YORKVILLE Audiopro 3400 Service Manual page 6

EMS Circuit
The Energy Management System circuit only applies to North American line voltage products. A North American AC
receptacle can provide 1850 watts before the wall breaker would trip, but how can a amplifier provide 2 x 1200 watts
out when only 1850 watts is avaliable out of the AC receptacle? A sinewave source connected to the amplifier driving
the amplfier to full power output will cause the circuit breaker on the amplifier to trip. Music driving the amplfier to
clipping can be equal an average power output between 12 and 30 percent of the amplifier?s maximum capable power
output. Under this condition less than 12 ampheres of current is drawn from the AC line. Some synthesized music
may go beyond the 12 to 30 percent level and make the amplfier draw more than 12 amperes from the AC line. If this
occurs the coil on board M1013A heats up to the point where through the heating of thermistor RV5, Q32 sinks
current through R109. On board M1011A Q11 starts conducting through the connection to R109. Q11 heats
thermistors RV2 and RV4 attenuating the audio signal going to each amplfier channel, thus decreasing the current
drawn from the AC line to the point where less than 12 ampheres is being drawn.
Fan Circuit
Looking at the schematic to
board M1013A, here is a quick
explaination of the fan control
circuit. There is a thermistor on
each M1002A board. When the
amplifier is first turned on, Q33
is saturated sinking the current
source through ZD10. As either
negative temperature coefficient
thermistor begins to heat up,
more current flows through D38
or D39 decreasing the conductor
of Q33 until the increasing VCE
of Q33 is enough to turn on Q34
and Q35. Further heating of the
thermistor causes an increasing
of Q33?s collector to emitter
voltage. Q34 and Q35 are a
common emitter stage with Q35
providing the drive current for the fan. To lower the dissipation of Q35, D42, D43, ZD12, ZD13 and R118 turn off Q35
when the full wave rectified voltage present of the collector of Q35 reaches approximently 50V by robbing current from
the base of Q34. The maximum fan voltage is 20.5 VDC. ZD14 and R120, R121 and R122 provide a current limiting
function. Figure #4 shows the current throught these resistors when there is 12VDC across the fan.
Thermal Shutdown Circuit
Test point 7 in the fan circuit is the measuring point for the shutdown voltage. As the temperature sensing thermistors that
control the fan circuit heat up the voltage on test point 7 rises until at 85 degrees celcius on the M1002A heatsinks the
voltage on test point 7 reaches 34.5 VDC and the amplifier must be shutdown to protect the output power transistors. ZD15
and D47 become forward biased and through the kill connection to board M1011A, Q37 on M1011A turns on turning the
relay off and muting the audio signal. After the amplifier cools down the kill voltage will decrease FIGURE #4 AP3400 Fan
Current Waveform until Q37 turns off turning the relay back on enabling the amplifier.
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Identifying Defective Boards in the AP-3400
STEP 1: VISUAL INSPECTION OF FRONT PANEL AND FAN
A. Check to see whether the green power LED is lit. If not, the amplifier has a power supply (M1013A board),
transformer, A.C. switch or line cord problem.
B. If the red protect LED stays on or samples off and on, this usually indicates a problem with one or both of the
M1002A current amplifier modules and possibly damage to the M1011A voltage amplifier board. Check for
misaligned pin connections or see if the ribbon cables have been cut or pinched through their insulation.
C. The fan running at full speed on power up usually indicates a problem with the fan circuitry on the M1013A board,
but it can also be caused by M1011A circuit problems. Erractic fan behaviour can be caused by damaged
thermistors located under the M1002A heatsinks.
D. No output on either or both channels can be caused by intermittent push switches at the input.
STEP 2: VISUAL INSPECTION OF INTERNAL CHASSIS
A. a) After removing the lid, look for any signs of smoke, charring or burnt components. If the M1002A boards have
such damage there may be some damage to the M1011A voltage amp board in the form of blown ICs, or possibly
open or shorted diodes, LED?s, resistors, transistors or capacitors. Exploded electrolytic capacitors indicate you
have major repairs to the M1002A, M1011A and M1013A boards.
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