Ampeg B200R Blowing fuses

[jpfx88]

I have a ampeg b200r that i am working on and it is now just buzzing when cut on then directly blowing the fuse, also the current limiter on the negative coming from wall is burnt. I am kind of in a panic trying to figure this out so any help/suggestions would be much appreciated.

thanks

[jpfx88]

just to add to this, I was adjusting the bias trimpot too while i was testing. I realize i shouldnt have done that. you think i blew some output transitors and whats the best way to find out?

[The Dude]

1) Unhook the speaker so as not to do further damage.
2) Build a light bulb limiter so that you can test the amp without further damage (search forums for "light bulb limiter").
3) Measure the output and report to us if the voltage is primarily AC or DC.

You likely have shorted output transistors since it's blowing fuses, so you can start by checking the output transistors for shorts.

[jpfx88]

Should I replace the the stock inrush current limiter or bypass it before I run the bulb in place if fuse?

[The Dude]

Your problem should be apparent either way. I would do a quick test of the output transistors before firing it up again anyway making sure there are no obvious shorts. It's easy enough to do.

[J M Fahey]

And the lamp bulb limiter is an external device with a current limited outlet where you plug your amp into.

No need to mess with its internal inrush limiter, fuse, anything.

Light Bulb Limiter

[jpfx88]

so i tried checking the mosfets with the diode setting while still in circuit, they are giving me little to no reading both ways. you guys think im getting a good enough reading like that or should i go ahead and desolder all of them to make sure?

[52 Bill]

so i tried checking the mosfets with the diode setting while still in circuit, they are giving me little to no reading both ways. you guys think im getting a good enough reading like that or should i go ahead and desolder all of them to make sure?

You need to be clearer in your communications here. What do you mean by "little to no reading both ways"?

Are you saying that you are getting a shorted reading or are you getting an open reading, or what?

[jpfx88]

well my multimeter shows "1" as an open reading and I am getting "002" which is the same reading as when i put the two leads together so i am assuming im getting a shorted reading.

[jpfx88]

all the other transistors in output section seem to be checking out fine just the mosfets giving me these readings. You guys agree that it seems like they are shorted?

[jpfx88]

update
I have desoldered 2 of the 4 mosfets just for starting my test, i then tried testing the mosfet. when negative is on source and positive is on drain my meter goes to to "500" then i bring the negative from source to gate then back to source it reads some like "56". Then when i bring the pos from drain to gate and back it shows "500" again. someone please help me with this, i am unfamiliar with mosfets and transistors.

The other one i tested shows "002" on pretty much any variation of how you place the leads, Shorted?

[Jazz P Bass]

It appears that the readings for the '500/56' mosfets show that the gate, when charged, turns the mosfet on (56 reading)
The '500 reading' shows that the drain & source are off.
Not too sure what meter you have.
Was it set for diode test?

The mosfets that show '002' are shorted.

FYI: Desolder and Test the Field Effect Transistors (MOSFET) in your amplifier | Decibel Car Audio the SPL Knowledgebase

[jpfx88]

I've heard your not suppose to use a desoldering iron to desolder these things but given I am going to replace all four and the fact that the board has been out a week you think it'll be fine?

[bsco]

The guy's here are experts....listen to them....it does sound like you have shorted mosfets...you could have other problems as well but you need to start eliminating things...take them out and check them....is there an insulator between the mosfet and the heat sink?? If so, take care not to damage them...and don't panic.....these guys here know what they are talking about...They have helped me out quite a few times here....
Cheers....

[The Dude]

I've heard your not suppose to use a desoldering iron to desolder these things but given I am going to replace all four and the fact that the board has been out a week you think it'll be fine?

I'm not sure where that came from. I use a desoldering station to remove most everything. Of course the same "rules" apply for any method of parts removal. Be careful not to put too much heat on a pad for too long and lift it from the board.

Now, assuming you have shorted FET's, which it seems you do. Also check all surrounding components- associated resistors that may have gone open, driver transistors that may have also shorted, diodes, etc. If you don't get everything sorted, you risk taking out your brand new FET's again. Be thorough!