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Mesa 400+ advice?

#21
Bad guitar cable.
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Its not the cable, I've tried many cables.

Bad input jack conection.
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Possibly, although I just made some sort of similar noise happen in the other channel. So it could be the input jacks, but it would have to be the exact same problem in both of them, which seems a bit less likely.

stop!
at this point you could be further damaging the amp.
unplug it and take it in to a tech, you've exhausted the "user serviceable" part of the program (unless you wanted to go out and buy all new tubes for it $$$
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Good advice. I've stopped. Also, I've tried replacing all the power tubes - had a backup set. Didn't do anything though.

I'm starting to think this must have something to do with the volume pots. Not something I can replace myself....but does anyone have advice for cleaning them before I shell out major bucks to ship this thing to a decent tech somewhere far away?

Thanks! I'm excited... feel like I'm starting to get somewhere!
 
#22
Naggon said:
WOAH!
Something really weird just happened.
I've been sitting here with the amp on and the tubes fired up for probably twenty minutes, and I just decided to plug in one last time just to test it. I plugged in my guitar instead of my bass, just because it happened to be closer to me, and my amp made some sound! It was really quiet (considering how loud it should have been - I had the channel 2 vol. at about 6 and the master at about 4), quite dirty, and just faded right down to nothing over about five seconds. Try as I might, I haven't been able to get it to make any sound again.

I'm totally stumped. Does anybody have any idea what this could mean???
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How old are the tubes? I think it's the tubes myself, but what do I know. Like I mentioned above, I'd clean out the sockets, inputs and pots while I'm at it.
 
#23
Like I mentioned above, I'd clean out the sockets, inputs and pots while I'm at it.
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I think this is exactly what I need to do. I've never cleaned pots on an amp before, and I dunno how comfortable I feel spraying some kinda cleaning solution up in my amp like these folks suggest doing to your passive bass: http://www.talkbass.com/forum/f38/how-do-i-clean-my-pots-824648/

Anybody know how you're supposed to clean sockets, inputs, and pots on an amp? I realize there's probably a thread for this already and I'm currently looking for it...
 
#25
If you can plug in a good preamped signal into the effects return and eliminate the preamp, that would be great. I don't know that a bass, specially a passive bass, would make anywhere near the required signal for the power amp to make a sound.

If you eliminate the preamp then the power amp main transformer is probably not feeding power to the output stage, since you swapped out the tubes already.

I don't know what harm could come from tubes not being supplied with the big voltage. I'm way too uninformed to be giving advice on this sort of stuff but the other guys seem to be doing family stuff at the moment.
 
#27
A bad or mis-seated preamp tube or oxidized/loose input or FX loop jack sounds like a big possibility given that you were able to coax a faded signal. With the jacks, even when using one input, the other is still active so oxidation could screw up the entire signal (ditto the FX loop, though IIRC it is a parallel loop on that amp and shouldn't matter as long as the FX volume is turned down). When you tried plugging directly into the FX return, did you make sure to turn up the FX volume control as well? I was able to get a decent signal on mine that way with a passive bass.
If there were a blown out component in the power amp I would think it would have caused a blown fuse as well at this point. Is the fuse the correct amperage? The guy that bought my 400+ a couple years ago received it and turned it on while forgetting to plug in his speaker cable and it blew the fuse immediately (fortunately for him!).

Could also be a bad pot or something else in the preamp chain (most likely). It seems you've tried most of the tricks at this point though apart from checking out the pre tubes and clening the input jacks with De-Oxit. Might be time to see a tech!
 

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