Circuit Theory 4-13
130446-1 Rev. A
MA-5002VZ Service Manual
©2000 Crown International, Inc.
4.6.8 Power Loss
The MA-5002VZ 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 on the ±15VDC keep this chip 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
immediate opening of the main relay, and reset of the
Soft-start circuitry. It is essential that C120 be discharged
immediately in the event that power is restored before
C120 would otherwise discharge. The power-loss cir-
cuit is common to both channels. Its output goes to
±LL and ±LH to immediately mute audio upon power-
down or power-loss, thus preventing turn-off audio noise.
4.6.9 Fan Control
The MA-5002VZ, unlike other members of the Macro-
Tech family, has two onboard fans. They are mounted
to the chassis divider assembly and pull cool air from
the front and discharge it across the output stage
heatsinks to the rear of the amplifier. Also unlike the other
Macro-Techs, the fans are fully ODEP proportional (they
operate in proportion to output stage temperature and
calibrated ODEP control voltage).
U713B combines channel 1 temperature and ODEP
level, U713A for channel 2. D706 and D707 form a di-
ode OR gate. The output of the OR gate drives one
input to U707B. The other input to U707B is from the
DC pulse width control circuit (U701B). U707B oper-
ates in a fashion similar to that of U707A, the Soft-start
control amplifier. A graphic example of the fan control
waveforms would look a good deal like those in Figure
4.7, except that the thermal drive would be unique from
that of the Soft-start ramp.
The fans will also be forced to operate at full speed in
the event a toroid transformer thermal switch trips open.
4.7 Display
Amplifier front panel indication includes a total of 7 LEDs.
These include Enable, ODEP, SPI/IOC, and ILOAD/ILIMIT.
The Enable indicator is an amber light which indicates
presence of the low voltage supply. It is powered by the
unregulated +24VDC supply. It will be on any time the
power switch is depressed (unless the low voltage fuse
blows).
ODEP indicators provide an on-line indication of ampli-
fier thermal reserve. The LEDs are amber (although they
may have a reddish appearance) and are normally on.
They dim and/or extinguish in the event that the
amplifier’s thermal reserve is exhausted. ODEP indica-
tors will also extinguish whenever the main supply re-
lays are open (such as a protection action being acti-
vated, or during Soft-start time-out).
Green SPI/IOC LEDs show signal presence (SPI) and
any form of distortion (IOC). They flash dimly with the
audio to show signal. In the event of an IOC condition
(output waveform differs from input by >0.05%, or input
overload) the light will be on brightly. An occasional flash
of IOC usually indicates clipping. If the IOC light locks
in, it usually indicates a protective action, or “hard” ODEP
limiting.
ILOAD/ILIMIT LEDs flash green with the audio when pro-
gram material is being delivered to a load. Its function
is similar to that of the SPI, except that SPI is voltage
driven and does not require a load. ILOAD comes on
when the amplifier is loaded, and its brightness is in
proportion to the output current. This is the ILOAD func-
tion. In the event of current limiting action, the light will
flash to red. This is the ILIMIT function.
4.8 Mono Modes
The MA-5002VZ has three main operating modes,
namely dual (stereo), bridge mono, and parallel mono.
There are a number of precautions which should be
taken when operating the amplifier in either of the mono
modes. The VZ mode switches for each channel must
be set to the same setting. Sensitivity, LOI, and Com-
pressor switches for channel 2 make no difference. The
input must be to channel 1 only. The input to channel 2
and controls for channel 2 are NOT defeated in either
mono mode, therefore no connection to channel 2 may
be made in either of the mono modes. The channel 2
level control should be turned down (counterclockwise)
fully in either mono mode.
Monaural amplifier operating modes are covered in
detail in the MA-5002VZ Reference Manual. The dis-
cussion below primarily aids in understanding how the
mono modes work for testing purposes.
4.8.1 Bridge Mono
Bridge mono is intended for loads of 4 ohms or greater.
The feedback loop for channel 1 also drives the input
to channel 2 in this mono mode. The input to channel 2
is, however, inverted. This causes the output of chan-
nel 2 to be of equal magnitude and opposite polarity
(for double voltage output). The output of the amplifier
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