Ampeg B200R Blowing fuses

[jpfx88]

I did replace all four output mosfets, before I did that my meter was showing a short on one of the mosfets, now it is not. The insulator seems good, whats a tell tell sign of a bad one? One thing I didn't do was put more thermal compound between heatsink and MOSFET though. When I am testing I am also testing with no load.

[bsco]

I did replace all four output mosfets, before I did that my meter was showing a short on one of the mosfets, now it is not. The insulator seems good, whats a tell tell sign of a bad one? One thing I didn't do was put more thermal compound between heatsink and MOSFET though. When I am testing I am also testing with no load.

you can test the mosfets with your DMM.... for N channel......connect the -probe to drain and the +probe to source.....note the reading.....now reverse the leads....note the reading...should be a low forward reading in one direction and OL in the other...........now go back to your low reading......touch the positive probe on the gate terminal and then return it to the source pin.....the reading should now be lower again.....then touch your finger on the gate and drain pins together....the reading will switch back to the original low reading. for P channel.....reverse the connections.....this is not a perfect test but will give you a fairly good indication if it is good or not.....

[jpfx88]

I'll double check them tonight but just assumed since all 4 are brand new now and not showing a dead short in them, they are fine. On another thing, the mica insulators look fine to me but what would show that they need to be replaced?

[bsco]

I'll double check them tonight but just assumed since all 4 are braotnd new now and not showing a dead short in them, they are fine. On another thing, the mica insulators look fine to me but what would show that they need to be replaced?

I got caught like that before....more than once I ended up with bad "Good"parts...now I test everything before I install it....there is a center hole in the insulator.....make sure that it is perfect.....no jagged edges or widening of the hole....also, make sure that none of the insulators are cracked or creased......when applying thermal compound, less is more...don't use too much...you only need a thin film.....if you still have the mosfets soldered in place and secured to the heat sink, do a resistance test between the center pin of the mosfet(drain) and the heat sink.If it shows short, lift that mosfet from the heat sink by removing the screw...if the short goes away, you have a bad insulator...let me know how you make out.......

[jpfx88]

alright, so i did check for shorts between the mosfets and the heatsink and everything checked out good. I guess I can now desolder them again to check to see if they are actual good parts. unless you guys have any suggestions as to where to go next. I appreciate all the help from you guys, im sorry for asking so many questions but im just fairly new at amp work in particular and the fact that this is not my amp im just being super cautious and trying to make the right moves.

[jpfx88]

think i should check the bridge rectifier?

[jpfx88]

If it was a snake, That bastard would've bitten the shit out of me. So I was really not wanting to disassemble this thing again so i got to thinking, maybe i should adjust the bias while i test it with light bulb to see if its just cranked up hot. Sure enough it needed adjusting back down, which tells me that me originally adjusting it is probably what blew out the mosfets anyway. Well, ya live and ya learn, now i get to go back to the amps original problem and see if replacing that capacitor a month ago fixed that.

[bsco]

like I had already said....I am no expert but if I was trying to repair this amp I would make sure that the new mosfets did not get damaged by another fault in the amp...that would be the first thing that I would do....I am sure that if somebody disagrees they will let us know....

P.S. didn't see your last post....check the speaker output jack now to make sure there is no DC voltage there....sounds like you are making progress....youi'll have this fixed up soon.....good learning experience isn't it....

[jpfx88]

So I checked it out with amp on, +probe on black/white wire and -probe on black wire, I am getting a dc draw. So it doesn't seem to be pushing any dc voltage just drawing.

[The Dude]

So I checked it out with amp on, +probe on black/white wire and -probe on black wire, I am getting a dc draw. So it doesn't seem to be pushing any dc voltage just drawing.

I'm not sure what you mean by that, so just to be clear.

1) No speaker hooked up
2) No signal to amp
3) Meter set to read DC voltage
4) Measure across speaker wires or jack

[jpfx88]

No speaker hooked up and no signal going in. I am testing on the dc voltage setting and getting a negative reading with -probe on black wire and +probe on black/white wire.

[g1]

What is the amount of negative DC?