Frustrated! Loud hum with no input. Really loud.

jakeisfine · #1 09-08-2010, 05:52 PM

It's an Ashdown EvoII 500watt head.
If it has power and its plugged into a cab...a constant hum is present.
There is no response if a bass is plugged in, but the vu meter registers input. With all knobs on zero and the mute button on...still super loud buzz in the key of B. Haha. No amount of fidgeting breaks the noise at all. I tried a different preamp tube...no change. I've tried all different combinations of cabs and cables with no change.
Every solder on both power and pre boards are legit.
Could it be the smoothing caps? Thats what Ashdown suggested. They are totally smooth with no bulging. The amp was built in '04. Seems too soon for caps to go out in 7 years. Please help!

RickenBoogie · #2 09-08-2010, 06:10 PM

Take it in to an amp tech. There's no other option.

craig.p · #3 09-08-2010, 07:29 PM

I think you're on the right track, Jake -- it's something in the power supply 'cause you're hearing either 60 Hz or (more likely, for full-wave rectification) 120 Hz. A tech will go in and look for AC voltage where there ought to be only DC. You could have a bad electrolytic or even a shorted rectifier diode. I'm assuming this is the 26-pound head and therefore has a "legacy"/linear power supply. Did this amp take a physical shock recently? Was it recently connected to an AC line where there were lots of transients present -- motor starts, for example? Or was it on when a thunderstorm came through the area? Could even be a component that was iffy back when it was first installed and it just now decided to let go. It happens (sometimes the suppliers don't hold up their end of things). We'll never know for sure.

I agree you've got to get it to a tech unless you're willing to take a crash course in analog circuit analysis/repair AND buy some test equipment to facilitate it AND learn how to de-solder/re-solder and...

billfitzmaurice · #4 09-08-2010, 07:48 PM

Quote:

Originally Posted by jakeisfine

With all knobs on zero and the mute button on...still super loud buzz in the key of B.!

At the least it would seem an output device is blown, it could be a blown power supply as well. This is not a fix for the inexperienced.

jakeisfine · #5 09-09-2010, 11:51 AM

Thanks. I appreciate your replies. I will have to take it to my amp guy soon.

fingertap · #6 09-09-2010, 12:26 PM

"B" note harmonics are very close to 30Hz or 60Hz or 120Hz, 240Hz, etc., the freq of the mains. That's a clue but it is a clue that only the experienced or a genuine tech could easily utilize.

Take it to a qualified tech. I'd bet it is simple to diagnose and it might be easy to fix. Yep. A qualified tech is the best option in my humble opinion. Good luck...

~

BassmanPaul · #7 09-09-2010, 02:13 PM

If it was the filter caps failing you would still get output. If the output transistors were shorted the main power fuse would blow. The power amp is drawing too much power. I'd say it's a blown transistor in the front end of the power amp.

Sorry Tech time!

Paul

johnk_10 · #8 09-09-2010, 03:25 PM

IMo, it's either a blown driver transistor, or one of the output transistors.

Bassmec · #9 09-09-2010, 03:41 PM

Sorry but its off to see the wizard the wonderful wizard of evo's, this is indeed a case of transistor perturbation Homes!.
And what's more I bet there is a stinky resistor or two in there
somewhere.

okcrum · #10 09-09-2010, 03:43 PM

Yeah, as suggested, an output device or the rectifier or filter cap for same. Ashdown's suggestion was the filter caps. They can be bad and not bulge or leak.

craig.p · #11 09-09-2010, 04:25 PM

Yep, that's absolutely true. Had one go bad on a chassis a couple decades ago, and it was a relatively new one. No physical evidence of failure. I took it apart and with a magnifying glass found a sharp metal sliver inside the "roll." All it took was enough jostling from the amp being moved from gig to gig for it finally to punch through the roll and create a short. I'm not saying this is the problem with the OP's amp, just relating one of those one-in-a-million stories.

jakeisfine · #12 09-09-2010, 07:29 PM

Quote:

Originally Posted by BassmanPaul

The power amp is drawing too much power. I'd say it's a blown transistor in the front end of the power amp.

If this is the case, could it be due to a low voltage problem? The amp is overworking itself to compensate?

johnk_10 · #13 09-09-2010, 09:59 PM

i don't think so.

BassmanPaul · #14 09-10-2010, 07:40 AM

No, It's something affecting the bias of the output stage IMO, but it iS something that can't be verified sight unseen. If you go in without knowing what you are doing you'll probably make things worse. Take it to a tech.

Paul