Ampeg BA blowing xistors

[kjkovarik]

A friend of mine has this Ampeg BA115 that is blowing line fuses, and I offered to fix it. I changed out both MOSFET's and the fuse, turned the power on, and the fuse blew. Checking the FET's in the circuit with a DMM revealed that one was shorted. All the zeners check good, and the transistors look good. All this was done with the speaker disconnected. I would appreciate any help or direction.

[stokes]

Although I am not very experienced at solid state circuits,I dont think it is a good idea to run it with the speakers disconnected.When you change the shorted component connect the speakers before testing it.

[Steve Conner]

If the output has gone to DC, that'll just trash the speaker too, so I'd not recommend it. Solid-state amps are supposed to be OK to test with no speaker, unlike tube amps. However, these circuits can be difficult to troubleshoot without experience.

The MOSFETs that Ampeg use (AFAIK) are pretty sensitive to biasing, and vary a lot between batches. Maybe the bias setting that was right for the old set causes the new set to "red plate" There should be a bias pot on the power amp board, and you should turn it to the minimum setting before testing with the new FETs, and then adjust it with the power on to get the manufacturer's recommended idle current. If you don't know, 100mA seems to be a good starting point. It may increase with temperature so recheck once the amp has warmed up.

[Mark Black]

Most transistor output stages are perfectly stable into no load and that would be my preferred initial check condidtion. Also I strongly recommend you build a "light bulb limiter" to help save those expensive new components and your stock of fuses. A decent explanation of how to build one is contained here:

http://www.fenderforum.com/forum.htm...-11-0505:27:17

Some Ampeg MOSFET basics have been covered before, although not for your specific model. Check out these threads with particular attention to Enzo's replies.

http://music-electronics-forum.com/s...ead.php?t=1078

http://music-electronics-forum.com/showthread.php?t=946

As he says in his comments there can be (and in my experience usually are) other components which have gone bad or been stressed by the initial failure. I've even found driver transistors which checked good in-circuit with simple testing but revealed themselves to have zero beta when checked out-of-circuit on a transistor tester. That's pretty rare (they usually dead short), but it happened to me and cost me a lot of time figuring it out.

These days I usually just replace most of the transistors in the amplifier circuit as a precaution after a power transistor failure in an Ampeg solid-state. Like Enzo says - they're cheap...

[Enzo]

Thanks.

And do not assume it is only the transistors. Make sure the rectifiers are OK in the power supply. and the unlikely but possible filter cap short.