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when FETs go bad


 
3/18/2006 8:44 PM
Patrick Smith
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when FETs go bad
Hey guys, i've done a lot of work on tube amps and know a lotta the tell tale signs when things go wrong, but I have a solid state Evans amp that has started acting up, kinda being a troubled child here lately :)  
 
1.) the input jacks and effects loop jacks were crappy and needed to be replaced...so i did that and everything was fine.  
 
2.) had a cracked solder joint, the amp would cut out and you could bang on the top and make it come back.....redid the solder joints on the main board and everything was fine  
 
THEN a few weeks later you could hit a note, let it sustain and get this staticy sound....last time i heard that (before i started working on amps my self) i took it to the tech and he said it was one of the FETs.....so that was my first thought, a bad FET. Definitely wasn't a solder joint, that problem was fixed.  
 
Here's the new twist. The amp now has a bad buzzzzzzzzz to it. Doesn't sound like a ground hummmmm but more of a buzzzzzzzz. My question is this, if an FET is going bad/gone bad would it cause a buzzzzzzzzzz like that? Never encountered this in any of my tube amps and don't know all that much about FETs and their problems/symptoms etc. I have a bunch of them on order from Mouser and at $2.88 each it will obviously be no problem to replace them all. Just curious if any of you have dealt with problem child solid state amps lately  
 
thanks in advance for any help  
 
PMS
 
3/19/2006 5:05 AM
Patrick Smith
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Wasn't the FETs. The amp in question is the Evans with an SOS in the thread above :)  
 
PMS
 
3/19/2006 3:42 PM
KB
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I think I worked on one of those that a Steel player brought in. Aren't they made in Shreveport,LA ? I fixed the input jack on this one. Fet's or jfets and Bi-polars in the low voltage small signal TO-92 package don't fail very often unless they get zapped by high voltage. Whether they are used as switches or ampflifiers they still test the same and have junctions or diode drops. Pots can cause the buzz if they loose their ground but usually hum. That input jack can also cause it so make sure it's ok. A loose solder joint or connection can do it also as well as the output transistors.  
 
KB
 
3/19/2006 6:33 PM
Patrick Smith
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"Aren't they made in Shreveport,LA ?"  
 
Indeed they are! and S'port is where this amp (and me) is/are. I'm going back in tonight, I'll let you know what i find.  
 
Thanks again  
 
PMS
 
3/20/2006 4:01 AM
Patrick Smith
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actually the correct term would be "were" made in Shreveport, now made in NC i think
 
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