN THE GPA-5300 UNLESS YOU ARE
TECHNICALLY OUALIFIEDTO Do so.

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,
Tests are performed after warming up the amplifier at idle with no load, for at least 60 minutes
All grounds during testing are referred to the ground of the black output terminal of the channel under
test.
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 loadl
Slew rate is measured with an inductive load and is derived with a dual-time-based oscilloscope
readIng the slope of a full power (125volts peak to peak) 5kHz square wave To avoid damaging
output network R-3 and C-4, DO NOT OPERATE THE AMPLIFIER AT FULL~POWER SINE WAVE
ABOVE 22kHz 0R FULL-POWER (125V PEAK TO PEAK} SQUARE WAVE ABOVE 5kHz.

not.

IMPORTANT: Before proceedihg with adjustments, be certain that the am'pIifier's
temperature has settled to a constant idle temperature near room temperature.

BIAS ALIGNMENT

1, With amplifier connected to an 89 resistive load, apply a 1kHz sine wave at the input to achieve
83.3w at the output.

Operate at 33.3w for 5 to 6 minutes.

Turn the unit off and disconnect the input signal and the load resistors

Disconnect the 3+ wire from the amplifier PCB and connect an ammeter between the B+ wire and the
B+ terminal on the amplifier PCB.

Turn amplifier on and allow a 10 to 15 minute settling period.

Adjust BIAS control (P 2) to obtain 800mA 1 SOmA

Allow unit to idle for 10 more minutes, and recheck adjustment for drift

99"

v.09-

DC OFFSET

1. Connect a DC millivoltmeter across the output binding posts. Adjust DC offset control (P1) until DC
offset = OmV (1 0.1 mV).

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.