Problem with OTop12

[Bruce Weldy]

Well, there's always something new.....

After using the system Tuesday night with no problem (as usual), we set up for the first Concerts in the Park show last night.

When I fired up the amp that runs the OT12s (mono, two amp channels, one per side, 2 OT12s per side), a strange noise came out of them. It wasn't hiss, nor was it hum. It almost sounded like a microphonic guitar pickup that was picking up a radio station....except, there was no station - just that electronic sound.

Immediately turned it off. Then we checked each side, then each box - turning it on for a second. Ended it up it was just one box. Turned it back off, put the cables back where they belonged, fired it up and it was fine. Certainly wasn't just a bad connection - I know those sounds.

My guess is that something is wrong with the crossover. I plan to pull that cab out and start tearing it apart. It worked fine the rest of the night.

Seems like I remember years ago blowing a capicitor in a crossover and I think it sounded like this, but it's been a long, long time ago.

Any thoughts? Anyone ever had this happen?

Here's a little snippet from last night.

https://www.facebook.com/bruce.weldy.3/ ... &ref=notif

[jimbo7]

From the limited knowledge I know of electronics; sometimes old or bad parts can make strange noises.

[Bruce Weldy]

From the limited knowledge I know of electronics; sometimes old or bad parts can make strange noises.

Substitute "old men" for "electronics" and you are definitely correct.

[Bill Fitzmaurice]

It's highly unlikely that it was the speaker. Electronics make random noise, speakers don't.

[Bruce Weldy]

It's highly unlikely that it was the speaker. Electronics make random noise, speakers don't.

Except that another OTop was parralleled with that one on the same amp channel and wasn't doing it.

It had me stumped.....just glad that it went away. But, I'll still be checking the crossover on that one.

But, that'll be another day......right now, I'm off to play some rock and roll tonight for my monthly Les Paul fix.

[Bruce Weldy]

I recreated the conditions from the other night. Plugged the OT12 into my amp rack. Powered it up - same result.....weird screeching sound.

Plugged it into one of the sub amps in the rack - same result. Considered that it might be a driverack problem (but why wouldn't it do it to the other speakers?), so I hauled out another spare crown amp and hooked it up. Same result.

I did all of those same tests with a Yamaha Monitor I have in the garage - quiet as can be.

So, it's definitely the cab. I took the back off the cab and one of the inductors had come loose from the crossover board and was resting on the bottom of the cab. I inspected the crossover, all looked good. Re-attached the inductor - I figured that when it fell maybe the wires were shorting out.

Tested again with the same result - no change.

I decided to do a video so that ya'll could hear it. Having trouble getting the video to download to my computer from my phone.....so, I don't have that right now. Keep in mind that I was turning the amp off as soon as the noise started. But, trying to walk over to the cab to video, it took a few seconds for me to get back to the rack to turn it off....it stopped right before I could turn it off. Turned it off and back on again - no noise.

This really has me perplexed. That's what happened the other day too....just started working. I figured that maybe it had shorted and messing around with it jostled it enough to lose the short. Obviously not the case now - I made sure all of the wires had room and weren't touching.

Here's some pics of the crossover (hard to believe that I built this about 10-11 years ago.) The components all look good as far as I can tell. Any ideas what this could be?

[Bruce Weldy]

Heres the video......you have to really crank the volume to hear it. It is much louder than that. I think the phone might have set a level based on my voice.

https://www.youtube.com/watch?v=2YkYKv_ ... BruceWeldy

[Bruce Weldy]

I guess the big question is - why would it stop after a about 6-8 seconds? That just makes me think something is on the way out. I'd like to fix it before that happens, but I don't have a clue what to attack.

Can a Piezo going bad do something like this?

[Bill Fitzmaurice]

I've never heard anything like that before. Try bypassing the high pass filter.

[Bruce Weldy]

I've never heard anything like that before. Try bypassing the high pass filter.

Just so I understand. Cut the signal that's going to the high pass filter, meaning that the piezos aren't getting power at all? Or, bypassing the high pass filter and letting the piezos run with out a high pass? Kinda' doubt I should hit the piezos without the filter, but just making sure.

Is just unplugging the piezos good enough, or do I need to take all of the high pass offline? It's all built on one board, so just want to make sure what I should isolate.

Thanks....

[Bill Fitzmaurice]

Yes, run them without the filter. They don't have to use one, they just work much better with it. That way you'll narrow it down to whether the filter of the piezos are the problem.

[Bruce Weldy]

Yes, run them without the filter. They don't have to use one, they just work much better with it. That way you'll narrow it down to whether the filter of the piezos are the problem.

After letting it sit awhile, I went back yesterday and fired it up. No noise. Came back and did it two more times in the evening - no noise.

What I haven't done is run some content through it. I'll do that this morning to see if it's even alive. Then go from there.

[Bill Fitzmaurice]

Poltergeist?

[Bruce Weldy]

Poltergeist?

Funkwarts...

[Bruce Weldy]

Well, it's definitely perplexing.....turned it on - no noise. Put some music through it - works fine. Array is working.

I'm gonna' keep running through this process for a couple of days.


I've been threatening for years to convert these to CDs like my other pair. Maybe that's my next move, regardless of what happens here.