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SERVICE MANUAL
2 CHANNEL
POWER AMPLIFIER
GFA-5002
TABLE OF CONTENTS
Introduction . . . . . . . . . . . . . . . . . . 1
Test Procedures . . . . . . . . . . . . . . 1
Parts List . . . . . . . . . . . . . . . . . . . . 2
Voltage Conversion . . . . . . . . . . . . 6
Specifications . . . . . . . . . . . . . . . . 7
PCB Layout . . . . . . . . . . . . . . . . . . 8
Wiring Diagram . . . . . . . . . . . . . . . 9
Schematic . . . . . . . . . . . . . . . . . . .10
10 Timber Lane
Marlboro, NJ 07746
USA
Tel: 732-683-2356
Fax: 732-683-9790
Web: http://www.adcom.com
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INTRODUCTION
Step
Coupling
Adjust
Adjust For
Plus Lead
Minus Lead
1
TP3
TP1
VR603
DC Voltmeter
2
TP4
TP2
VR604
reads 10mV
This service manual is intended to assist trained and qualified technical personnel in verifying the
performance of, adjusting, and repairing the ADCOM GFA-5002 amplifier. The procedures described here
are not intended for persons unfamiliar with the appropriate safety and test procedures.
WARNING
THERE ARE POTENTIALLY LETHAL VOLTAGES WITHIN THE GFA-5002 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-5002 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-5002 UNLESS YOU ARE TECHNICALLY QUALIFIED TO
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.
• 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.
IMPORTANT
BEFORE PROCEEDING WITH ADJUSTMENTS, MAKE SURE AMPLIFIER IS AT ROOM
TEMPERATURE.
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
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a GFA-5002 SERVICE PARTS LIST
Amplifier PCB
AF705514 and AF705524
Schematic Part Number Description
Location
C603, C604 12001260 Capacitor
C607, C608 12001400 Capacitor
C609, C610 12005470 Capacitor
C611, C612 12001420 Capacitor 22pF 125V
C613, C614 12003070 Capacitor 39pF 125V
C615, C616 12005325 Capacitor
C617-C620 12003080 Capacitor 68pF 125V
C621, C622 12001450 Capacitor
C623, C624 12001170 Capacitor
C625,C626 12001450 Capacitor
C901-C904 12005450 Capacitor
D001,D002 16001254 Distortion LED (Yellow) LTL-1254
D003,D004 16001204 Thermal LED (Red) LTL-1204
D601-D608 16004148 Diode 1N4148
D610-D613 16003001 Diode DSC30TC
D801,D802 16004148 Diode 1N4148
D901,D902 16008030 Bridge Rectifier PBPC903
F601-F604 19009400 Fuse 4A 250V
Q601-Q606 33002362 NPN Transistor 2SC2362K
Q607,Q608 33001016 PNP Transistor 2SA1016F-T
Q609,Q610 33001376 PNP Transistor 2SA1376L-T
Q611,Q612 33009700 PNP Transistor 2SA970B-T
Q613,Q614 33001376 PNP Transistor 2SA1376L-T
Q615,Q616 33003478 NPN Transistor 2SC3478L-T
Q617,Q618 33006000 NPN Transistor 2SD600KE
Q619,Q620 33003478 NPN Transistor 2SC3478L-T
Q621,Q622 33001376 PNP Transistor 2SA1376L-T
Q623,Q624 33003902 NPN Transistor 2SC3902
Q625,Q626 33001507 PNP Transistor 2SA1507
Q627,Q628 33001047 NPN Transistor 2SD1047
Q629,Q630 33008170 PNP Transistor 2SB817
Q631,Q632 33001047 NPN Transistor 2SD1047
Q633,Q634 33008170 PNP Transistor 2SB817
Q801,Q802 33001016 PNP Transistor 2SA1016F-T
R603,R604 27004260 Resistor
R605,R606 27004135 Resistor
R607,R608 27004050 Resistor
R609,R610 27004120 Resistor
R611,R612 27004420 Resistor
R613,R614 27004350 Resistor
R615,R616 27004225 Resistor
R617, R618 27004140 Resistor
R619,R620 27004160 Resistor
R621,R622 27004120 Resistor
R623,R624 27001550 Resistor
2.2µF 100V
1µF 100V
47µF 100V Electrolytic
4.7µF 50V Electrolytic
0.22µF 100V
0.1µF 100V
0.22µF 100V
6800µF 63V Electrolytic
2.74KΩ 1%MF
4.99KΩ 1%MF
1KΩ 1%MF
22.1KΩ 1%MF
33.2Ω 1%MF
825Ω 1%MF
475Ω 1%MF
332Ω 1%MF
47.5KΩ 1%MF
22.1KΩ 1%MF
121Ω 1%MF
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R625,R626
27004330 Resistor
5.62KΩ 1%MF
Schematic
Part Number Description
Location
R627,R628 27004225 Resistor
R629,R630 27004420 Resistor
R631,R632 27004260 Resistor
R633,R634 27004540 Resistor
R635-R638 27004235 Resistor
R639-R642 27004320 Resistor
