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Evans amp help!

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3/18/2006 10:03 PM

Evans amp help!

I have an Evans amp (200 watts FET thing) that developed a buzzzzzzzzzzzzzzzzzzzzzz, pretty loud actually.

Here's a funny note about the buzz. When the amp is on and nothing is plugged into it, it's just fine, no buzz. put something in the effects loop and there it is. When you just plug into the amp with no effects loop it was still there but not as bad.

I replaced all 4 FETs to rule those out and it did the same thing. THEN i flipped the power switch (goes 2 ways for grounding) back and forth and it would pop. I was thinking hmmmmmmmmmm maybe a problem with the switch or the cap on the switch when ALL OF A SUDDEN the noise went away! Unfortunantly so did most of the volume of the amp.

It's wierd! It sounds just like it always did, except maybe 1/5 of the volume. No cutting out. The tone controls work, reverb etc.

The thing works just fine except it has lost most of it's volume.

One more lil' quirk to this problem. If you use the effects loop it's really quiet, the loss of volume i'm talking about. BUT if you you don't use the effects loop and just run into the amp the volume is dropped even further.

I'm at a loss with this thing. The amps i know are the old Marshalls and Fenders. I've never had an amp do this, sound like it always does just with a massive volume cut. Output capacitor/s maybe? I'm lost on this one.

Guys ANY help or suggestion would be most appreciated, so thanks bunches in advance.

PMS

3/18/2006 11:30 PM
Don Symes	The timing of that symptom does suggest something in the PSU let go. I'm thinking something like a rectifier ... or maybe that grounding cap? If you have a DVM, make sure you still have full voltage going into the PT, and then try to work out if the recto and first filter are still functioning. My next thought would be the FX return jack, but I have no real basis for that. Good luck!

3/19/2006 7:48 AM

It could well be parasitic oscillation. With power MOSFETs, that could well be at over 100MHZ, so many tech scopes might not even see it. The described symptoms seem to fit.

Check the gate cirucits of the MOSFETs and ensure that (a) there are stopper resistors of 100R to 1K and that (b) if they're there, they are right at the gate pin and that they're not shorted.

Unless the PSU is switching, I would not suspect power supply.

3/19/2006 11:29 AM
Patrick Smith	Thanks guys, i really really appreciate the help on this. I'll go in tonight and see what i find. I may be in over my head on this one, i'll let you know what if anything i find. thanks again very much PMS

3/19/2006 9:00 PM

Well i think we have multiple events happening with this amp.

First there was the noizzzzzzzzzzzzzzzzzz

then i flipped the power switch back and forth (it pops) and the noizzzzzzzzz went away but so did most of the volume of the amp

Well i went back in tonight and the resitors to the pins are fine, both measure 100 and no shorts. Trouble shot solder connections, nuttin'.

THEN i find the ground connection to the effects loop OUT had completely come undone. So i fixed that. I'm thinking that would explain the noizzzzzzzzzzzzzzzzzzzzz when the effects loop was used. So i put the amp back together and it has no noizzzzzzz but no volume either.

So now i'm thinking in my switching the power switch on and off like that i did something.

It's wierd, all the controls work, reverb works, master vol works, preamp vol works, everything works....it's just running at maybe 1/5 of the volume it should. What could flipping the power switch back and forth screw up? A bad power switch wouldn't account for a vol drop would it? That doesn't make sense to me but it sure popped when i switched it back and forth.

Any help/info is greatly appreciated as always but i'm getting to the exasperation point with this one. It's definitely on the injured reserve list for steel guitar gigs and the modded twin reverb is warming up in the bull pen

3/19/2006 11:43 PM

quote:
"A bad power switch wouldn't account for a vol drop would it?"

Pretty unlikely ... especially if there's no difference between switch sides.

The cap off the switch common? Get it fully charged, then pop it to the opposite end of the opposite phase, doubling the voltage it faces and yanking current out (or slamming it in).

Something similar in the PSU front end?

Some of the solid-state amps have these 'jumping' power supplies where the rails to the output devices go up and down with the size of the intended swing - there could be some funk therein provoked by a bunch of spikes.

We're well out of my depth here, if you can't tell.

Oh - are there any MOVs in the input - those wear out.

Hope there's some help in this.

3/20/2006 8:17 AM
Patrick Smith	Don i very much appreciate the help! We're getting way out of my depth here too, unfortunantly. This is the wierdest thing i've yet to encounter with an amp. I'll keep hunting for a while and see if i can find anything. Thanks again guys, i really appreciate the input PMS

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