I'm looking to repair an amp for a buddy,
All I know about it is that it's a Crate, 50 watt bass amp. When played, it apparently "farts". When he uses headphones, it's fine though.
He reckons it's a problem with the transformer, and that it needs to be replaced.
I'm good with a soldering iron, and used to repairing things, including replacing components in amps, so I am pretty confident in my ability to perform the repair, but I'm just unsure how to go about properly diagnosing the problem. A lot of the other stuff I have done has been replacing pots, capacitors, jacks, switches etc that I knew were problematic so there was no diagnostic work in that.
Any help that people could provide, with that limited information, would be greatly appreciated. Even a quick checklist for how to diagnose problems would be cool if anyone was willing to share, because I would definitely like to improve in this kind of thing for future reference.

Thanks a Million.
Blame it on the dog.

Nah, but seriously if you've got the amp on you surely you could get a better idea of what the actual issue is? As far as I know, farting means driving the speaker too hard, which would make sense given that it doesn't happen when using headphones, but isn't necessarily a fault with the amp.
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I'll get the amp over the weekend.. For now I'm just trying to speculate and gather theories and ideas on how to properly find the issue. Apparently it only started suddenly enough, and had been fine beforehand. I'll ask if he plays it maxed out or anything.
I assume if the speaker being overdriven is the issue, I would have to replace the speaker or something? Assuming of course he doesn't run a load of stuff in front of the amp to make it happen.
Usually when a bass player complains that his amp is "farting out" it means that his speaker isn't able to handle whatever the amp is putting out. You can usually put that down to the speaker itself, the amount of power he's using, or the cabinet design.

Entirely possible that he has a whole different definition of what that means to him, however...
Could be a mismatched port, too small of a cab, a torn speaker, a loose badfleboard, or too much bass/volume. If it is actually the transformer i would be interested in hearing about that.
I'll come back with more details when I can, just waiting for him to get back to me at the moment..