Swaping preamp boards on JL amps - diyAudio
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Old 10th January 2009, 08:28 AM   #1
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Default Swaping preamp boards on JL amps

Hi, I accidentally burned one of the preamp boards (with leds on it) on the JL 300/2. I don't have time to find replacement components, I found another JL 300/4 that is willing to donate one of its daughters. After I swap the boars do I have to adjust anything?
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Old 10th January 2009, 06:21 PM   #2
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You should check your bias to the outputs. Other then that I am fairly sure you will be OK. There is a two pin jumper style plug on the board for each channel a simple comparison with a voltmeter is a good start. I like to O-scope the outputs on all channels just for good measure keeping a eye out for crossover notch distortion.

And unless your good to excellent at de-soldering the daughter board will be difficult to remove, and you could damage the main board thru-holes if your not careful. Its simpler to just cut out the old board then remove the header pins one by one safely. I cut the right angle as the pins bend over and go into the daughter board. This will ease the pin removal and reduce your chances of damaging the thru-holes on the double side main board. I have seen many boards damaged by others and its not a pretty sight, and it can pretty much trash the amp if your not careful....That's about it, Good luck on your project....
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Old 11th January 2009, 02:16 AM   #3
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ChipQuik ...

The best thing since sliced bread.

You'll never damage another via (almost guaranteed).

If you use it, apply the flux liberally and use a length of the CQ alloy about 3/4 the length of the row of pins. Run the iron with the CQ along the pins to distribute it evenly. After it's distributed, place your iron's tip on top of each header pin and gently move the pin around in the through hole. This will work the alloy down into the via.

Run the iron back and forth along the pins (without EVER touching the board or pads). This will heat all of the pins at once. With a relatively hot iron (~750F), the pins will pull free with essentially no force after about 6-10 pass of the iron along the pins.

If you can't quite get them all hot enough at once, use a heat gun to preheat the board. This will help prevent board from soaking so much heat from the pins.

After using the alloy, you must remove as much of it as possible. Desolder the pads and pins. Add new solder and desolder once again.

I agree, the bias will probably need to be adjusted. Since JL doesn't publish the recommended voltage across the test points, measure the voltage on the other 3 channels and set the 4th channel to match. Don't try to measure from the top of the board, there's too great a risk of shorting to the busbars. Measure the voltage from under the board.
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