JL 300/2 with offset rail voltage - diyAudio
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Old 23rd August 2013, 09:57 PM   #1
Randy62 is offline Randy62  United States
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Default JL 300/2 with offset rail voltage

This amp had a piece of wire inside it that shorted out the leads on the positive rectifier.I replaced both rectifiers and all 4 50v 2200uf caps.It powered up fine.I benched it at 171watts RMS on both channels with a 4 ohm resister load.Then I tested it bridged into a 4ohm resistor load and it did 342 watts RMS(37 volts) right before clipping.I reduced the power to about 25 watts (sine wave at 1khz) and let it run for about 5 minutes,then ran it back to full power.After about 5 seconds the negative wave started clipping at about 50 watts.

Now I have +67volts and -21volts on the rails.There is a +DC voltage and a pulsed negative on the outputs of the rectifiers.So I thought the negative rectifier or new caps was at fault so replaced both rectfiers and all 4 caps again.
The amp tested good again.Until it got to about 140 degrees F,then it did the same thing again.+67 and - 21 volts with the negative half showing a pulse and the positive rectified at 67 before the coils.Rail voltage is still +67 and - 21.I replaced the parts again but this time it didn't help.

This would normally make me suspect a shorted winding in the transformer but I have good +/-53 volts and +/-15 volts in the rest of the amp.

I have been working on this thing for 8 days now.I have pulled every part on the rails but the driver boards and outputs to no avail and used about 15 feet of braid already.Im afraid that all the parts mounted to the heat sink are going to start breaking off the board from all the flipping over and over.

I am baffled at this point.Anybody got any ideas what I am missing?
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Old 23rd August 2013, 10:11 PM   #2
Randy62 is offline Randy62  United States
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Oh,I forgot to mention,the amp is only pulling about 2 amps at idle and if I turn the idle bias on ether channel it will cause the rail voltage to only change by a couple of volts but its still no were near normal.
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Old 23rd August 2013, 10:31 PM   #3
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The inductors tend to short to each other and often have broken leads at the board.

Do you see the same amplitude square wave on the input to all legs on both rectifiers?
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Old 23rd August 2013, 11:04 PM   #4
Randy62 is offline Randy62  United States
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Yes.The wave is the same.Its running at a very low duty cycle but they are both symmetrical (about 70 volts to center,140 P-P).Im glad you mentioned the inductors.They are separated by a black rubber silicone,but on a hunch I bypassed the one with the low voltage and as soon as I powered it up the amp pulled 20 amps.I removed the jumper and then pulled all the caps(little yellow ones) I could find on the neg rail.No help.
I tried to put a resistor on the 12 volt power wire to trouble shoot it with the inductor jumped but these JL's wont even fire up like that.I even took all the diodes out of the circuit on the small yellow transformer but all that did was burn out the fqp65no6 mosfet and the .1 ohm white resistor.
I pulled the inductors out and checked them over.Couldnt find anything wrong with them.

Last edited by Randy62; 23rd August 2013 at 11:30 PM. Reason: Add
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Old 24th August 2013, 04:10 AM   #5
Randy62 is offline Randy62  United States
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Ok,Since you referred me back to the inductors I pulled them back out.I had epoxied them in,but got them back out fairly easy.I yanked a pair of output filter inductors from a Memphis 1000d and put them in place of the originals.They are only .3mh compared to the 3.6mh of the others but now I get +67volts and -62volts.
Much closer.There is a small voltage spike on the - rail compared to the + one.
I am now concerned about the 50 volt rating of the caps.Im not sure if that is the right ones.Someone before me replaced the 2 caps on the + rail but the - ones looked original.The negative power supply pulse is now gone,its more DC except for the spike.Im not really comfortable running it long like this or connecting speakers yet.
The idle current has dropped to about 1 amp now.

Any thoughts?
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Old 24th August 2013, 05:14 AM   #6
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What is the DC voltage measured directly across the rail caps?
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Old 24th August 2013, 05:37 AM   #7
Randy62 is offline Randy62  United States
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The readings Ive been referring to are on my scope,but with a meter I get +65.8 and -62.1.Across both I get 127.6.
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Old 24th August 2013, 05:42 AM   #8
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Is that on the rectifiers or on the filter caps?
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Old 24th August 2013, 05:56 AM   #9
Randy62 is offline Randy62  United States
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That's at the caps.However when I swapped out the inductors I removed the 4 50 volt caps and put in a single 50volt 1000uf on each rail.I did this to eliminate the caps being the problem,easier to replace the inductors and temporary testing.
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Old 29th August 2013, 12:18 AM   #10
Randy62 is offline Randy62  United States
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Well,I changed the caps to 63 volt Elna's,replace the inductors with similar ones and replaced the negative rectifier.I basically got the same results.This time I got a real bad ripple(ringing) on the negative rectifier.With the scope set on AC only,the trace is dead center on the positive rectifierl(after it settles)and I get +/- switching on the negative rectifier.So I removed the amp section from the circuit with just the caps,inductors and rectifiers on the transformer with a small resistive load and still have the same results.So I believe its the transformer even though I have a good +/-53 and +/- 15volts everywhere else on the board.
I guess its done for.
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