Arcam FMJ P7 Service Manual page 17

P7 Controller Circuit Description
The Amp controller PCB Panel consists of 6 PCBs.
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Controller PCB
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Display PCB
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Connector PCB
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Mains Switch PCB
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2x Wire Clamp PCBs
Overview
The controller PCB contains the microprocessor and most
of the circuitry for controlling the P7 amplifier. The
display PCB provides the LEDs and resistors for the
simple 7 LED display for the front panel of the unit. The
mains switch PCB provides a means of supporting the
front panel mounted mains switch. The connector PCB fits
on the base of the chassis near the rear panel and provides
a means of connecting the 7 amplifier modules to a
connector which is then linked to the controller PCB. The
connector PCB also provides a means of mounting the rear
panel mounted trigger input connector and connecting its
signals to the mains controller PCB.
The cable clamp PCBs are used to hold the transformer
power cables neatly to the side of the chassis base.
The controller PCB
The controller PCB provides the following functionality
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Mains power distribution, switching and soft-start
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Voltage selection for 115/230V operation.
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+5V (for logic) supplies
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+22V (for relay and fan) supplies
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Variable speed fan drive
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Relay control for soft starts and sequenced power up/
power down for switch on, switch off and fault
conditions.
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Heatsink temperature measurement for fan speed
control
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Transformer Thermal trip monitoring
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Mains supply monitoring for output muting
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Soft start resistor monitoring to prevent soft start
resistor overdissipation
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Amplifier module fault status monitoring for module
over-temperature, module DC offset, module VI
limit
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Output muting relays drive circuit
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Amplifier status display drive circuitry via the front
panel mounted LEDs.
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Watch-dog failsafe mechanism which activates in the
case of microprocessor failure
Mains power distribution, switching and
soft-start
The mains input supply is connected directly to the control
PCB via SK100. The mains supply is routed through the
power relay RLY100 and through the 3 parallel resistors
R100, R101, R102. These form a high power rated 50-
Ohm resistor that is used to limit the inrush current into the
2 large power toroids. These toroids are mounted beneath
the transformer tray at the front of the unit. The 50-Ohm
series resistor limits the inrush current to 5A and is
allowed to remain in circuit for a few hundred
milliseconds after power on. After the power amplifier
module electrolytic capacitors have charged to full supply
voltage the current through the resistor falls to a low value
(which is sensed by the circuitry around IC105 as
described below) and the soft start resistor is then switched
out of circuit by relay RLY101. This arrangement of
power relays ensures that relay RLY101 never switches
high voltages and its contacts remain clean.
Voltage selection for 115/230V operation
The control PCB provides a means of configuring the unit
for 115V or 230V operation. This is achieved by having 2
connectors marked with the appropriate voltage for each of
the units 3 transformers (1 auxiliary transformer which
powers the control PCB and 2 large toroids each of which
powers 3 ½ amplifier modules). The connectors SK101,
SK102 for the aux TX and SK103, SK104 for the Right
hand power toroid and SK105, SK106 for the left hand
power torroid are configured to place the two primary
windings of each transformer in parallel for 115V
operation and in series for 230V operation. The
arrangement of fuses (6 in total which are all fitted)
ensures that each winding has a series fuse when
configured in parallel for 115V operation and 1 of each
pair of fuses is used to fuse the series configured windings
when set for 230V operation. In this way there are 2
parallel fuses for each transformer when set for 115V (to
allow twice the current as required). The fuses are time
delay types.
C121 is X rated and provides suppression for rectifier
switching transients for EMC conducted noise compliance.
RC100 is a series RC network to prolong relay life.
+5V supply (for logic)
The auxiliary transformer has 2 secondary windings.
These are rectified by full wave rectifiers to form the DC
supplies.
The +5V supply is rectified by D101, D102, D105, D106
and smoothed by C102. The supply is then regulated by
the 3 terminal 5V regulator REG100. The supply is
designed to provide 9.5V DC to th input of the regulator
when fed with a nominal 230V. This allows 8V at the
input to the regulator (so that it remains within regulation)
when fed from 195V mains supply. The regulator is fitted
with a heatsink. C103 is provided to eliminate input
transients. C107 ensures regulator stability.
+22V supply (for relay and fan)
The +22V supply is formed in a similar manner by D103,
D104, D107, D108 and C101. The supply voltage is
chosen to give 22V when driven from a mains voltage of
230V. This gives around 24V when the mains supply is at
its maximum value (265V). This is due to the relays
having a maximum specified input voltage of 24V.
Variable fan speed drive
The fan for the amplifier is required to be very quiet. This
is achieved by having the fan speed proportional to
temperature. The temperature is measured by the
microprocessor (as described below) and when the
temperature exceeds a programmed threshold then the fan
is switched on at its lowest speed (approx 7V). As the
temperature of the unit increases the fan speed is increased
until it is operating at full speed (12V). This is achieved by
having the microprocessor output a PWM 5V pk to pk
square wave on line PWM_OUT. The square wave is
filtered by R112, C108 to form a DC level. The amplitude
of the DC level is thus proportional to the duty cycle that
is under microprocessor control. The filtered DC signal is
then amplified by the simple 4 transistor amplifier formed
around TR100, TR113, TR114 and TR115. The gain of the
amplifier is set by R117, R118 to around 6. This ensures