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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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