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Thack 29th November 2006 04:02 AM

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.

Leolabs 30th November 2006 12:33 AM

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.

Thack 30th November 2006 01:59 AM

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)

Leolabs 1st December 2006 01:21 AM

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.

Thack 1st December 2006 02:44 AM


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.

Leolabs 1st December 2006 01:59 PM

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.

Thack 1st December 2006 10:08 PM

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.

Leolabs 2nd December 2006 12:33 AM

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.

Thack 5th December 2006 02:36 PM

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.

Leolabs 6th December 2006 12:34 AM

Possibly the MPSW42/92s already shorted.You can measure them with a diode test function on a DMM.

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