QSC-ISA750-pwr-sb1 维修电路原理图.pdf
1ISA0001Title:ISA Turn-on HangupBulletin #:ISA0001Issue Date: April 4, 2002Models Affected:ISA 750, 800T, 450, & 500TBulletin Revision:AProduction Range: 01/200004/2002 (serial # 0100 xxxxx0402xxxxx)Service BulletinDescriptionThe circuitry that controls protective current cutback in ISA amplifiers may in some instances cause a channelusuallychannel 1to “hang up” at turn-on. This problem is most pronounced in the ISA 750 and ISA 800T models, but may alsooccur in some ISA 450 and ISA 500T amplifiers.SymptomsWhen 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 theamplifier, 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 engagedA DataPort accessory powered by the ampLow AC line voltageCold ambient temperaturePassing an audio signal of a sufficient level through the affected channel will usually clear the problem; the level requiredgenerally depends on the severity of the hang-up.Background: the cause and the remedyEach channel has an operational amplifier (op amp) driving its output stage. Its supply rails furnish just enough current fornormal operation by themselves and use the output signal to help replenish them. When the output stage tries to put out toomuch 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, sodoes 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 valuesquickly enough after turn-on. The op amp output swings to one rail, causing it to collapse and in turn causing the bias to notsettle 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 recalibratethe current-limit trimpots (R139, R140, R239, and R240).Temporary work-aroundsUntil 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 outputInstructionsTools 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 cuttersRadioFans.CN 收音机爱 好者资料库2ISA0001 Desoldering equipment or solder braid Four 4.7-volt -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 inthe ISA 450 and 500T are single-sided. Therefore, to minimize disassembly, this bulletinrecommends two different procedures among these models:In the ISA 750 and ISA 800T, 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 500T, the new zener diodes will be soldered “piggyback”-style to the old ones. The instructionsfor this procedure begin after those for the ISA 750 and ISA 800T.0.37.6 mm0.410.2 mmD107 & D210:D207 & D110:0.150.23.85.1 mmProcedure: Replacing zenerdiodes in an ISA 750 or ISA 800Tamplifier1.Disconnect the amplifier from AC and wait at least 10 minutesfor 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 thatfasten the heat sink to the chassis.4.Remove the three screws that attach the channel moduleassembly to the chassis standoffs.5.Lift up the front of the channel module assembly. Rotate it sothat channel 1s module (the lower one) is on top; set theassembly down in the chassis with the heat sink resting onthe standoffs (Figure 2).6.Disconnect the ribbon cable from the channel 1 module.7.Locate zener diode D107. It is near where three wires aresoldered to the circuit board and secured with hot glue. If thehot glue covers the diode, use the tip of your hot solderingiron and a small screwdriver to melt and clear the glue away(Figure 3).8.Notice the orientation of D107. Unsolder and remove it; becareful not to damage the through-plating in the holes. Installand 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 moduleassembly back over so that the channel 2 module is on top.10. Locate zener diode D207 and notice its orientation. CarefullyTools and materials required (continued):Figure 1. Prepare the leads of thenew zener diodes.Figure 2. Prop the channel module assembly up on thechassis standoffs.Figure 3. You might need to clear hot glue away fromdiode D107. Use your hot soldering iron tip.RadioFans.CN 收音机爱 好者资料库3ISA0001unsolder and remove it. Install and solder a new zener diode oriented in thesame direction. Do the same with diode D210.11. Check your work. Dont reattach the channel module assembly and heat sink tothe chassis yet. Instead, continue with the bias and current-limit calibration.New 4.7-volt zenerOld 5.6-volt zenersno i t arb i l aCt su j dA&054ASIT005ASI&057ASIT008ASI741Rs so r caega t l ovCD: sa i b1l ennahC131RV21 . 0V70 . 042Rs so r caega t l ovCD: sa i b2l ennahC7132RV21 . 0V70 . 0dao lde t r ohso t n i t ne r r uct up t uO041R&931R: 1l ennahC042R&932R: 2l ennahCA54A95 . 8*dao lde t r ohsgn i v i r dnehwt ne r r ucCA041R&931R: 1l ennahC042R&932R: 2l ennahCA5 . 55 . 4A5 . 65 . 5) SMR (smho2o t n iega t l ovgn i pp i l C931Rt su j dA: 1l ennahCy r t emmy sr o f932Rt su j dA: 2l ennahCy r t emmy sr o fV5 . 735 . 33V9444) kaep (smho2o t n iega t l ovgn i pp i l C931Rt su j dA: 1l ennahCy r t emmy sr o f932Rt su j dA: 2l ennahCy r t emmy sr o fV354 . 74V3 . 962 . 26; e r u t a r epme tt ne i bmat a (*dnamedCAe l d I) t ohnehwr ehg i h, A4 . 0%01A6 . 0,%01. V001r o f2 . 1ybr oV032r o f5 . 0ybt ne r r ucy l p i t l um; s r e i f i l pmaV021r o fe r anwohsse r ug i F*Procedure: Installing new zener diodes in anISA 450 or ISA 500T amplifier1.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 twonew 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 withthe bias and current-limit calibration.Procedure: Bias and current-limit calibrationNote: 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 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 amplifiers 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 assemblyand rest it on the heat sink so the bottom circuit board is accessible. Note that the trimpots for the bottom channelmodule are located on asmall circuit board next tothe top module.3.Plug the amplifier into thevariable transformer.4.Turn the gain controls allthe way down.5.Turn the amplifier on andgradually turn up thevariable transformer tothe amplifiers normaloperating voltage whileyou monitor its AC currentdraw (it should notgrossly exceed the idlecurrent figures in Table 1).Make sure the amplifier isfunctioning correctly.Bias adjustment (performonly with unit at ambienttemperature)Note: This calibration must bedone with the amplifiercircuitry at ambient tempera-ture. If the amp has begun towarm up, you must turn it offFigure 4. Solder a new zener diode“piggyback”-style onto each old one.Table 1.4ISA0001and 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 adjustmentNote: Read through these instructions before proceeding. To avoid tripping the amps circuit breaker, work carefully butquickly, and dont run the amp too long at full power into 2 ohms. If you dont have the equipment to do this procedureproperly, 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 1s input. Connect channel 1s output to a2-ohm load, and connect the oscilloscope probe across channel 1s output.10. Turn the amplifier on. Turn up channel 1s gain control partway. On the oscilloscope you should see the amplitude of thesine wave increase accordingly.11. Turn the gain control back down and apply a short circuit across the output terminals of channel 1. Clamp a currentprobe either onto one of the brown wires running to the AC switch or onto the gray output wire from channel 1s module.12. Turn the gain control all the way up. Adjust trimpots R139 and R140 equally until the current measured falls within therange 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 gaincontrol back up until the output clips. The voltage at which the signal starts to clip should fall within the range shown inTable 1. If the clipping is asymmetrical, that is, the signal clips on either the positive or negative side first, adjust R139to 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 proceedingwith 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 checkthe amps idle current. If the amplifier is still at about room temperature, the idle current should match the value shownin Table 1.Finishing the repair17. 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 informationIf you need any further information regarding this service procedure, please contact QSC Technical Services at the addressesor numbers below.Telephone:1-800-772-2834 (within USA only)+1 (714) 957-7150Fax:+1 (714) 754-6173E-mail:tech_Web site: (product info and support) (on-line technical services and accessory sales)Postal and parcel address:QSC Audio Products, Inc.Technical Services Group1665 MacArthur Blvd.Costa Mesa, CA 92626 USA
