JL 300/2 with offset rail voltage - Page 2 - diyAudio
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Old 29th August 2013, 07:15 PM   #11
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Location: Louisiana
On which points are you getting 53v?
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Old 30th August 2013, 07:02 AM   #12
Randy62 is offline Randy62  United States
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Join Date: Dec 2011
Location: Cincinnati area
Its the voltage coming from the smaller power supply with the yellow transformer going to the last 2 pins on each driver board.
As with most amps the center tap on the secondary is referenced via the outer shield(ground) on the RCA input and isolated from the 12 volt input ground.But this amp has the 12 volt ground,secondary center tap,and the RCA preamp output shield all connected,and the RCA input shield is isolated.I assume it has balanced inputs.
So all voltages share the same ground regardless were I probe.

To me,it just seems that a shorted winding would cause major obvious problems.I mean,there should be some major current draw at idle even with no common ground RCA's plugged into it.

This is why I thought it was a bad cap somewhere on the rails,so I started pulling them.That didn't help.

The last time I worked on it I had the idle bias fully counterclockwise.As I started to increase the bias I could see the Negative rail voltage decrease just like the bias pot controlled the rail voltage.

Now Im thinking ok,maybe the Mosfets on the positive rail are open.But since the amp worked perfect till it got hot on 2 previous attempts I decided to isolate the power supply from the rails first.

That's when I checked the center pins on the rectifiers in AC mode.I get 0 volts on the positive rectifier and I see about 140 volts +/- 70 AC on the center pin of the negative rectifier.In DC mode the negative rectifier has a lot of ringing and if I push it hard against the heat sink with my finger the ringing almost disappears.Im sure it has a lot to do with the LCR circuit on it but the positive rectifier doesn't do that.And the more I increase the idle bias(which brings the voltage down)the more there is a dead spot like one of the diodes is open in the rectifier.Ive already replaced it 4 times prior to this test.Maybe I will change it one last time.
I don't know what else to do.I have been repairing audio gear for the last 30+ years,15 of which as my full time business.According to my records Ive done at least 4000 amps.
I wish I could put a different transformer in it,but of all the stacks of scrap amps I have nothing comes close to physically matching regardless of voltage.

Any and all suggestions/comments are welcome.
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Old 30th August 2013, 09:57 AM   #13
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The same windings feed both rectifiers so it's not likely to be a transformer related problem.

Are you replacing the negative rectifier from the same batch of parts? If so, try one from a different batch or even one pulled from a junk amp.
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Old 31st August 2013, 12:12 AM   #14
Randy62 is offline Randy62  United States
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Join Date: Dec 2011
Location: Cincinnati area
Good point.
All the rectifiers I installed were pulled from other amps.The original rectifier that shorted was defective and one of the others I replaced were defective(second batch).The last one I pulled before isolating the rails acted funny on one leg.It would start at .45 volts then go up to .7 then back down to .45 within about a second.So I put a different one in.I am not going to do anything with it for a few days.I got to backed up messing with it plus its labor day weekend.
I will put a new rectifier in it one more time before giving up on it.If it is frying rectifiers,what would cause this?
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Old 13th September 2013, 05:51 PM   #15
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Join Date: Feb 2010
Location: TAGANROG
too bad the problem is not solved. Have the same problem in JL audio450.4. When shorting a throttle voltage aligned)))) What is the reason can not understand. Choke tried rewinding does not work (((
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