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Old 13th August 2008, 01:42 AM   #1
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Default Jbl Bp1200

i have never seen an amp with so many of these transistors. i am still unclear how to know what part to replace with when the component is a complete melt down.

is this normal for so many of these to die at once? i have 10 that are a totally destroyed. also should i be looking to replace the resistors along with the transistors? all the stripes are melted off but im sure i can asume they are all the same.

also i found other burnt components i think its a diode?

is there anything else i should be looking for? i havent even hooked power to this yet.
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Old 13th August 2008, 01:43 AM   #2
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the other 5
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Old 13th August 2008, 01:45 AM   #3
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possible diodes? or somesort of transistor
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Old 13th August 2008, 02:32 AM   #4
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The burnt fets in the power supply should be IRF3205's. The resistors r743-747 and r753-757 are carbon film 22ohm 1/5w 5%.
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Old 13th August 2008, 03:09 AM   #5
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Burnt power supplies go hand in hand with shorted output Fets near those burnt resistors. A fairly common failure mode of just about ANY class D amp built in the past years gone by.

There was a fella on E-bay selling the JBL documents for this amp. I suggest you get them if your serious about repairing this amp.

If the output fets are not shorted I would start worrying about the Toroid transformer located near the burnt fets being shorted on itself.

All of the above have been seen inside these particular amps over the years... Good luck on your rebuild of the JBL
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Old 13th August 2008, 04:08 AM   #6
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Q741/751 are the driver transistors for the power supply FETs (the burned ones). The part number is 2SA1266. You should replace both of them. D741/751 probably survived but you should replace them also. They are 1N4148 diodes.

You need to clean the area around all burned components. After cleaning, examine the areas that were blackened to determine if there is any damage to the board (especially around the driver transistor).

Before you order or replace anything, you need to check the outputs (previously suggested). If you read anything near 0 ohms between any of the legs of any individual transistor, the transistor is likely defective. Since the outputs are in parallel groups, if one is defective, all in that group must be replaced.

If one or more outputs have failed, there's a good chance that the audio driver transistors have failed. There's also a good chance that at least one of the 12v Zener diodes has failed (in the audio driver circuit).
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Old 14th August 2008, 12:19 AM   #7
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i am measuring the resistors and they are all measuing around 7 ohms. are you sure that i should be ordering 22 ohm ones to replace them? (R742-r747) and (r752-r757). i found this part number from mouser but it is only 1/4 watt not the reccomended 1/5th watt

http://www.mouser.com/Search/Product...3Mo17tIQ%3d%3d

i found these to replace the burnt out power transistors. will they be the same for both sides of the amp

http://www.mouser.com/Search/Product...nOvtUewg%3d%3d

all of the biggest transistors (MUR304OPT) X4 all have continuity between all legs. i tested them in the board im not sure if i should pull to test or not.

as for the cluster of burnt parts at location. location Q751 is cracked and burnt ontop of the board and also underneath and im guessing Q506 is its twin? Q506 isnt cracked or burnt but should it be replced too? should i replace the capacitor at location R511 or the resistor underneath it? i cleaned it up and ill try post a better picture.

this part number isnt coming back from mouser as valid part number and mine is too melted to get a good number from.

where do i find these parts audio driver transistors or my zener diode on this board.
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Old 14th August 2008, 12:40 AM   #8
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Did you check the outputs to determine if any were shorted?
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Old 14th August 2008, 12:51 AM   #9
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Quote:
Originally posted by scampo77
[B]i am measuring the resistors and they are all measuing around 7 ohms. are you sure that i should be ordering 22 ohm ones to replace them? (R742-r747) and (r752-r757). i found this part number from mouser but it is only 1/4 watt not the reccomended 1/5th watt
742 and 752 are 1k ohm and 743-747 and 753-757 are 22ohm according to the service manual.
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Old 14th August 2008, 01:19 AM   #10
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are the ouputs the transistors #MUR304OPT ?

"go hand in hand with shorted output Fets near those burnt resistors"

those transistors are right next to the burnt ones so i figured they would be it. i thought i just needed to check for resistance between all the legs? how else do i check them? the outer legs of all 4 measure at 0 ohms the others, when i apply the test leads they keep going higher and higher and all 4 respond the same way.
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