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Old 2nd August 2013, 02:44 AM   #1
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Default JL 500/5 Blown channels

Hey guys,
To be honest this is my first attempt at an amp repair. I have looked through many of the other JL 500/5 amp repair guides and have seen a few of the common problems and where to check. I have not had the opportunity to hook the amp up to a power supply( don't have a desktop power supply) and have been told that the amp has a blown front and rear channel.

I have disassembled the amp to the point where I can access all the power transistors and output transistors. I have tried my best to search visually for cracks or other visible signs of damaged electrical components, but nothing sticks out at being problematic.

I first testing all the bias resistors on both the power and output transistors and they all seem to be to spec. I have noted the results from checking the output transistors using the continuity setting on a digital multimeter. The results are posted below.

Output Transistors IRF540
Q202 Q203 Q102 Q104 Q402 Q403 Q302 Q303 Q406 Q405 Q422 Q408
G-D open 1870 open 1869 open 1863 open 1863 1476 open open 1747
G-S open 1008 open 1010 open 1005 open 1003 662 open open 664
D-S open open open open open open open open open open open open

D-G open open open open open open open open open open open open
S-G 1858 1634 1851 1620 1855 1595 1876 1606 1333 771 772 1330
S-D 529 529 527 527 527 527 527 527 527 527 524 525

From what I can tell it seems as though I have one bad transistor in every pair. Does this seem right? Also is there any reason why the right transistor in every pair is the one that seems to have readings with the leads reversed?

Any Ideas as to what the problem could be and how to go about locating the issue? I have read Perry's beginner guide and I have some electrical knowledge.
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Old 2nd August 2013, 06:11 PM   #2
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This is one amplifier that you would not want to learn/start repairing on. Very complex, not to put you down or make you give up. Start on a much simpler amplifier such as those from the flea-market that way you wont cause much damage ($$ wise).
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Old 2nd August 2013, 10:03 PM   #3
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I agree this is a tough one to learn on. Plus JL has flat rate repairs for these thru JL dealers and its a good deal most of the time. If any of the channel driver cards are bad you will likely damage the main board trying to remove them for servicing. Seen that bunch of times.

As for your posted test info, it appears that all of the power devices are Ok. They tend to short out and read 10 ohms or less. A blown front and rear channel does not sound typical, but it is possible. You will need a way to power the amp up on some form of current controlled 12 volt source, and all of those power devices must be clamped to the heat sink bar on the rear of the amp to do any real testing to find faults on this amp.
You need to diagnose it under power on situations if possible just to find the issues and verify them to be correct. Then you need to tech those problems out.

Your problems can be caused by all sorts of issues. like bad input board, or bad channel driver board, or shorted outputs. Perry's trainer tells you how to use current limiters on 12 volt power sources. Your going to need that setup to work on this amp, just to pin point the failures. Or you will be teching out the entire amplifier chasing the needle in the haystack.

"From what I can tell it seems as though I have one bad transistor in every pair. Does this seem right? Also is there any reason why the right transistor in every pair is the one that seems to have readings with the leads reversed?"

If you try to measure fets or transistors that are paralleled like in all car amps your meter will be reading across any sets that are connected together as pairs or sets, your readings will be of all of those devices in the sets or pairs, due to the fact they are paralleled in circuit together. For beginners this will be very misleading, especially since you lack a factory document to describe how everything interconnects on the board. This will cause you to chase ghosts like most inexperienced tech's do on their first attempts at repair....
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Old 2nd August 2013, 10:49 PM   #4
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Be aware that FETs will show odd reading dependent on the on or off state of the junction. Not necessarily a faulty semiconductor!
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Old 6th August 2013, 12:05 AM   #5
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Were you checking the transistors in the circuit or out of the circuit?
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Old 7th August 2013, 02:38 AM   #6
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Quote:
Originally Posted by Perry Babin View Post
Were you checking the transistors in the circuit or out of the circuit?
I was checking them while in the board first to see if that would lead me anywhere. after rereading your help manual it makes sense as to why the readings would not be true for each individual FET if they are paralleled like most of the other members have pointed out.



Just to clarify. I got the amp for free from my brother and was told that if i can fix it then i would be able to keep it. Therefore, i am really not that worried about compounding the problem, but if its not too much soldering then i think i should be able to complete it successfully.

As for my soldering skills. I think i might have found a good test subject. I had bought a Klipsch promedia 2.1 computer speaker set up while i was in middle school and blew one of the channels. I remember taking the amp apart that was in the sub enclosure and replaced the fried FET, only for that to blow up another component. This led me to continue chasing the problem, before i finally decided to give up. I still have the circuit boards from that, so i could use those to practice.


I assembled the amp back together tonight and am planning on hooking the amp up to a car battery and trying to diagnose the problem that way. I have a could of questions before I do so.

1). I've seen that i could just jump the + lead to the remote wire instead of having to fish behind the dash for a remote wire. would this work or would i risk damaging more internals of the amp?

2). Could I use the RCA input on the amp for the front and rear channels and a 3.5mm to RCA cable to find the blown channels? If so i remember reading in the manual that I would probably have to change the input voltage on the amp to LOW instead of High.

3). I plan on taking off the bottom portion of the amp to make sure that the 3 LEDs on each of the individual sound cards are illuminated, but is there anything else that I should test while the amp is hooked up to the car? I have a spare speaker and subwoofer that I could hook up to each of the channels.
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Old 7th August 2013, 03:48 AM   #7
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I'll try to help with the JL but not the Klipsch.
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Old 7th August 2013, 04:05 AM   #8
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internal photos of the amplifier will help....
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Old 7th August 2013, 05:00 PM   #9
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If the klipsch repair is going to be done on the forum, it needs to be done in another thread.
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Old 7th August 2013, 05:20 PM   #10
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Quote:
Originally Posted by Perry Babin View Post
If the klipsch repair is going to be done on the forum, it needs to be done in another thread.
Makes sense to me. I wouldn't want to clutter the thread.
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