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Old 29th November 2006, 04:02 AM   #1
Thack is offline Thack  United States
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Default Problems with Carvin PB500 Amp

I'm having problems with this amp. I replaced the main MOSFETs with IRFP240/IRFP9240. I found that I don't have -15v past R108. The resistor is hot to the touch and loses -5v. At R58 and R8 It's down to -10v and past them it's down to -5v. So what is draining my voltage. Could Q1 and Q51 be the problem? This site wont let me upload the schematic so I have to show the link instead.



http://www.carvinmuseum.com/pdf/proa...owermodule.pdf
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Old 30th November 2006, 12:33 AM   #2
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First,you have to disconnect the DC15V rail from the load(ie the power amp and the protection).See the voltage go normal or not.
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Old 30th November 2006, 01:59 AM   #3
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Yes, when I take the 15v leads(QC3 & QC4) from the transformer off the board they meausure 15v but it is alt. voltage. The diodes d101 thru d108 make the voltage DC. The -v drops -5v right after R108 and it gets hot to the touch. could P101 be the problem. It looks like it balances the voltage to the IC 102A (Realy Threshold)
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Old 1st December 2006, 01:21 AM   #4
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No.That's the wrong way to tackle the problem.You should disconnect H1-3,+15V,H1-4,-15V,H1-5 from the power amp section and protection circuitry.
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Old 1st December 2006, 02:44 AM   #5
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Quote:
You should disconnect H1-3,+15V,H1-4,-15V,H1-5 from the power amp section and protection circuitry.
Those are all on a connector that's 10 pins long. I can't isolate individual pins. Should I take the whole harness off? H1-5 & H1-3 do measure 15V.
I took off the whole harness and the voltage drops are still there. There must be a circuit or semiconductor that is draining the voltage. Like I said earlier there is a 5v drop on the negative side thru R108 and it's only a 22 ohm resistor. Something downstream of the -15V rail is draining the voltage. Any ideas?
I called up Carvin and they won't fix it. They said it's too hard to get parts considering it's 15 years old.
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Old 1st December 2006, 01:59 PM   #6
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Ok.Definitely problem with the loads.Try to desolder all the OPAMPs from the board and check the voltages again.If things getting normal,then 1 of the OPAMPs are damaged.If not,maybe problem with the bias stages.
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Old 1st December 2006, 10:08 PM   #7
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thanks for your help. Is there a way I can check the chip with an volt meter and chip specs to determine if a ground is present where it shouldn't be? Those chips are surface mounts and look difficult to remove.
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Old 2nd December 2006, 12:33 AM   #8
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Because something is 'loading' the voltage rail and don't know who is the culprit.All the suspected components should be removed to determine which is the problem here.
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Old 5th December 2006, 02:36 PM   #9
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Update: I just realized my output voltage at the speaker output is maxed out at +70V. Somehow my input stage is getting voltage to turn on the mosfets. I also measured +40 at A2 IC pin #2. The max input voltage for that chip is +/-18V. So I have some problems. I suspect a diode somewhere has failed and is letting some of the high rail voltage (70V) get to my input stage.
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Old 6th December 2006, 12:34 AM   #10
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Possibly the MPSW42/92s already shorted.You can measure them with a diode test function on a DMM.
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