R643-R650 27004170 Resistor
R651,R652 27004140 Resistor
R653,R654 27004120 Resistor
R655,R656 27004610 Resistor
R657-R664 27001500 Resistor
R665,R666 27003035 Resistor
R667,R668 27004650 Resistor
R669,R670 27004455 Resistor
R671,R672 27004120 Resistor
R673,R674 27004200 Resistor
R675,R676 27003105 Resistor
R801,R802 27004670 Resistor
R803, R804 27004120 Resistor
R805,R806 27004320 Resistor
R807,R808 27004090 Resistor
R901-R904 27001590 Resistor
475Ω 1%MF
33.2Ω 1%MF
2.74KΩ 1%MF
499Ω 1%MF
68.1Ω 1%MF
47.5Ω 1%MF
10Ω 1%MF
332Ω 1%MF
22.1KΩ 1%MF
22.1Ω 1%MF
0.33Ω
5.1Ω (2W)
750Ω 1%MF
133Ω 1%MF
22.1KΩ 1%MF
10KΩ 1%MF
1Ω (1W)
100Ω 1%MF
22.1KΩ 1%MF
47.5Ω 1%MF
121KΩ 1%MF
5.6KΩ (1W)
S601,S602 32005400 Thermal Breaker B-1002B
VR603,VR604 35001090 Bias Pot SFKPV220R
Protection PCB
AF705534
Schematic Part Number Description
Location
C801 12005100 Capacitor
C802-C804 12005610 Capacitor
C701, C702 12005640 Capacitor
C703 12005610 Capacitor
C704 12005020 Capacitor
C705 12002020 Capacitor
C706 12005610 Capacitor
C707 12005610 Capacitor
C905, C906 12005070 Capacitor
C907, C908 12005130 Capacitor
C909 12005360 Capacitor
10µF 50V Electrolytic
33µF 50V Electrolytic
0.22µF 50V Electrolytic
33µF 50V Electrolytic
100µF 25VElectrolytic
0.01µF 50V
33µF 50V Electrolytic
33µF 50V Electrolytic
1000µF 35V Electrolytic
220µF 25V Electrolytic
100µF 50V Electrolytic
C910 12005700 Capacitor 4700pF 250V
D701-D706 16004148 Diode 1N4148
D707 16004003 Diode 1N4003
D803 16004003 Diode 1N4003
D804 16004148 Diode 1N4148
D903 16001530 Bridge Rectifier W-02G
D904 16004148 Diode 1N4148
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D905 16004003 Diode 1N4003
F902
19009100 Fuse 1A 250V
Schematic Part Number Description
Location
IC701,IC702 21004558 Dual Op-Amp NJM4558DD
IC703 21005550 Timer IC NJM555D
IC801 21001237 Protect IC UPC1237H
IC901 21007912 Regulator IC NJM7912
IC902 21007800 Regulator IC NJM7812
PT902 24005002 Standby Transformer TT-345-U
Q701,Q702 33005360 NPN Transistor 2SC536G-T
Q703 33006000 NPN Transistor 2SD600KE
Q704 33003399 Digital NPN Transistor 2SC3399-T
R701,R702 27004200 Resistor
R703,R704 27004210 Resistor
R705,R706 27004630 Resistor
R707,R708 27004400 Resistor
R709-R711 27004210 Resistor
R712,R713 27004465 Resistor
R714,R715 27004200 Resistor
R716 27004210 Resistor
R717 27004070 Resistor
R718 27004530 Resistor
R719 27004600 Resistor
R720,R721 27004120 Resistor
R809 27004290 Resistor
R810 27001590 Resistor
R811 27004485 Resistor
R812 27003150 Resistor
R813 27004210 Resistor
R905 27003095 Resistor
10KΩ
100KΩ
221KΩ
1.82KΩ
100KΩ
909KΩ
10KΩ
100KΩ
2.21KΩ
1.21KΩ
4.75KΩ
22.1KΩ
18.2KΩ
5.6KΩ (1W)
82.5KΩ
820 Ω (1W)
100KΩ
33Ω (1W)
RY801 28001140 Relay VB-24SMCU-E
RY901 28001150 Relay G4W-1112P-VD
Input PCB
AF705544
Schematic Part Number Description
Location
C601, C602 12001390 Capacitor 125V 330pF
JP601, JP602 22001020 RCA Jack 4TR-2709-3
R601,R602 27004050 Resistor
1KΩ
SW601 37001350 Bridging Switch SX4TR-2886
VR601,VR602 35005002 Input Level Pot 4TR-3379
DC Trigger
Board
AF705554
Schematic Part Number Description
Location
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JP701 22001570 DC Trigger Input Jack YK-5103A
SW901 37005006 Ground Switch SS004-P022
Other Hardware
Schematic Part Number Description
Location
30005002 Speaker Binding Post 4TR-3357
19000500 Fuse - 115V Models (U.S.) 5A 125V
19008250 Fuse - 230V Models (Euro) 2.5A 250V
24005003 Transformer TT-342-U
37005002 Vacation Switch R19-33FM2-2N
16002249 POWER/STANDBY LED VRPY3349S
25001060 AC Inlet 4TR-2786
13005004 Printed Rear Chassis TRP-750A
13001020 Foot 4TR-2823#1 24FX14H
47005002 Alum. Shield Board 4TR-3400
13005002 Front Panel 3TOZ-1 TFP-824
11005002 LED Ring (for Power LED) 4TR-2209B#1 9F
13005003 Top Cover 3TQA-5
15001110 AC Power Chord 115V (U.S.) 4TR-3403 SJT AWG16
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a GFA-5002 Voltage Conversion
115V to 230V
1) Unplug the power cord and remove the top cover.
