Arcam FMJ P7 Service Manual page 19

it can be indicated on the front panel display) each module
also provides an address line. A fault occurring on a
particular module will cause both the fault line to be low
and also the address line. In this way the amplifiers can be
monitored by the controller through a total of 10 lines.
The address lines are called:
FAULTn* (where n = 1 to 7 as per amplifier module
number)
The fault lines are:
FLT_DC* to indicate a DC offset error
FLT_VI* to indicate VI limit protection
FLT_TEMP* to indicate that the temperature of the output
transistors has exceeded the 110C trip temperature.
Output muting relays
The muting relays are on the amplifier modules. The
relays allow the amplifier to be disconnected from the
loudspeaker load. This is required for muting switch on
and switch off thumps and in the event of fault conditions.
The relays are driven from the control PCB by the simple
open collector transistor drive arrangement formed around
TR102, TR104- TR110. The diodes D111-D118 protect
the transistor collectors against over voltage spikes
occurring when the inductive relay coils are switched off.
The open collector transistors are driven through the
74HC259 type addressable latch. This configuration
allows 24 output lines to be provided from 7
microprocessor lines. The outputs of one of the 259s
(IC103) are used to drive the muting relay transistors. The
remaining packages (IC101, IC102) are used to drive the
display LEDs as described below.
Control of the 259s is as follows. Each has 3 address bits
S0, S1, S2 which allows one of 8 outputs to be selected, a
DATA input which allows a 0 or a 1 to be latched to the
output and a GATE input which transfers the signal on
DATA to the selected (addressed) output of the package.
The GATE input is also used as a chip select to select
which of the 3 packages is being controlled. This allows
the S and DATA inputs to be connected in parallel on
multiple packages.
Amplifier status display
The amplifier status is indicated via the front panel
mounted LEDs. IC101, IC102 are used to directly drive
the tricolour LEDs which are mounted on the LED PCB.
Each channel has a single Tricolour LED. The LED has a
red and green LED built in and the third colour (yellow) is
produced by having both Red and Green LEDs ON at the
same time.
IC101 is connected to the red LEDs and IC102 is
connected to the green LEDs.
Watch-dog
The watch dog circuit provides a failsafe mechanism
which places the amplifier in a safe (OFF) state in the
event of the microprocessor failing or crashing. The
circuitry monitors the average
WATCHDOG line and if it falls outside its required
voltage then it removes the power supply to the relays
which has the effect of muting the amplifier outputs and
removing the power to the power amplifier toroids.
The line WATCHDOG is toggled by the software in the
micro. Every time it completes a program loop it toggles
the state of the line. This results in a 50% duty cycle signal
voltage on the
that is averaged to 2.5V by the filter R114, C110. The
voltage is then fed to the window comparator formed
around IC104. The network R105, R106 and R107 sets the
upper and low thresholds at 75% of 5V and 25% of 5V
respectively.
The outputs are open collector and if the filtered
WATCHDOG signal exceeds the upper threshold or falls
below the lower threshold then the b-e junction of TR103
is pulled low and TR116 is then turned off – which
removes the power to the relays.
The filtered WATCHDOG signal can only move outside
the window if the WATCHDOG line sits high (or low)
continuously. This can only occur if the microprocessor
breaks or the software crashes.
This protection scheme also includes the fan fuse (F101).
If the fan fuse blows then TR103 gets no base current that
also immediately removes the power supply to the relays.
LED PCB
The LED PCB contains the 7 bicolour LEDs and their
current limiting resistors. The LED PCB also provides a
means of routing the mains switch wiring to the control
PCB via the LED PCB to control PCB flexfoil cable.
Connector PCB
The connector PCB provides a means of joining the
amplifier modules to the control PCB via the 22way
flexfoil cable. The PCB also includes circuitry for the 12V
trigger circuit.
The trigger circuit is formed around SK201. The 12V
trigger signal is converted to a 5V logic level compatible
signal by R200, D200. C201 and C202 provide EMC
suppression (since the signal is ultimately routed to the
Control PCB which contains a microprocessor). R202
isolates the ground pin of the jack socket from the
amplifier ground. This prevents possible ground loop
problems with other hifi components connected in the
system.
The zener diode clamped signal is limited to 4.7V pk
amplitude. This is fed to the microprocessor via line
TRIG/RC5
Fault conditions
Fault D.C
When a D.C fault is created on an amplifier all the LEDS
turn red accept the module with the fault that flashes
green.
Fault V.I
When a V.I fault is created all the LEDS turn red accept
the module with the fault which flash red.
Therm fault (amplifier overheating)
When a therm fault is created all the LEDS stay green and
the amp module with the fault flashes amber.
Thermal trip (TX overheating)
When a thermal trip is created all the LEDS flash amber,
stating that it is a transformer problem.