Table 1.

unsolder and remove it. Install and solder a new zener diode oriented in the same direction. Do the same with diode D210.

11.Check your work. Don’t reattach the channel module assembly and heat sink to the chassis yet. Instead, continue with the bias and current-limit calibration.

New 4.7-volt zener

Old 5.6-volt zener

Procedure: Installing new zener diodes in an ISA 450 or ISA 500T amplifier

Figure 4. Solder a new zener diode

“piggyback”-style onto each old one.

1. Disconnect the amplifier from AC and wait at least 10 minutes for internal voltages to bleed down. Remove the top cover.

2. Remove the four screws on the bottom of the amplifier that fasten the heat sink to the chassis.

3. Remove the three screws that attach the channel module assembly to the chassis standoffs.

4. Locate zener diodes D107 and D110 on the channel 1 module (the upper circuit board). Bend and trim the leads of two new zener diodes and solder them piggyback-style to D107 and D110, oriented in the same direction (Figure 4).

5. Turn the channel module assembly over and repeat step 4 with zener diodes D207 and D210.

6. Check your work. Don’t reattach the channel module assembly and heat sink to the chassis yet. Instead, continue with the bias and current-limit calibration.

Procedure: Bias and current-limit calibration

Note: During the current-limiting part of this procedure, the amplifier will draw high amounts of AC: approximately 22 am- peres at 120V, or 11 amperes at 230V. Be thorough, but work quickly to avoid tripping the amplifier's circuit breaker.

1. Start with the variable transformer turned all the way down to zero, the amplifier turned off, and its top cover still removed.

2. On the amplifier's circuit boards, locate resistors R147 and R247, and trimpots R131, R231, R139, R239, R140, and R240. To reach R147 on the ISA 750 and ISA 800T or R247 on the ISA 450 and ISA 500T, rotate the chassis module assembly and rest it on the heat sink so the bottom circuit board is accessible. Note that the trimpots for the bottom channel module are located on a

small circuit board next to the top module.

3.

Plug the amplifier into the

Calibrations

Adjust

ISA 450 &

ISA 750 &

 

 

variable transformer.

ISA 500T

ISA 800T

 

 

 

 

 

 

 

 

 

 

 

 

 

4.

Turn the gain controls all

 

Channel 1 bias: DC voltage across R147

R131

0.12 V

0.07 V

 

the way down.

 

 

 

 

 

 

 

 

 

 

Channel 2 bias: DC voltage across R247

R231

0.12 V

0.07 V

5.

Turn the amplifier on and

 

gradually turn up the

 

 

 

 

 

 

 

 

 

Channel 1: R139 & R140

 

 

 

 

variable transformer to

Output current into shorted load

4–5 A

8.5–9 A

 

Channel 2: R239 & R240

 

the amplifier's normal

 

 

 

 

 

 

 

 

 

 

 

 

 

operating voltage while

AC current when driving shorted load*

Channel 1: R139 & R140

4.5–5.5 A

5.5–6.5 A

 

you monitor its AC current

Channel 2: R239 & R240

 

 

 

 

 

 

draw (it should not

 

 

 

 

 

 

 

 

 

Channel 1: Adjust R139

 

 

 

 

grossly exceed the idle

 

 

 

 

 

current figures in Table 1).

Clipping voltage into 2 ohms (RMS)

for symmetry

33.5–37.5 V

44–49 V

 

Channel 2: Adjust R239

 

Make sure the amplifier is

 

 

 

 

 

 

for symmetry

 

 

 

 

functioning correctly.

 

 

 

 

 

 

 

 

 

 

 

Bias adjustment (perform

 

Channel 1: Adjust R139

 

 

 

only with unit at ambient

Clipping voltage into 2 ohms (peak)

for symmetry

47.4–53 V

62.2–69.3 V

Channel 2: Adjust R239

temperature)

 

 

 

 

 

for symmetry

 

 

 

Note: This calibration must be

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

done with the amplifier

Idle AC demand* (at ambient temperature;

 

0.4 A, ±10%

0.6 A, ±10%

circuitry at ambient tempera-

higher when hot)

 

 

 

 

 

ture. If the amp has begun to

 

 

 

 

 

 

 

*Figures shown are for 120V amplifiers; multiply current by 0.5 for 230V or by 1.2 for 100V.

 

 

warm up, you must turn it off

 

 

 

 

 

 

 

ISA0001

 

 

 

3

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QSC Audio ISA 750, ISA 500T, ISA 800T, ISA 450 manual Procedure Bias and current-limit calibration, Calibrations Adjust ISA
Page 2 Page 4

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.
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