QSC ISA 800T - Manual

1

ISA0001

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)

Service Bulletin

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