JL Audio 500/5

[RockfordFIx]

I have a JL Audio that has came in for repair. The sub channel plays clean audio but the rear speaker are distorted and front speaker have no audio. With panel removed I see 4 driver boards. The 2 front boards have additional LEDS lit. There are no visible burn marks and amp is not in shutdown. Do these LEDS not being indicate driver board fault.

 

[Perry Babin]

All 3 LEDs should be lit. The first thing to try is to desolder and resolder the large 47k resistors.

 

[RockfordFIx]

Is this a common problem with these amps?

 

[Perry Babin]

Yes. It's likely the most common problem that don't involve blown outputs. You can see the crack that develops in the attached image.

There are other problems but this is the first thing to do.

If you don't desolder be fore resoldering, the problem will often reoccur because the oxide layer isn't completely removed.

If you have ChipQuik, use it to remove the driver boards if you remove them to do the work.

 

[RockfordFIx]

I removed the resistors one driver board resistors pads were coming off, so I installed some 1/4 watt resistors. The driver board to the left was okay I cleaned the pad and reinstalled them. When I powered the amp up all 4 driver boards were on and then the right back board started flickering

 

[RockfordFIx]

It’s definitely on the driver board. Going to remove all the transistors and clean the pads and test components outside of the circuit now

 

[RockfordFIx]

I confirmed the driver board by swapping position and the problem followed

 

[Perry Babin]

Ia the 47k resistor next to the 2 LEDs getting hot (normal)?

What is the DC voltage across each of the 2 LEDs?

 

[RockfordFIx]

I just found a 10ohm resistor open, all other components diode checked okay.

 

[Perry Babin]

Is it working now?

 

[RockfordFIx]

Yes the driver board is working correctly now. I did however short the outputs one channel. It over heated checking signal with no load and FETs weren’t clamped. I just repacked them and bias those outputs. During disassemble 2 of the FET clamps broke at the bend. I am assuming because it was 5c in my garage during disassemble. Do you know when I can flirt these at?

 

[RockfordFIx]

Her is a picture of the clamp and the driver board. The bottom 2 left resistors are 10 ohm. The far left one was open.

 

[Perry Babin]

Someone may have other options. I used to buy the 4525 aavid clips but they're hard to find now. Salvaging from other amps is another option. Be careful if you use clips from a different brand amp. They often look the same but don't hold as well.