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JL Audio 450/4 repair - Need help please!!
JL Audio 450/4 repair - Need help please!!
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Old 3rd January 2018, 06:31 PM   #1
ResonateEngineering is offline ResonateEngineering
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Default JL Audio 450/4 repair - Need help please!!

Hello all, new to the forums. I am in need of somone who is familiar with JL Audio slash amps, 450/4 in particular. I apologize in advance for lacking the knowledge that some of you have. I am new to amp repair diagnostics. I donít have a good local source for repair so I am debating on attempting myself. The amp is useless to me in its current condition so I donít have much to lose.

I ran too low of an ohm on the front channels and fried some power fets I believe. The low ohm light never came on to my knowledge. Now those channels have severely low output with distortion. I originally thought it was stuck in protect mode however after opening it up I see some visual damage.

Mainly what I need is info on what parts to use for replacement. There are 6 fets burnt and some small diodes or something that are right in front of the fets and burnt as well. Everything else appears to be okay to my untrained eye.

Anybody interested in giving me some direction?

Thanks in advance!!
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Old 3rd January 2018, 08:44 PM   #2
Perry Babin is offline Perry Babin  United States
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It sounds like you blew the power supply FET and the gate resistors burned. You also need to check the output transistors (IRF540s) to see if any are shorted.

I'd use the IRF3205 for the power supply FETs. The gate resistors are 47 ohms, 1206 size.
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Old 3rd January 2018, 09:35 PM   #3
ResonateEngineering is offline ResonateEngineering
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Thanks Perry was you da man!
Do you do any repair work on the side I am still debating on sending this out or attempting it myself.
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Old 3rd January 2018, 10:13 PM   #4
Perry Babin is offline Perry Babin  United States
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I'm not doing any repairs right now but when I'm working, I only do work for a local shop (nothing shipped to or dropped off at my shop).
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Old 3rd January 2018, 10:26 PM   #5
ResonateEngineering is offline ResonateEngineering
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Are there other members on here that you know of that work on amps? Also, where will I find the output transistors? Are they clamped yo the heat sink as well?
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Old 3rd January 2018, 10:35 PM   #6
ResonateEngineering is offline ResonateEngineering
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Does it tell you anything since the channels still had audio output (weak and distorted). Didn’t know if that was a clue or offered insight to someone like yourself with JL experience. Can the outputs short and still have audio output at speaker terminals? Thank you for your time, I can’t find any detailed information like this on the web...
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Old 3rd January 2018, 10:39 PM   #7
Perry Babin is offline Perry Babin  United States
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They are on the other end of the amp, on the heatsink. Did you remove the clamps?
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Old 3rd January 2018, 11:02 PM   #8
ResonateEngineering is offline ResonateEngineering
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I only took off the clamps on the 6 burnt fets so far. I’m hoping all the damage is centralized to those and the gate resistors.
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Old 3rd January 2018, 11:03 PM   #9
ResonateEngineering is offline ResonateEngineering
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Do you have a schematic of this particular unit you could share?
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Old 3rd January 2018, 11:12 PM   #10
Perry Babin is offline Perry Babin  United States
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JL doesn't release diagrams.

You have to check the outputs. If you repair the power supply and the outputs are shorted, the power supply will fail again.

Search this forum for both the 450/4 and the 500/1 to see how other repairs have gone and the tips for repairing those amps.
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