Arcam FMJ P7 Service Manual page 18

that the PWM_OUT signal duty cycle can move the output
of the amplifier over the required range. C104, C113
provide suppression for EMC and C112 provides amplifier
loop compensation. The supply to the fan is fused by F101
this fuse limits current in the event of the fan stalling. If
the fuse blows the watchdog timer cct immediately
switches off the power relays to ensure failsafe condition.
Relay control
The power relay and soft-start relay (RLY100, RLY101)
respectively are driven by microprocessor signals
MPOWER and MSOFT_ST*. Transistors TR108, TR111
buffer the microprocessor outputs. Diodes D115 and D119
prevent damage to the collectors of the transistors by the
inductance of the relay coils at coil switch off.
The power for the relay coils (+22V_SW) is provided
through TR116. This transistor is off (hence relays off)
when the watchdog circuit detects no microprocessor
activity (as described below) or the fan fuse F101 is blown
(as described above).
Heatsink temperature measurement
The microprocessor monitors the temperature of the
heatsink using the circuitry around TR112. Capacitor
C114 is alternately charged through fixed resistor R122
(2K2) and thermistor TH100. The time taken to charge the
capacitor is measured by the microprocessor. The ratio of
the time taken to charge the capacitor through the 2K2
fixed resistor compared with the time taken to charge the
capacitor through the thermistor allows the microprocessor
to calculate the resistance of the thermistor. The software
is then able to establish the temperature of the thermistor
from its resistance.
Three microprocessor lines
measurement. TREF, TTEMP, C_DISCH.
A measurement cycle proceeds as follows.
C_DISCH goes high to discharge C114. After a delay to
ensure C114 is fully discharged C_DISCH then goes Low
and TREF is set as an output and goes high. TTEMP is set
as an input. This allows C114 to charge through R122.
The time taken for C114 to charge to the input high
threshold of the micro is measured by timing through input
TTEMP.
When the threshold is reached, C_DISCH is again taken
high to discharge the capacitor.
After a delay to ensure C114 is fully discharged C_DISCH
then goes Low and TTEMP is set as an output and goes
high. TREF is set as an input. This allows C114 to charge
through TH100. The time taken for C114 to charge to the
input high threshold of the micro is measured by timing
through input TREF.
When the threshold is reached, C_DISCH is again taken
high to discharge the capacitor.
This cycle is repeated continuously and the ratio of
capacitor charge times allows the ratio of resistance R122
to resistance TH100 to be measured. The measurement is
immune to variation in value of C114, saturation voltage
of TR112 and average supply voltage on the 5V rail. The
variation of input voltage 1 threshold of the TREF and
TTEMP inputs is not compensated for but in practice this
is usually found to be minimal. The method should give a
measurement accuracy of a few degrees C that is all that is
needed for fan speed control.
Transformer Thermal trip monitoring
The main power transformers have thermal trips built into
them. These are normally closed and go open when the trip
temperature is exceeded. The trips are put in series and
are involved in this
pulled to 5V through R131. In the event of 1 (or both)
transformer trips going
TX_OVTEMP goes high and the microprocessor is then
able to power the amplifier down as required.
Mains supply monitoring for output muting
In order to prevent thumps through the loudspeaker when
the amplifier is switched off it is necessary to detect the
removal of the mains supply so that the amplifier modules
can be muted.
This is affected by the circuitry around TR101 and the
microprocessor. The AC supply for the 22V rail is sensed
by R109. At the positive peak of the mains supply TR101
is turned on and saturates thereby discharging C109. In the
event of the AC supply disappearing then C109 will not be
discharged and will instead charge to 5V through R132.
The time constant R132 . C109 = 47mS sets the time taken
for line PWROK* to go high. So if the mains supply
disappears for around 5 capacitor charge cycles then
PWROK* will notify the microprocessor which will then
mute the amplifier outputs.
Soft start resistor monitoring
The soft start resistor is used to limit the inrush current
into the large power transformers. The resistor is designed
to be in-circuit only for the duration of the inrush current
at switch on. The resistor must be shorted out by the relay
before the amplifier is configured to deliver output current.
Also in the event of a fault (e.g. a short across one of the
amplifier module reservoir capacitors) then the resistor
might be exposed to a situation which might cause over
dissipation.
This is avoided by having the microprocessor monitor the
voltage across the resistor so that in the event of a fault the
amplifier can be switched off before damage to the resistor
occurs.
The voltage across the soft-start resistor is monitored by
the circuitry around IC105. When voltage is present across
the soft-start resistor (only the positive half cycle is
sensed) then the opto-transistor is turned on. This
discharges C100.
The RC time constant R104, C100 = 220mS dictates that
line SSPROT* will be low until around ¼ of a second after
the voltage across the soft-start resistor has fallen to zero.
Amplifier module fault status monitoring
!
Module over-temperature
!
Module DC offset
!
Module VI limit
The amplifier modules contain circuitry that senses the
above fault conditions. The fault status is indicated to the
control PCB by means of open collector transistors on
each module that are ON when the fault exists.
The means of connecting the fault lines to the control PCB
requires explanation.
There are 7 amplifier modules, each of which has 3 fault
lines. If these were tracked individually then this would
require 21 lines to the control PCB.
In order to reduce the number of lines required then the
connections are changed so that the 3 fault lines from each
of the are 7 modules are connected in parallel. So that e.g.
the VI limit fault line is pulled low when any one of the 7
modules exhibits a VI limit fault. In order for the
controller to establish which module has the fault (so that
open circuit then line