JL Audio 500/1 - low distorted output, melted LED wires

[craig469]

I bought a used JL slash amp off ebay, worked great for a couple months, now it powers up but the speaker output is very low (<1v) and seems distorted.

I dissasembled the amp and noticed that there is a small board (soldered vertical perpendicular to the main board) that has been removed or replaced previously(prior to my purchase). I also noticed that 2 or 3 of the wires going to the status LEDs on the top of the amp had some melted insulation and may have been pinched under the solder pins on that (vertical) board that I mentioned previously. The area around that vertical board smells a bit burnt too.

I'm a mechanical engineer with limited electronics experience sooo.... I've got a DVOM and maybe access to an oscilliscope. Any help on where to start would be greatly appreciated, thankyou!!!!

 

[Perry Babin]

With the amp on and the black probe on the amplifier's ground terminal, what is the DC voltage on the positive and negative speaker terminals?

 

[craig469]

speaker terminal voltages

with 12.3v dc power &
DVOM black lead connected to ground terminal:


input voltage set to high and input sens @ 1/3:
.85VDC on the + speaker terminals
.78VDC on the - speaker terminals

input voltage set to low and input sens @ 1/3:
.86VDC on the + spkr terminals
.79VDC on the - speaker terminals

Thank you so much, from what I've read you are an amp wizard!

 

[Perry Babin]

If the amp was powered up and the green LED was lit and you have less than 1v on the speaker terminals, the power supply is likely blown. Look at the resistors R600-R607. Do any look blackened?

 

[craig469]

Yes R600 - R607 are burnt.

q507 also looks like it burnt a spot on the heatsink.

 

[Perry Babin]

That means that all of the power supply FETs (Q600-Q607) are likely blown and Q506 and Q507 are likely blown.

All of those (and any others that are found to be defective) will need to be replaced.

 

[craig469]

Thankyou very much! If possible can you recommend any good sources to purchase the replacement components from (R600-R607 and Q506,507 and Q600-607)? Is there a part number for the resistors, or are they just a standard item (I think I read on the forum that they were 47ohms)?

 

[Perry Babin]

Digi-key and mouser are good distributors.

If this is a v1, the SMD resistors are 1206 size, 47 ohm. The originals are 1% tolerance.