no output

[Jdixon]

I have an Ampeg BA210 SP that quit working. I took amp apart and found that the fuse was blown. Replaced fuse, tried to turn on but fuse popped again. Took amp to a shop, and they said the MOSFET was blown. The output. They want $200 to fix it, but i am a broke bass player. However i am kind of handy, and i think i could replace mosfet circuit, but i am not sure which circuit is the mosfet one. The shop said the mosfet was expensive, but $200. Nothing on the whole board looks burnt, i staired at the thing forever, and i cannot find anything that looks abnormal, but i am also no tech. Can anyone help me out?????

[Enzo]

SOmetimes parts burn up, but most bad parts don't look any different.

Your MOSFETs are the power transistors screwed to the heat sink inside. Two are IRFP9140 and two ar IRFP140.

Mouser sells IRFP9140PBF for $1.73 each (The PBF on the end just means "lead-free")

They also sell the IRFP140PBF for $4.47 each.

When the amp blows fuses, the most likely cause would indeed be those parts shorting out.


Having said that, it is quite possible other parts were damaged in the process, and if we just solder in new MOSFETs, they can just blow up the instant we turn on the power. This really does belong in the hands of someone who knows what he is doing. $200 sounds a bit steep to me, but I don't know where you are. If you are a block from Times Square, that sounds reasonable.

[PepeSilvia]

Enzo, what is more than likely the "other parts" that would be damaged that would cause the MOSFETs to blow up again? The high-power source resistors?

[J M Fahey]

*Many* others.
An open 5 cent 1/4W resistor which makes them overbiased can kill them in a jiffy.
That's why troubleshooting skills are needed for repair; if you replace 99% bad parts, itīs still not repaired.
Beyond parts themselves, a bad/cracked solder, bad/dirty connector, a trimmer pot wrongly set, all can kill it again.
So when somebody asks "which parts should I replace to repair it?" they are in fact asking only half the questions .... and remote control step by step answers are often impossible, so here you will usually read "guidelines" such as check bias or check for shorted transistors or the like, can't get more precise than that.
Sorry.

[Enzo]

A good rule of thumb is that when outputs blow, replace the drivers that drive them - even if they test "good." The check every resistor in that part of the circuit for damage. That open 5 cent resistor is the perfect example.