QSC Audio Car Amplifier ISA 450 User Guide

Service Bulletin  
Title:  
Bulletin #:  
ISA Turn-on Hangup  
ISA0001  
Issue Date: April 4, 2002  
Models Affected: ISA 750, 800T, 450, & 500T  
Bulletin Revision:  
Production Range: 01/2000–04/2002 (serial # 0100xxxxx–0402xxxxx)  
A
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 amplifiers 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  
1
 
New 4.7-volt zener  
Old 5.6-volt zener  
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. Dont 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 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. Dont 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.  
Table 1.  
ISA 450 & ISA 750 &  
3. Plug the amplifier into the  
variable transformer.  
Calibrations  
Adjust  
ISA 500T  
ISA 800T  
4. Turn the gain controls all  
the way down.  
Channel 1 bias: DC voltage across R147  
Channel 2 bias: DC voltage across R247  
R131  
R231  
0.12 V  
0.07 V  
0.12 V  
0.07 V  
5. Turn the amplifier on and  
gradually turn up the  
Channel 1: R139 & R140  
Channel 2: R239 & R240  
Output current into shorted load  
45 A  
8.59 A  
variable transformer to  
the amplifier's normal  
operating voltage while  
you monitor its AC current  
draw (it should not  
grossly exceed the idle  
current figures in Table 1).  
Make sure the amplifier is  
functioning correctly.  
Channel 1: R139 & R140  
Channel 2: R239 & R240  
AC current when driving shorted load*  
4.55.5 A  
5.56.5 A  
Channel 1: Adjust R139  
for symmetry  
Channel 2: Adjust R239  
for symmetry  
Clipping voltage into 2 ohms (RMS)  
Clipping voltage into 2 ohms (peak)  
33.537.5 V  
4449 V  
Channel 1: Adjust R139  
for symmetry  
Channel 2: Adjust R239  
for symmetry  
Bias adjustment (perform  
only with unit at ambient  
temperature)  
Note: This calibration must be  
done with the amplifier  
circuitry at ambient tempera-  
ture. If the amp has begun to  
warm up, you must turn it off  
47.453 V  
62.269.3 V  
Idle AC demand* (at ambient temperature;  
higher when hot)  
0.4 A, 10% 0.6 A, 10%  
*Figures shown are for 120V amplifiers; multiply current by 0.5 for 230V or by 1.2 for 100V.  
ISA0001  
3
 
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 amps circuit breaker, work carefully but  
quickly, and dont run the amp too long at full power into 2 ohms. If you dont have the equipment to do this procedure  
properly, it is better to skip it and leave the current-limit trimpots as is than to risk misadjusting them.  
9. Set the audio sine generator to 1 kHz at 1 volt RMS and connect it to channel 1's input. Connect channel 1's output to a  
2-ohm load, and connect the oscilloscope probe across channel 1's output.  
10. Turn the amplifier on. Turn up channel 1's gain control partway. On the oscilloscope you should see the amplitude of the  
sine wave increase accordingly.  
11. Turn the gain control back down and apply a short circuit across the output terminals of channel 1. Clamp a current  
probe either onto one of the brown wires running to the AC switch or onto the gray output wire from channel 1's module.  
12. Turn the gain control all the way up. Adjust trimpots R139 and R140 equally until the current measured falls within the  
range shown in Table 1.  
13. Turn the gain control all the way down and remove the short circuit so the channel drives the 2-ohm load. Turn the gain  
control back up until the output clips. The voltage at which the signal starts to clip should fall within the range shown in  
Table 1. If the clipping is asymmetrical, that is, the signal clips on either the positive or negative side first, adjust R139  
to make it symmetrical.  
14. Turn the gain control down. If the amp has begun to warm up, shut it off and let it cool a few minutes before proceeding  
with channel 2.  
15. Repeat steps 8 through 12 for channel 2. Use trimpots R239 and R240 to adjust the current limiting in steps 11 and 12.  
16. Turn both channelsgain controls all the way down. Clamp the current probe onto one of the brown AC wires to check  
the amps idle current. If the amplifier is still at about room temperature, the idle current should match the value shown  
in Table 1.  
Finishing the repair  
17. Turn the amplifier off and disconnect all input and output cables.  
18. Re-install the top cover. The amplifier can be returned to use.  
Contact information  
If you need any further information regarding this service procedure, please contact QSC Technical Services at the addresses  
or numbers below.  
Telephone:  
1-800-772-2834 (within USA only)  
+1 (714) 957-7150  
Fax:  
+1 (714) 754-6173  
E-mail:  
Web site:  
Postal and parcel address:  
QSC Audio Products, Inc.  
Technical Services Group  
1665 MacArthur Blvd.  
Costa Mesa, CA 92626 USA  
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ISA0001  
 

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