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Old 19th July 2012, 09:23 PM   #1
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Default JL 300/4 Correct output transistors

Hey guys!

Just paid a little over $180 for this (300/4) amp and got frustrated the minute I tested and I had DC on Left-Rear output (good thing I was using a 2A bench supply otherwise I'd burn my test speaker). Went back to the ebay listing and the PO DID mention that the channel sometimes worked and sometimes didn't? whaaat? I guess it's a once cooked it can't go back

So I looked at several posts and saw that some of you guys have replaced the IRF540 output transistors with other components part number, however, it looks like the IRF540PBF is the correct one right?

Perry's picture mentions the number 4 belongs to left-rear channel right?
http://www.bcae1.com/temp/jlaudio300...doutputs01.swf


BTW: Where are the test points for side bias (4mV)

THANKS in advance!
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Old 20th July 2012, 04:01 AM   #2
RiLoWa is offline RiLoWa  United States
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Yes, use the IRF540PBF. The PBF means it is the lead free version.

It may depend on a board revision, but the 300/4's that I have worked on have the bias points labeled on the component side of the board near the buss bars in front of the outputs.

They are best measured from the opposite side of the components so that you don't short your test leads against any of the buss bars.
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Old 20th July 2012, 04:17 AM   #3
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Awesome! I just orderd them.
I'll post on progress, thanks!
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Old 27th July 2012, 06:48 PM   #4
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Unhappy No success

I swaped transistors yesterday night and no success, DC is still at that same output, meaning that there something wrong upstream on that channel I suppose...

I monitored the idle current and took about 40 seconds from 0 to 3 amps. Another (non-heatsinked) IRF540 that's close to the power supply controller got really hot.. Other 3 channels are OK, so I suppose it's a channel preamp stage problem, what'cha say?
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Old 27th July 2012, 08:31 PM   #5
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If all of the LEDs are lit on the driver board, the problem is likely the two differential amplifier transistors on the top-left corner of the board. There is also a 620 ohm resistor that often burns when those transistors fail.
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Old 27th July 2012, 08:57 PM   #6
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Quote:
Originally Posted by Perry Babin View Post
If all of the LEDs are lit on the driver board, the problem is likely the two differential amplifier transistors on the top-left corner of the board. There is also a 620 ohm resistor that often burns when those transistors fail.
Are you talking about this (also took notes on the 473 resistor from other thread) (notes on picture)

Let me check that and I'll post on how it goes, I think all the LED's were ON for all the driver boards...

In your experience, what causes this to fail?
Attached Images
File Type: jpg 300Slash4-Driver-Board.jpg (846.5 KB, 70 views)

Last edited by sameoldme; 27th July 2012 at 09:05 PM.
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Old 28th July 2012, 04:14 AM   #7
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OK! just got home and reconnected the amp in the bench..
The 2 top lights in the defective channel board are not lighting up...
:S
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Old 28th July 2012, 05:16 AM   #8
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OK (again ) I just read in other posts, that the 2 top-left transistors are BC850C's I noted that one had different voltages than the other 3 boards. I removed it and now all the lights turn ON and the amp is no longer demanding a lot of current...
Tiny little critters!! I guess order and replace, see what happens right?
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Old 29th July 2012, 09:46 PM   #9
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I just ordered these from Mouser: 726-BC850CE6327
Can't wait for them to get home... I checked the resistors that Perry recommended and they were OK, I guess it was just one of the transistors and the resistors didn't melt, hopefully that does the trick
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Old 3rd August 2012, 03:37 AM   #10
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Just a quick update guys, and PERRY, THANKS A LOT!!!
I fixed the AMP; she's up and running, thanks for all the help!!

It was the "B" little transistor in Perry's diagram, that was it. Fired it up and it runs like a champ, channels seem like they are the same level all and it sounds clean and crisp!

If you guys need part numbers or anything else, let me know..

Here's a Pic with all the lights on and the amp running

Thanks again Perry! and sorry for being impatient and posting in somebody else's thread
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File Type: jpg DSC00540.jpg (627.2 KB, 48 views)
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