QSC Audio ISA 500T, ISA 450, ISA 800T, ISA 750 User Manual
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.
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).
This is a normal and vital part of the amplifier’s protection against short circuits.
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
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Tools and materials required (continued):
• Desoldering equipment or solder braid
• Four 4.7-volt ¼-watt zener diodes (Microsemi DZ840731ET/R; QSC part number QD-0004.7-ZT)
Test equipment required:
• Oscilloscope
• 2-ohm resistive load (rated for at least 1200 watts)
• Wire or other means of shorting the amplifier output
• Variable AC transformer (e.g., Variac, Powerstat, etc.) rated for 25A (120V) or 12A
(230V). Make sure your AC supply is appropriate for the amplifier.
• 1 kHz audio sine wave generator
• Digital multimeter
• AC current probe (e.g., BK Precision model CP-1)
• Trimpot adjustment tool (non-conductive)
Note: The circuit boards in the ISA 750 and ISA 800T are double-sided, while those in
the ISA 450 and 500T are single-sided. Therefore, to minimize disassembly, this bulletin
recommends two different procedures among these models:
D107 & D210:
D207 & D110:
0.3"
7.6 mm
0.4"
10.2 mm
Figure 1. Prepare the leads of the
new zener diodes.
• In the ISA 750 and ISA 800T, the zener diodes will be unsoldered and soldered from the component side. The instructions for this procedure begin below.
• In the ISA 450 and ISA 500T, the new zener diodes will be soldered “piggyback”-style to the old ones. The instructions
for this procedure begin after those for the ISA 750 and ISA 800T.
0.15–0.2"
3.8–5.1 mm
Procedure: Replacing zener
diodes in an ISA 750 or ISA 800T
amplifier
1. Disconnect the amplifier from AC and wait at least 10 minutes
for internal voltages to bleed down. Remove the top cover.
2. Prepare the new zener diodes as shown in Figure 1.
3. Remove the four screws on the bottom of the amplifier that
fasten the heat sink to the chassis.
4. Remove the three screws that attach the channel module
assembly to the chassis standoffs.
5. Lift up the front of the channel module assembly. Rotate it so
that channel 1’s module (the lower one) is on top; set the
assembly down in the chassis with the heat sink resting on
the standoffs (Figure 2).
6. Disconnect the ribbon cable from the channel 1 module.
7. Locate zener diode D107. It is near where three wires are
soldered to the circuit board and secured with hot glue. If the
hot glue covers the diode, use the tip of your hot soldering
iron and a small screwdriver to melt and clear the glue away
(Figure 3).
8. Notice the orientation of D107. Unsolder and remove it; be
careful not to damage the through-plating in the holes. Install
and solder a new zener diode oriented in the same direction.
Do the same with zener diode D110.
9. Reattach the ribbon cable and turn the channel module
assembly back over so that the channel 2 module is on top.
10. Locate zener diode D207 and notice its orientation. Carefully
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Figure 2. Prop the channel module assembly up on the
chassis standoffs.
Figure 3. You might need to clear hot glue away from
diode D107. Use your hot soldering iron tip.
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