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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
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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
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New 4.7-volt zener unsolder and remove it. Install and solder a new zener diode oriented in the same direction. Do the same with diode D210. Old 5.6-volt zener 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. Figure 4. Solder a new zener diode “piggyback”-style onto each old one. Procedure: Installing new zener diodes in an ISA 450 or ISA 500T amplifier 1. Disconnect the amplifier from
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and wait until it cools down to about room temperature. 6. Measure the DC voltage across R147. Adjust bias trimpot R131 to reach the correct voltage shown in Table 1. 7. Next, measure the DC voltage across R247. Adjust bias trimpot R231 to reach the correct voltage shown in Table 1. 8. Turn off the amplifier and reattach the channel module assembly and heat sink to the chassis. Current limit adjustment Note: Read through these instructions before proceeding. To avoid tripping the amp’s circuit b
