Ab international 1200 c service manual

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INTRODUCTION

The model 1200c from AB International is the only totally modular Power Amplifier. It
features LED output power display and speaker limiting protection, as you would expect
from a high-end professional amplifier. But what really sets the 1200c apart is it's com-
pletely interchangeable output sections. Each of the output tunnels is independent of
the other and each output module packs its own variable D.C. Fan to make sure your
sound system stays cool even when the music's hot.

CIRCUIT DESCRIPTION

To assure absolute long term reliability, the output section of each channel incorporates
multiple Motorola Power Transistors, which provide 3 times the amplifiers rated power
in watts of dissipation per channel. The output stage is arranged in a fully-complementa-
ry format for class AB / 2 operation. The bias current is evenly distributed among all out-
put devices. Bias thermal compensation is accomplished by thermally mating a bipolar
semiconductor junction to the heat-producing output device. Triple defused high power
driver transistors are employed along with high speed, high voltage annular devices for
the predriver and inverter stages. Utilization of these components provides the required
separation of Ft break points for absolute stability. Fully complimentary drive and load-
ing is utilized throughout. Only 20 dB of negative feedback is used to reduce forward
transfer distortion to minimum levels. VI type energy limiters are incorporated for short
circuit protection of the amplifier. Due to the unusually large safe operating area of the
output stage, the limiters do not actuate until driving a forty-five degree reactive load of
under 1.4 ohms at full power.

CONSTRUCTION

The amplifiers are designed on an all-modular concept permitting rigorous preassembly
module testing and maximum service accessibility. Each functional module is fully tested
before final assembly. Although components of the highest quality are used throughout,
each amplifier is burned in, prior to shipment, at the worst case operating point to elimiv
nate any possibility of component malfunction. All chassis components are precision ma-
chined from high quality aluminumand sheet steel stock and finished with a highly
durable, baked powdercoat finish. The entire packaged concept is directed toward maxi-
mum efficiency of space and structure, accounting for the compact size.

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INSTALLATION

All AB International amplifiers are designed for mounting in a standard 19 inch equip-
ment rack. The amplifiers require 5-1/4 inches of vertical panel space, with 16-3/ 4 inches
required behind the panel. Total depth, including handles is 18-1/4 inches. Front panels
are machined from solid aluminum stock, with a black finish and sturdy rack mount han-

dles.

Placement of the amplifier is not critical for normal operation, provided sufficient air flow
is allowed to reach the internal heatsinks. If the unit is to be placed on a shelf or a similar
enclosed area, allow four inches behind the rear panel to permit air flow through the sys-
tem. For installations in a cabinet, allow an additional two inches above and one inch be-
low the amplifier to permit air to be drawn from the rear. If the amplifier is to be mounted
in an equipment rack or cabinet with heat producing equipment, be sure that environ-
mental operating temperatures do not exceed 55' C (131 ' F). Should overheating occur be-
cause of inadequate ventilation, the temperature protection circuitry will automatically
protect the amplifier. When safe operating temperature is restored, the amplifier will re-
turn to operation.

Because the amplifiers are capable of delivering high power from a relatively small pack-
age, considerable heat can develop in cabinets containing several instruments. A good
rule of thumb to adopt is to provide forced air cooling to any enclosure containing four or
more instruments.

POWER CONNECTIONS

Professional Series power amplifiers are specified for operation from 120/ 240 Volt 50/60
Hz mains supply.

Model 1200c amplifiers are prewired for the line voltage that is indicated on the rear panel
of the amplifier. Follow step 8 of Rear Panel Features for proper power cord connections.

In new installations and portable sound systems, or any situation in which the main pow-
er is suspect, it is wise to confirm appropriate voltage and polarity BEFORE connecting
the instrument to power sources.