QSC ISA 450 User's Manual
Contents
1. Title ISA Turn on Hangup Bulletin ISA0001 Issue Date April 4 2002 Models Affected ISA 750 8001 450 amp 5007 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 e High pass filters low frequency filters engaged e A DataPort accessory powered by the amp e Low AC line voltage e 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 the2. small diagonal cutters SA0001 Tools and materials required continued Desoldering equipment or solder braid Four 4 7 volt 44 watt zener diodes Microsemi DZ840731ET R QSC part number QD 0004 7 ZT Test equipment required Note he circuit boards in the ISA 750 and ISA 800T are double sided while those in 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 eN Bi Ta Digital multimeter E 3 8 5 1 mm AC current probe e g BK Precision model CP 1 D207 amp D110 i Trimpot adjustment tool non conductive the ISA 450 and 500T are single sided Therefore to minimize disassembly this bulletin Figure 1 Prepare the leads of the recommends two different procedures among these models new zener diodes Procedure Replacing zener diodes in an ISA 750 or ISA 800T amplifier 1 In the ISA 750 and ISA 8001 the zener diodes will be unsoldered and soldered from the component side The instruc tions for this procedure begin below In the ISA 450 and ISA 5007 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 8007 Disconnect the amplifier from AC and wait at least 10 minute
3. zener diode oriented in the New 4 7 volt zener 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 T the chassis yet Instead continue with the bias and current limit calibration Figure 4 Solder a new zener diode Procedure Installing new zener diodes in an 99vtack style onto each old one 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 standofts 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 On 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
4. 75V 44 49 V Make sure the amplifier is Channel 2 Adjust R239 functioning correctly for symmetry Bias adjustment perform nile 1 Adjust R139 only with unit at ambient Clipping voltage into 2 ohms peak eae aie R239 47 453 V 62 2 69 3 V temperature for symmetry Note This calibration must be done with the amplifier idle AC demand at ambient temperature 04A 10 06 A 10 circuitry at ambient tempera higher when hot ture If the amp has begun to warm up you must turn it off Figures shown are for 120V amplifiers multiply current by 0 5 for 230V or by 1 2 for 100V SA0001 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 amp s circuit breaker work carefully but quickly and don t run the amp too long at full power into 2 ohms If you don t 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 cha
5. ill 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 OSC 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 tech_support qscaudio com Web site www gscaudio com product info and support www qscstore com on line technical services and accessory sales Postal and parcel address QSC Audio Products Inc Technical Services Group 1665 MacArthur Blvd Costa Mesa CA 92626 USA 4 SA0001
6. nnel 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 channels gain controls all the way down Clamp the current probe onto one of the brown AC wires to check the amp s idle current If the amplifier is st
7. 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 e Quickly turning it off and on CAUTION this will cause an audible pop in any loudspeaker connected to the amp e Unplugging and re connecting speaker load from the channel output Instructions Tools and materials required e 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
8. s for internal voltages to bleed down Remove the top cover Prepare the new zener diodes as shown in Figure 1 Remove the four screws on the bottom of the amplifier that fasten the heat sink to the chassis Remove the three screws that attach the channel module assembly to the chassis standoffs 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 Figure 2 Prop the channel module assembly up on the assembly down in the chassis with the heat sink resting on Chassis standoffs the standoffs Figure 2 Disconnect the ribbon cable from the channel 1 module 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 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 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 Figure 3 You might need to clear hot glue away from 2 diode D107 Use your hot soldering iron tip ISA0001 unsolder and remove It Install and solder a new
9. to avoid tripping the amplifiers 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 5007 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 Table 7 the top module 3 Plug the amplifier into the Hit tt ISA 450 amp ISA 750 amp variable transformer eaa nons anusi 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 5 Turn the amplifier on and Channel 2 bias DC voltage across R247 2 bias DC Channel 2 bias DC voltage across R247 across R247 R231 0 12 V ogy 07 ogy gradually turn up the Channel 1 R139 amp R140 variable transformer to Output current into shorted load Channel 2 R239 amp R240 8 5 9 A the amplifier s normal operating voltage while o N Channel 1 R139 amp R140 you monitor its AC current AC current when driving shorted load Channel 2 R239 amp R240 45 55 A 55 65A draw it should not grossly exceed the idle o 1 Adjust R139 current figures in Table 1 Clipping voltage into 2 ohms RMS ae 33 5 3
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