Fried my amplifier

comfortably_numb · March 30, 2014, 10:00:10 AM

Tried this in the Lounge with no response.

Tech 21 Trademark 60 - solid state amplifier

So the other day I noticed my amp making weird sweeping phasey sounds at high gain. I don't usually play it at high gain, so I thought maybe it was just inherent in the amp or my line power. I was unsettled by it, but didn't give it too much thought. I did notice that it came and went, and imagined my wife had plugged in a fan or something putting oscillations into the house power. I've experienced this to greater effect with a drill motor.

Just now though, I was playing and it popped really loudly (initially I didn't even recognize this because I was playing with a Slow Filter effect which I never use and it has a pretty deep pop type sound as the filter starts to open) but then just loud 60Hz hum and a burning smell. The amp was still on, still capable of switching channels via footswitch, but no guitar signal whatsoever.

What the hell happened and where do I start? After unplugging and shorting the big caps out of course.

Oh - forgot to mention. This thing has an XLR out that I record through. Today I turned on my mic pre before noticing the kids had been messing with it and was supplying 48V phantom power to the amp. Great. Didn't seem to bother it at the time, and the weird phasey stuff started before this, but I imagine it may have something to do with it?

I opened her up and she's got lots of SMD components. I'm not very hopeful about my chances of fixing it, but maybe?

Jdansti · March 31, 2014, 01:40:58 AM

Others on the forum are more knowledgable than I about amps, so hopefully they will chime in. However, if it were me, the first thing I would do would be to just do a visual inspection looking for burned components and boards. As you said, do this with it unplugged and caps discharged (or just stay away from them). For safety, just look but don't touch. It would help if you could post photos of any damage you see. A schematic would be very helpful if you have one. You might have to call Tech 21 to see if they'll provide it.

comfortably_numb · March 31, 2014, 07:08:58 AM

I may well call Tech21 though I doubt they'll provide me with any such thing. I did open it up but didn't see anything obvious. Lots of SMD components which cause me further doubt concerning my ability to do anything with this.

GibsonGM · March 31, 2014, 08:12:35 AM

I hate when I see all the SMD stuff

( 9 times out of 10, if it's not something very simple (a jack pulled off the board etc) then I can't fix it. Everything in there is all digital effects, and it's a real bear to try and diagnose them (not worth the time, frankly). Keep trying, maybe it's simple!! A cap or something, if it was over-voltaged...

comfortably_numb · March 31, 2014, 04:48:42 PM

It's not digital at all. But lots of SMD.

GibsonGM · March 31, 2014, 05:50:06 PM

Sorry, my bad....thought you referenced playing with some built-in effects, which would/could have been DSP (is Line 6?), but I see you were using an outboard filter. SMD is a pain no matter what is going on inside!

)

Digital Larry · March 31, 2014, 06:38:37 PM

I have a Trademark 60 also. Opened it up once for laughs. The power amp is a (largish) single chip and it shouldn't be that hard to replace it. Question is, why did the other one fry? You wouldn't want to replace the power amp just to have it sizzle out again.

First thing I'd do would be to check the power supplies. Not that I can tell you exactly how to do this!

Ice-9 · March 31, 2014, 06:39:15 PM

If the amp was still switching channels etc, and there was a burning smell I would at first suspect an output transistor has blown. I don't have a schematic for this amp .

What confuses me though is you mention that it has an XLR output, but you have a mic connected to it. I may have misunderstood but why would a mic be plugged into an output socket.

EDIT-- Digital Larry you must have posted as I was. So its a single chip amp and not transistors. Pictures would be of benefit.

Digital Larry · March 31, 2014, 06:41:44 PM

It was plugged into his mic pre-amp, not a mic. And the 48V phantom power might have done something nasty to it.

Ice-9 · March 31, 2014, 06:45:11 PM

Quote from: Digital Larry on March 31, 2014, 06:41:44 PM
It was plugged into his mic pre-amp, not a mic. And the 48V phantom power might have done something nasty to it.

Gotcha

Digital Larry · March 31, 2014, 06:49:10 PM

Loud pop and 60 Hz hum - I'm thinking a cap in the power supply went kaboom. That should be fairly obvious.

Seljer · March 31, 2014, 07:14:46 PM

^Yep, thats sounds like a likely diagnosis. Then you just have to worry if the power supply exploding also took with it any other components.

Look for a large black power amp chip, it's probably heatsinked or at least screwed to the side of the case. Measure the voltage on it's pins (look up it's datasheet to get the pinout) and you should quickly see what you're dealing with as two pins are most likely tied directly to the power supply. Look for any asymmetry of the power rails or DC voltage on the output pin.

I figure the engineers at Tech21 probably foresaw the accidental phantom power scenario and were smart enough to use bipolar caps with a high enough voltage rating to decouple the XLR output (or even used a transformer for complete isolation)....but who knows.

comfortably_numb · April 01, 2014, 04:38:55 PM

Wow. Thanks for all the input guys. I'll give it another look soon and see. I was expecting an expanded or blown cap, but didn't see anything obvious, but I was also really intimidated by all the SMD and didn't really look very carefully I guess.

GibsonGM · April 02, 2014, 07:38:55 AM

That's what I was talking about

/ Kind of daunting when you look in and see what I call "all that digital crap", LOL.

Best you can do up-front, really, is to see if you can find the power supply and check its output. Find a schematic online...without that, you're toast. It might have test point voltages....gotta start somewhere!!

pappasmurfsharem · April 02, 2014, 11:56:15 AM

Quote from: comfortably_numb on April 01, 2014, 04:38:55 PM
Wow. Thanks for all the input guys. I'll give it another look soon and see. I was expecting an expanded or blown cap, but didn't see anything obvious, but I was also really intimidated by all the SMD and didn't really look very carefully I guess.

Pics

guitarkill · April 02, 2014, 03:30:53 PM

My vote is for a blown power transistor, and maybe a bad power supply cap too. But the loud pop sounds like one of the power transistors blowing up. I had it happen to me with a computer power supply once, I know that sound (and smell).

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Fried my amplifier