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QSC Audio ISA 800T, ISA 500T manual Description, Symptoms, Background the cause and the remedy

Models: ISA 750 ISA 450 ISA 500T ISA 800T

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Service Bulletin

Title:

ISA Turn-on Hangup

Bulletin #:

ISA0001

Issue Date: April 4, 2002

Models Affected:

ISA 750, 800T, 450, & 500T

Bulletin Revision:

A

 

Production Range: 01/2000–04/2002 (serial # 0100xxxxx–0402xxxxx)

Description

The circuitry that controls protective current cutback in ISA amplifiers may in some instances cause a channel—usually channel 1—to “hang up” at turn-on. This problem is most pronounced in the ISA 750 and ISA 800T models, but may also occur in some ISA 450 and ISA 500T amplifiers.

Symptoms

When the amplifier is turned on, one or both channels remain inoperative, with their respective clip LEDs lit. In some cases, the problem will clear by itself after several seconds. This problem is merely a nuisance and presents no danger to the amplifier, the speakers, or the rest of the sound system.

Any one or more of these situations will tend to aggravate the problem:

•High-pass filters (low-frequency) filters engaged

•A DataPort accessory powered by the amp

•Low AC line voltage

•Cold ambient temperature

Passing an audio signal of a sufficient level through the affected channel will usually clear the problem; the level required generally depends on the severity of the hang-up.

Background: the cause and the remedy

Each channel has an operational amplifier (op amp) driving its output stage. Its supply rails furnish just enough current for normal operation by themselves and use the output signal to help replenish them. When the output stage tries to put out too much current into a short circuit or abnormally low load impedance, it clips prematurely at a very low voltage; as a result, there is not enough output signal to replenish the op amp supply rails. The starved supply rails collapse, and with them, so does the op amp output. This is a normal and vital part of the amplifier’s protection against short circuits.

A channel hangs when the op amp rails and the bias in the output transistor circuitry do not settle to their correct values quickly enough after turn-on. The op amp output swings to one rail, causing it to collapse and in turn causing the bias to not settle correctly. As a result, there is no negative feedback to force the op amp to resolve itself to zero volts.

To correct the situation, replace the 5.6-volt zener diodes (D107, D110, D207, and D210) with 4.7-volt diodes and recalibrate the current-limit trimpots (R139, R140, R239, and R240).

Temporary work-arounds

Until the amplifier can be serviced, one of these actions will usually clear its hang-up and get it working properly:

•Quickly turning it off and on (CAUTION: this will cause an audible pop in any loudspeaker connected to the amp)

•Unplugging and re-connecting speaker load from the channel output

Instructions

Tools and materials required:

•Soldering iron with fine tip (recommended range 25 to 60 W)

•Rosin-core solder (60/40 or 63/37 eutectic type)

•Long-nose pliers

•#1 and #2 Philips screwdrivers

•Small diagonal cutters

ISA0001

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QSC Audio ISA 800T manual Description, Symptoms, Background the cause and the remedy, Temporary work-arounds, Instructions
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ISA 750, ISA 450, ISA 500T, ISA 800T specifications

QSC Audio, a renowned name in professional audio solutions, offers a range of power amplifiers designed to deliver high performance, reliability, and user-friendly operation. Among their product line, the ISA series stands out, particularly the ISA 800T, ISA 500T, ISA 450, and ISA 750 models. These amplifiers are tailored for various applications, making them ideal for sound reinforcement in live events, small venues, and installations.

The ISA 800T is a powerful amplifier featuring a robust design that can deliver up to 800 watts of power per channel at 4 ohms. This model uses advanced feedback topology and has built-in thermal and short-circuit protection, ensuring reliability and longevity in demanding environments. The ISA 800T also provides features like selectable low-frequency filter, high-pass filter, and variable speed fan control, which enhance its versatility in different applications.

The ISA 500T model, offering 500 watts per channel, is perfect for smaller venues, yet it doesn’t compromise on quality. This amplifier also employs the same advanced feedback topology, providing pristine audio reproduction while maintaining low distortion levels. The ISA 500T offers a variety of input options, including XLR and TRS connections, allowing easy integration into existing audio systems. The compact form factor makes it a favorite for mobile setups and permanent installations alike.

The ISA 450 is another noteworthy addition to the series, designed to cater to smaller systems. It offers 450 watts per channel at 4 ohms and features a similar well-engineered design as its higher-powered counterparts. The ISA 450 is equipped with thermal and overload protections, enabling operation without worry in challenging environments. A unique characteristic of this model is its efficient power consumption, making it an eco-friendly option.

Finally, the ISA 750, with 750 watts per channel, bridges the gap between the ISA 500T and ISA 800T, providing a perfect balance of power and flexibility. It incorporates intuitive controls and connectivity options while maintaining the hallmark features of the ISA series.

All ISA series amplifiers utilize QSC’s rigorous engineering, ensuring they not only meet but exceed the demands of audio professionals. Each model is designed for modularity and durability, ready to endure the rigors of tour life or permanent installation. With their combination of power, reliability, and advanced features, the QSC ISA series amplifiers are an excellent choice for anyone seeking outstanding performance in sound reinforcement applications.