JL 300/4 - First Amp Repair

[greg_gorrell]

Hey, this is my first post on the forum but I've been around for a little while reading and learning quite a bit, especially from Perry and his tutorials. I have been getting interested in electrical circuits lately, especially with car audio but never had the desire to tear my fully functional amps apart until now. My amp to my components went into protect mode so I figured I'd fix it myself. Its a 300/4 from JL and I'm only using the two front channels so the bad channel wasn't hard to find. I only replaced the fets and after hooking it back up, they took a crap again after 20 minutes or so of playing music. I'm gonna be replacing those again today, along with some resistors. I'm also gonna resolder the driver resistors, check zener diodes on the driver board, and also replace the 5 watt .1 ohm resistor. Is there anything else I should do to make sure these would go on me again? What even causes these JL amps to go through output fets so much? Thanks in advance guys.

[Dimon74]

Probably you have to adjust BIAS on that channel where you changed mosfets.There are four test points for BIAS.Check BIAS on working channel (Measure it w/o speaker connected and no input signal applyed) then adjust it to same voltage on that channel.

[Perry Babin]

If all of the LEDs are lit on the driver boards and it was producing clean audio, the driver board components are likely OK.

The solder connections on the resistors marked 473 (on the driver board) should be resoldered (actually, add new solder, desolder and resolder). The failure of the solder connections on those resistors generally causes either distorted output or the amp will go into protection.

I've never seen the large 0.1 ohm resistors fail. They're likely OK. You can check them after you remove the shorted FETs.

When you reassembled it, did you remove the old heatsink compound and replace it with new compound? When you add new compound, only apply it to the transistors. Don't apply too much (no more than on the transistors in the attached image). When the transistors are clamped, make sure that the compound is displaced evenly around the entire perimeter of the transistor.

When you adjust the bias, you'll either need to insulate your probes so they don't short on the busbars or you'll have to touch the probes to the test points on the bottom side of the board.

When adjusting the bias current, have the transistors clamped to the heatsink and have a 10 amp fuse in the B+ line. Turn the bias pot slowly.

[greg_gorrell]

Ill be going to work on this right now... thanks for the help.

[greg_gorrell]

I got the two 47k ohm driver resistors resoldered and the old output fets out today and it went a lot easier than the last time. While I have a thread going I would like to clear a few things up. To adjust the bias im going to turn the pot by the driver board and check the reading where the two holes are by the fets labeled bias, right? As I understand, this is changing the resistance of the signal going to the fets. Also, where can I find some more information on how the path the signal takes through the amp? From your tutorial Perry, I have read that after leaving the voltage amplifier the signal would go to the driver transistors, followed by the output transistors before the speakers. Im lost on how it gets here from the transformers. Other than this, you have a great intro to repair and I plan on gettting your full tutorial eventually as im in the middle of buying a house and my job is becoming more uncertain as the days go on. But thanks for all the help so far.

[Perry Babin]

I got the two 47k ohm driver resistors resoldered

**** 2 per driver board?

and the old output fets out today and it went a lot easier than the last time.

**** They'll drop out easily if you add enough new solder to allow the soldering iron to heat all 3 terminals when you lay the tip of the iron across them.

While I have a thread going I would like to clear a few things up. To adjust the bias im going to turn the pot by the driver board and check the reading where the two holes are by the fets labeled bias, right?

**** Yes. This file shows the relationship of output to driver board.
http://www.bcae1.com/temp/jlaudio300...doutputs01.swf

As I understand, this is changing the resistance of the signal going to the fets.

**** No transistor switches on the instant that a tiny fraction of a volt is applied to it. This type of FET typically takes just over 3v to start to pass current. If the outputs aren't at (or very near) that threshold (~3v), the drive circuit will have to be infinitely fast to swing from 0v to 3v to prevent distortion. The bias sets the voltage so that the outputs are just beginning to conduct. The voltage across the test points is the voltage from one output's source to the other output's drain. It's probably ~4mv.


Also, where can I find some more information on how the path the signal takes through the amp? From your tutorial Perry, I have read that after leaving the voltage amplifier the signal would go to the driver transistors, followed by the output transistors before the speakers.

**** It varies from amp to amp but that's essentially correct, preamp level in >> differential amplifier >> voltage amplifier stage >> drivers and then outputs.

Im lost on how it gets here from the transformers.

**** Audio doesn't pass through transformers on most car amps.

[Dimon74]

Hi Perry.About LED lights ,you is not right.I had repaired two amps 500/5 and 300/4 you know drivers boards are the same.After boards were done,two LED lights didn't work,but the board itself was working fine.Probably SMD LEDs were broken

[Perry Babin]

I know I'm a rookie here and shouldn't be posting. Thanks for the heads up.

[Dimon74]

Sorry if I said something wrong

[Perry Babin]

If you re-read the post, I said that the driver board was likely OK if the LEDS were lit. How was that wrong?