Need help with amp repair.

[jeremy.flaugh]

Hello,

I came across a 12Ch amp for whole house audio. The amp has 6 boards with 2Ch each. 1 of the 12 Channels was not working just outputting 49Vdc. So far in this order I have replaced the main mosfet IRFI4212H then the mosfet driver IRS2092STRPBF and finally the Opamp MC33079DT. But I still get 49Vdc on the output the R channel does work I would like to get this amp up and running 100% I attached pictures of the board I would appreciate if anyone has any ideas I'm tired of throwing pars it this. I can say the parts are inexpensive the shipping is the killer.

The reason I replaced the opamp is because of this bulletin I found on the manufacturers website I thought the Opamp might have been overdriven. https://www.snapav.com/wcsstore/Exte...Tech_Notes.pdf

[Enzo]

Is the channel that puts out 49vDC getting the -49v for its other side out at the transistors?

[jeremy.flaugh]

Is the channel that puts out 49vDC getting the -49v for its other side out at the transistors?

It looks like it does if I took some measurements and edited and attached a pic. If I understand correctly the Mosfet switches the output to the speaker between - and + to make sound.
I did not realize how bad the solder job on the mosfet looks. It looks like chicken poop I plan on cleaning it up.

Thanks for your time.

[jeremy.flaugh]

I took some measurements all with the DC range selected.

[g1]

Aren't all the 5 other working boards the same as this one?
You can get all the correct voltages by measuring one of the working boards.

[jeremy.flaugh]

Just did that I think I found my problem.

I'm not sure why but this is the 2nd Mosfet "I ordered 2" in this board this one has a short between 3 and 5 and the causing the +42 to puke thru. I just tested the first replacement that have still laying around and pins 1,3 and 5 are all shorted which would explain why the amp blew the fuses the second I turned it on.

The original for some odd reason seams fine no shorts between 1.3 and 5 so it might go back in.

[glebert]

Did you buy the transistors on Ebay? Lots of counterfeit transistors coming out of China.

[jeremy.flaugh]

Did you buy the transistors on Ebay? Lots of counterfeit transistors coming out of China.

No digi-key lol! I feel like either I got a bad batch or I did something wrong on installation.

But that blows my mind that someone would counterfeit something that costs $1.50.

[Enzo]

People counterfeit things that cost 3 cents. You sell a million of them it still is real money.

When output transistors fail, they often are joined by driver transistors, resistors, diodes, and other parts in the immediate circuit. Also, people want to focus on "bad parts" ignoring the rest. A broken copper trace on the circuit board is the same as a bad resistor to the circuit.

[g1]

Fet's can be tricky to measure, under certain conditions at certain pins, a short is normal.
Again, I would compare with some on a working channel.

[jeremy.flaugh]

With the help of the datasheet
https://www.infineon.com/dgdl/irfi42...535623fc841f7a
And this video
https://www.youtube.com/watch?v=gloikp9t2dA

I was able to verify that the fets I replaced had shorts between s1/d1 and s2 I'm not sure why or how they got in that degraded state i'll test the new batch before installing. The original fet will not close between S1/D2 and D1 on the other side the fet behaves correctly.

The max input on the gate for these guys is +-20V so I measure to see if there is something blown ahead of the fet.

[The Dude]

Fet's can be tricky to measure, under certain conditions at certain pins, a short is normal.
Again, I would compare with some on a working channel.

^^^^^^ THAT.