JL Audio 500/5 repair advice, please

[wjyoung]

Hello All,
I have a 500/5 that had a bad channel because one of the bias transistors (Q202 and Q203) shorted out. I replaced both with IRF540's that I sourced from ebay. I made sure not to use IRF540N or IRF540Z parts.

When I hooked the amp back up in the car, several transistors (IRFZ44R) burned up, unfortunately. Google tells me these are N-channel MOSFETs, so I'm guessing they're output transistors.

Can anyone tell me what I might have overlooked? Any idea why those transistors burned out? I don't see any obvious shorts from the board(s) to the chassis. The speakers all read 4-6 ohms. I did run a brief test at home, but I only tested two channels at a time into 8 ohm speakers.

I'd rather learn how to fix this than send it in for repair, so any advice would be greatly appreciated. I'm ok with ordering parts and replacing components, but I'd like to avoid having to do it over and over again.

Thanks in advance,
Wayne.

[timwebb]

its a few threads about these amps that might help u

[Perry Babin]

Q201 is the bias transistor for Q202 and Q203.

The 540s are the output transistors.

The Z44s are the power supply transistors. You'll need to replace all of the power supply transistors and probably all of their gate resistors in the power supply that failed (there are multiple power supplies). Which of the Z44s failed?

Do NOT buy semiconductors from eBay. There are too many fakes and too few legitimate sellers. Buy from a reputable distributor like Digi-Key or Mouser.

Replace the 540s that you purchased from eBay with those from one of the distributors I listed.

You may have had the bias set too high in the channel that you replaced the parts in.

How long did the amp work in the vehicle after you reinstalled it?

What were you using for a power supply on the work bench?

[wjyoung]

Originally Posted by Perry Babin

Q201 is the bias transistor for Q202 and Q203.

The 540s are the output transistors.

Thanks for clarifying, that makes more sense. I couldn't imagine why the bias transistors need to be so hefty.

Originally Posted by Perry Babin

The Z44s are the power supply transistors. You'll need to replace all of the power supply transistors and probably all of their gate resistors in the power supply that failed (there are multiple power supplies). Which of the Z44s failed?

Q904 and 906 completely burned out. The ones around them (Q903, Q908, Q907) are covered in oily soot. As are the nearby components on the motherboard.

Originally Posted by Perry Babin

Do NOT buy semiconductors from eBay. There are too many fakes and too few legitimate sellers. Buy from a reputable distributor like Digi-Key or Mouser.

Replace the 540s that you purchased from eBay with those from one of the distributors I listed.

Ok, lesson learned, will do.

Originally Posted by Perry Babin

You may have had the bias set too high in the channel that you replaced the parts in.

Not sure how that could have happened, unless it was the act of installing the ebay 540's that did it?

Originally Posted by Perry Babin

How long did the amp work in the vehicle after you reinstalled it?

What were you using for a power supply on the work bench?

I started with a 0-30V, 0-3A current limiting power supply I built as student long ago. It was set at 12V of course. When it seemed to be unable to supply enough current to the amp, I switched to a computer power supply which I had converted for bench use. The amp seemed to run fine with that - no funny smells. I only ran it for a few minutes though, so maybe not long enough for problems to develop.

In the car, I connected the front speakers first, tested them for about 15 seconds, then proceeded to the rear speakers. That's when a fuse blew. Figuring that it was an old fuse, I replaced it. Instead of the fuse blowing, the amp started to smoke. Guess I should have taken the hint and stopped when the stopping was good.

Thanks again for taking an interest, and for the good advice.

[Perry Babin]

When you get the amp repaired and reinstalled, use nothing larger than a 20 amp fuse and confirm that the amp is working perfectly at low to moderate volume before going back to the normal fuse. The fuse that's recommended offers essentially no protection for the amp. This applies to many of the amplifiers on the market.

If I'm not mistaken, the FETs that failed are for the sub and rear channels. The FETs you replaced are for the front channels. One should not have affected the other unless both supplies failed.

Go back and check every transistor on the heatsink. Which ones have failed (you don't have to list the 900 series FETs that you already found)?

[wjyoung]

Originally Posted by Perry Babin

Go back and check every transistor on the heatsink. Which ones have failed (you don't have to list the 900 series FETs that you already found)?

Ok, I put my multimeter in continuity mode and a probe on each of the outer legs while the transistors are still on the board. Of the transistors along the heatsink, Q302 is the only one that appears to be shorted out.

Not sure if this is the correct way to do things, please let me know if there's a better way.

Thanks again for your interest and rapid replies!

Cheers,
Wayne.

[Perry Babin]

That's certainly not definitive but it could be enough to cause the amp to fail. Q302 and Q303 will have to be replaced also.

Did you check the other Z44s to see if any were shorted?

[wjyoung]

Originally Posted by Perry Babin

That's certainly not definitive but it could be enough to cause the amp to fail. Q302 and Q303 will have to be replaced also.

Did you check the other Z44s to see if any were shorted?

Yes, the other Z44s aren't shorted out. I even checked the little guys, Q409, 421, 420, 411, 419, 407, 404.

I had a look on Digikey and Mouser for Z44Rs and 540s. Of the two, it seems Digikey is most willing to deal in small quantities.

Finding a replacement for the 540s is not straightforward. The original HEXFET part has been discontinued by IRF, and other manufactures have created variations of it. I'm going to have a closer look at the datasheets, but can you tell me which parameters I should try hardest to match up?

Thanks again,
Wayne.

[wjyoung]

..let's see if this uploads.

[Perry Babin]

I don't think these amps are as sensitive as others for the output transistors. If you use the same part (with the same suffix -- ignoring the 'pbf' that may be part of the suffix), they should work.