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PQatPIT 19th February 2009 03:35 PM

JL Audio 300/4
 
Hi! Problems repairing JL 300/4 rev 1.

Right rear channel had shorted positive side fet IRF540 (Q400) and a bit toasted 0R1 5W resistor (R412). After replacing these there is dc voltage creeping up to -6.5 volts or so, then I shut it down. Do not know how far it would have gone, thought that it would be a lot too much anyway.

Driver boards have three LEDs, they all light up in other boards but two leds that are just below JL Audio text do not on this board (U402). Did check incoming voltages on pins 1 (+44.5V) and 2 (-44.5V), they are ok on all boards just like schema tells us. Also found that dc voltage on input pin 6 is creeping to -6V and keeps going on. No dc on other boards on these pins. From schematic I get that this dc voltage could come from output since this input is linked to amp output via 10k resistor (R416).

Could it be that negative side fet IRF540 (Q401) really is damaged, but didn't show anything odd when measured with DMM?


Thank you in advance!

jol50 19th February 2009 04:10 PM

I always do the whole channel, have had them be bad and not show. Also check drivers if it has them. You could always swap parts from a good channel to verify if you don't have new.

Perry Babin 19th February 2009 11:52 PM

The first thing you should do is to resolder the connections on the large 47k ohm resistors (marked 473).

If that doesn't work, check the two 10 ohm resistors above pins 1 and 2 of the driver board.

PQatPIT 4th March 2009 12:10 PM

Quote:

Originally posted by Perry Babin
The first thing you should do is to resolder the connections on the large 47k ohm resistors (marked 473).
That was the trick, now it plays ok. I found that bias voltage at test points is about 3.8 mV to 4.5 mV when sound is good.

It was kinda hard to resolder those resistors, it was ok only after I sucked old solder away and soldered some new in. Of course I did it to all other channels, too.


BIG Thank You for Perry!

Perry Babin 4th March 2009 01:34 PM

Any time you have a bad solder connection, you should add new solder (or flux), desolder completely and resolder the connection. If you only try to resolder the connection, you may leave a layer of oxidized solder in the connection which will make it fail again.

I don't see many of these but if I remember correctly, 4mv is a common setting.

PQatPIT 4th March 2009 03:06 PM

Quote:

Originally posted by Perry Babin
[B]Any time you have a bad solder connection, you should add new solder (or flux), desolder completely and resolder the connection./B]
Yep, this is what I usually do.

This time it didn't even try to flow, and old solder with some new with a bit flux added was just heating and nothing more. I have soldered many things in my years but this was my first time when it was like this bad.

Well, happy end now. Thanks!


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