Hi
I was hoping that someone could help me help a colleague of mine to find the fault on this bass amplifier. If it is not OK to ask a question like this, or if it should have been posted somewhere else, ´m sorry for that. I have not used my analog electronic skills much since school some 20 years ago, and seems to be a bit rusty.

The fuse F1 and F2(see the included schematics) blows instantly(one or both) when power is applied to the amplifier. What I have done so far is measuring the output of BR2 to be 170v when there is no load. The input to BR2 is 60V on both AC1A and AC2A measured to ground. Measuring between "output side" of F1 and F2 shows about 1ohm between them. My first thought was shorted D11/D12. Measuring D11 and D12 when they are mounted on the PCB shows them to be open both ways. When removing them they are not. So apparently there is something else that is shorting.

PROFET_51_52.pdf

 

I would suspect the mosfets are blown Ef1 to 10 on the output, check for shorts between Drain and Source..

 

Should be

I would suspect the mosfets are blown Ef1 to 10 on the output, check for shorts between Drain and Source..

Thank you for you sugestion. I will check that when I come back to work on monday

Also I started to get a bit confused about the correct word to use in english. -in my first post I say that the diode is open both ways. What I mean is that it lets current trough both ways, or that the circuit is closed. How should I say that the correct way?

 

Diodes conduct one way only, if its conducting both ways its faulty, test them out of circuit, note the fets also have diodes inside them.

 

I would suspect the mosfets are blown Ef1 to 10 on the output, check for shorts between Drain and Source..

Hi Dodgydave
All of the mosfets except two of the irf9640 where shorted. The two last ones seemed ok. Guess that they should be replaced as well. Is there anything else I should check ? I'm a bit afraid that they will all blow again Really appreciate the help I have gotten so far.

 

Take out the bad guys and make sure the drive line (collector Q16) is hanging around zero DC with no input signal. This is basically a huge op-amp so think in those terms. You might have to fake a feedback path to base of Q13 to get the amplifier to balance out at zero DC without its power transistors working.

 

Check the voltage at the junction of D9,10 it should be half of the supply.

 

Check the voltage at the junction of D9,10 it should be half of the supply.

That's a good place to look for zero DC but there is more to it. R88 forces the base of Q11 to be what is called "circuit ground" (and I called it zero DC). That means the base of Q13 must be zero DC for the amplifier to be in balance (give or take a few millivolts of mis-match between the transistors Q11,Q13). That stack of diodes (D9 D10 Z3 Z4) must have the zero voltage level surrounded by less than 7.4V to 7.5V in both directions. Looking for zero DC at the junction of D9,D10 might get you a zero because there are no output transistors left in operating condition. I'm saying: check the drive voltages before the gates of the output transistors to be sure (Q11,Q13,Q14,Q15,Q16) the input stages didn't go wonky and destroy the output transistors, and this is how:

Disconnect the speakers. Take out all the power output transistors. Alligator (connect) the gate driving ends of R107 and R108 to the output line. Measure for zero DC. If it is there, you are probably OK. You can add a small audio signal to the input (Pin6 of CN2B) and see if it shows up on the output line and centered around zero DC. If that works, you have it whipped. Don't forget that RV1 is there to adjust the idle current when there is no input signal and the output transistors are installed. I hope the method of adjustment is in your literature.

You are allowed to ask questions. I can't assume you know all of my jargon.