APPENDIX A
Initial Test Setup:
AP2020 ChA \ ChB Gain pots max clockwise.
FILTER switch out (Flat).
Limiter Defeat Switch In (Limiter Disabled)
Mono/Stereo switch (Stereo Mode).
GND LIFT switch out.
Impedance Switch to 4 ohm
Cover the AP2020 with a lid to ensure proper cooling.
Visual Inspection:
•
Make sure power supply wires are not blocking air flow from fan and they pass over the fan as per the
model. The wires must not be pushed into the frame of the fan whenn the lid is put on or the fan will
deform and become noisy.
•
Make sure the multiconductor ribbon cable (M1129) is not popping out of the header socket ... use large
foam pieces between the header and the chassis.
•
EMI Shield does not contact XLR conductors at M1129 Input board.
•
Both Rectifier bridges have been fully soldered.
•
Damaged Volume Controls ( 360 degree rotation )
•
Bumpers are mounted on the fishpaper
•
XLR Input jacks are centered relative to the chassis cutout holes
Schematic Diagrams
:
M1129 Preamplifier and Filter
M1130 Current & Voltage Amplifier
M1131 Power Supply and Fan
M1132 Volume Control and Activity Leds
New Circuits
M1131 Dual Gain Fan Drive Circuit
OP amp U1 has a Common-Base transistor amplifier (Q6) connected across its feedbacak path. At low heatsink
temperatures ( and low dc voltage at TP1 ) this opamp acts as a high gain comparator with reference voltage set by
R7 and R9. When the voltage at TP1 exceeds the Reference voltage the dc output voltage at pin 1 swings negative . At
approximately - 7 Vdc Q6 is forward biased and R15 is inserted in the negative feedback path. The op amp circuit
switches to a low gain amplifier which gradually increases fan speed as the voltage at TP1 gradually increases. The fan
continues to run linearly even after the SHutdown circuit puts the Power Amp into PROTECT.
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