Problems with Carvin PB500 Amp

[Thack]

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/proaudio/500wstereopowermodule.pdf

 

[Leolabs]

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]

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]

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]

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]

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]

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]

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]

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]

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

 

[Thack]

I'm just going to replace all of those transistors. There are so many back circuits on this board. THey don't test shorted but they do allow current in both directions, like i said could be back circuits. Capacitor C3 and C53 look like they are bulging a little. Why are those caps so big for an input signal of only 3Vrms max?
I'm going to replace those too along with the IC op amps.