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Old 24th October 2013, 12:37 AM   #1
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Default JL Audio 500/1 Rev2

I have a JL Audio 500/1 Rev2 amp that when powered on it has a loud buzzing coming threw the outputs until I touch the RCA's ground. The RCA's are connected good and I found one transistor burned on what looks to be the PWM board. I'm not familiar with the design/layout of JL amps so I could be wrong on that.

The amp will produce audio fine unless I remove my finger on the RCA's.

I'd like to know the value of Q706 and if there is anything else that could go out with it.

Would that transistor cause a feedback like I am getting?

Here's a picture of the transistor:
Click the image to open in full size.

Here's the entire board for reference:
Click the image to open in full size.
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Old 25th October 2013, 01:42 AM   #2
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Q706 actually tested good when pulled. I figured out that it read 3B.

I ordered BC856B,215 to keep on hand in case I need them, but I probably should have went with MMBT918, right? I used The SMD Code Book to find the part number.

So now I am back to the same issue with no leads. All outputs and power supply FET's are good. What could it be?

Last edited by Zookz; 25th October 2013 at 01:47 AM.
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Old 25th October 2013, 04:37 AM   #3
kds9591 is offline kds9591  United States
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Sounds like you have a floating ground, possibly caused by poor pin contact from the top filter board to one of the main bottom board sockets. Use some contact cleaner on the sockets and re-seat the top board several times and see if that helps.
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Old 25th October 2013, 08:45 PM   #4
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The BC856 is probably the right part number. I doubt that that transistor or the associated circuit could cause the problem.

Have you tried operating all potentiometers and switches through their entire range to see if that made any difference?

I'd suggest clamping transistors to the heatsink if you're going to be doing any troubleshooting.
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Old 29th October 2013, 10:29 PM   #5
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I cleaned the contacts on all pins and there is a ground issue but I am just not sure what is causing it.

There is ~1.8Mohms between the ground terminal and the RCA's for the input. There are no open traces that I notice and I don't have a schematic to trace it back.

Adding a jumper wire between the ground and RCA's solve the issue but I'm still curious what the underlying problem is for my own knowledge. Should I just run a jumper wire to ground for the repair or add a small resistor to ground? Would that be a sufficient repair?
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Old 29th October 2013, 10:48 PM   #6
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A low frequency hum is generally caused by the 12v power supply. Are you sure that the 12v DC from the supply is clean?

Have you tried it on the car?

You cannot leave a wire jumper. It will cause noise when you install it in the vehicle if the signal source is powered by the vehicle's charging system (like a standard head unit).
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Old 30th October 2013, 12:37 AM   #7
kds9591 is offline kds9591  United States
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Quote:
Originally Posted by Zookz View Post
I cleaned the contacts on all pins and there is a ground issue but I am just not sure what is causing it.

There is ~1.8Mohms between the ground terminal and the RCA's for the input. There are no open traces that I notice and I don't have a schematic to trace it back.

Adding a jumper wire between the ground and RCA's solve the issue but I'm still curious what the underlying problem is for my own knowledge. Should I just run a jumper wire to ground for the repair or add a small resistor to ground? Would that be a sufficient repair?
You can check continuity to the amps power ground on the following connector pins on the top board.

Since the pins are not numbered on the connectors the top left pin will be referenced as pin one if you are facing the front of the amp. Top row will be the row towards the center of the amp. All slide switches pushed to the right on both boards.

The following connector pins should be connected to the amps power ground.

Connector P4- top left pin top row.

Connector P3- Pins 4, 5 top and bottom rows.

Connector P1- Pins 4,7 top row pins 3,4,7 bottom row.

If you don't have continuity between the amp power ground and pins 4,5 top row P3 you have a open on the bottom board.
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Old 30th October 2013, 01:37 AM   #8
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On a 500/1 I have here, I read 2.1M RCA shield to ground. If I reverse the leads, I read 2.5M ohms.
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