2) Locate the standby transformer, PT902, on the circuit board in the middle of the unit.
3) Near PT902, cut jumpers J1, J2, J4, J5 - see figure 1 below.
4) Make jumpers J3 and J6 - see figure 1 below.
5) Change the rear panel fuse to 2.5A 250V and place a 2.5A sticker above the fuse holder so that the
new value is indicated.
F902
PT902
115V
J4
230V
RY901
115V
115V
J6
J5
J1
J3 230V
J2 115V
Figure 1: Jumper position for 230V operation. Main fuse is 2.5A 250V.
230V to 115V
1) Unplug the power cord and remove the top cover.
2) Locate the standby transformer, PT902, on the circuit board in the middle of the unit.
3) Near PT902, cut jumpers J3 and J6 - see figure 2 below.
4) Make jumpers J1, J2, J4 and J5 - see figure 2 below.
5) Change the rear panel fuse to 5A 250V and place a 5A sticker above the fuse holder so that the new
value is indicated.
F902
PT902
115V
J4
230V
RY901
115V
J5
J6
115V
J1
J3 230V
J2 115V
Figure 2: Jumper position for 115V operation. Main fuse is 5A 250V.
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a GFA-5002 SPECIFICATIONS
Power Rating (To FTC Requirements)
50 watts continuous average power per channel into 8 ohms at any frequency between 20Hz to 20kHz with
all channels driven at less than 0.05% THD
75 watts continuous average power per channel into 4 ohms at any frequency between 20Hz to 20kHz with
all channels driven at less than 0.05% THD
Bridged Power: 175 watts continuous average power (one channel) into 8 ohms at any frequency between
20Hz to 20KHz with less than 0.09% THD
IM Distortion (SMPTE)
1 watt to 50 watts into 8 ohms..................................................................................................................................... ≤ 0.05%
1 watt to 75 watts into 4 ohms..................................................................................................................................... ≤ 0.05%
IM Distortion (CCIF, Any Combination from 4kHz to 20kHz)
50 watts into 8 ohms.................................................................................................................................................... ≤ 0.01%
75 watts into 4 ohms.................................................................................................................................................... ≤ 0.01%
Frequency Response @ 1 Watt into 8 ohms (10Hz to 20kHz).................................................................................. +0, -0.25dB
Power Bandwidth (-3dB)............................................................................................................................................ 5Hz to 130kHz
Dynamic Headroom into 4 ohms............................................................................................................................................ 1.8 dB
Signal to Noise Ratio, “A” Weighted (50 watts into 8 ohms).......................................................................................... ≥ 100dB
Gain...... .......................................................................................................................................................................................... 29dB
Input Sensitivity ....................................................................................................................................................................... variable
Input Impedance.......................................................................................................................................................................... 17kΩ
Damping Factor (20Hz to 20kHz)............................................................................................................................................. ≥ 400
Rise Time (5kHz, 90V, peak-to-peak square wave, 20% to 80%)....................................................................................... 2.7µS
Power Consumption (Continuous, All Channels Driven)
Quiescent.......................................................................................................................................................................... 30VA
Maximum......................................................................................................................................................................... 600VA
Power (Available in 230VAC on special order).................................................................................................... 115VAC - 50/60Hz
Chassis Dimensions.................................................................................................................................................. 3” x 17” x 11.5”
Maximum Dimensions............................................................................................................................................ 3.5” x 17” x 12.5”
Weight.............................................................................................................................................................................................. 18lb
Weight, Packed............................................................................................................................................................................... 21lb
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