Common Mode Output Current
Common mode current
¡n the
output stage
can
only
be due to an
output
stage
faílure,
or
full
power output
of RF
energy. Common mode current
occurs when
a
high current
level
exists
in
both
the
positive
and the
negative
halves
of the
output stage.
U115
is a
specíalty
device.
It
serves
as
both
an
Opto-SCR,
and as a
conventional SCR.
It
must
have both
an
optic gate
and
conventional
gate
firing
at the
same
time
in
order
to
latch.
The
conventional
gate
is
fired
by
current sense
of the
output stage Low-side.
The
optic gate
is
fired
by the
High-side current sense.
If
high currents exist
¡n
both sides
simuitaneously,
the SCR
will
latch
on, and
remain
on
until
the
unit
is
turned
off.
When
the SCR
latches,
low
voltage causes
the red LED
(labeled Output
Module)
to
light,
and
places
a low on the
FAULT
signal
line.
A low on
the
FAULT line
is
sensed,
via
D112,
by
C120. Once again,
a low
here
discharges
C120
and
shuts
down
the
amplífier.
Output Thermal
Output
over-temperature
protection
was
covered
in the
ODER
Circuitry
section
of
this
text.
The
calibrated temperature sense
f rom the
positive
half
of the
output stage drives
an
over-temperature
amp,
U117A.
If
heatsink
temperature exceeds
a
limit
of
about
130°C,
the
amplifier will
go
into hard ODEP.
This
does
not
shut down
the
amplifier,
but
does
clamp
the
audio.
Transformer
Thermal
The
main powertransformers have
built-in
thermal switches which open
in the
event
of
transformer over-temperature.
In the
event that
the
thermal switch opens
in the
channel
1
toroid, Q709 turns
on
causing
U707D
to go
low. When
¡t
does,
the
over-voltage/thermal
switch
LED
on
the
control module
is
energized
and the OV1
signal
is
tripped. This
results
in
amplifier
shutdown
by way of
shared
over-voltage
circuitry.
FET
Thermal
The
MOSFET
switches
in
the
VZsupply
have
built-in
thermal protection.
In
the
event
of
over-heat
they
functionally open, locking
the VZ
supply
¡n
low
voltage (high current)
mode,
regardless
of the
control signal.
Power
Loss
The
MA-5000VZ
has the
ability
to
sense
a
"brown-out"
condition
on the
AC
service.
This
is
accomplished
by
sensing
the
low-voltage
power
supply
at
U111A.
Local
capacitors
onthe
±15VDC supply
to
this
chip
keep
it
powered
for a
short
time
after
the low
voltage supply drops
out
on
power-down/loss.
An RC
network, consisting
of C1 and R6,
will cause
the
output
of
U111A
to
shift
low the
instant
low
voltage
is
lost.
This negative potential
discharges
C120
causing immediateopening
of the
main relay
and
reset
of
the
Soft-start
circuitry.
It is
essential thaí
C120
be
discharged
immediately
¡n the
event that power
is
restored before
C120
would
otherwise
discharge.
Thepower-losscircuit
is
common
to
both channels.
Its
output
also
goes
to ±LL and ±LH to
immediately mute audio upon
power-down
or
power-loss,
thus preventing
turn-off
audio noise.
Notes
MA-5000VZ
- 27
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