Problem with OTop12

[Tom Smit]

Is it possible that a cap is at fault? I wondered that maybe it is causing oscillation because I have seen caps used as an oscillation tank for DIY induction heaters.

[Bruce Weldy]

Is it possible that a cap is at fault? I wondered that maybe it is causing oscillation because I have seen caps used as an oscillation tank for DIY induction heaters.

Good question. Neither cap looks damaged at all.

Tested it a few more times this afternoon and tonight.....still no noise. Quite perplexing.

[Seth]

It seems to me that if there's no signal going into the amplifier, there would be no signal nor voltage at the outputs, yet sound is made. There's definitely something in the crossover that's interacting with the output stage of the amplifier. The only thought that comes to me is the cap, which could have a charge capable of causing that sort of interference on the amp output. Maybe there's some breakdown of the electrolytic paper causing a minute internal short of the capacitor? Everything else in the crossover is just made of wires. Cap is my first and only thought. They do go bad before the look like they've gone bad.

That's all I got. I look forward to learning of the solution and also to a follow up build, converting them to CD. Hoping you're otherwise doing well, Bruce.

[Bruce Weldy]

Well, Tom and Seth.....you're right!

So, been testing it and it's been good everytime. I decided to jut button it up and go from there. As I was checking one last time to make sure the crossover was secured properly, I decided to feel the end of the caps to see if anything was bulging....then I saw the problem...

Right now, you're probably wondering if the doddering old fool is blind, stupid, or both......well, you're wrong!

Here's the pic I took while it was acting up.

The lead from the cap was burned through. I guess that's when it gave up the ghost. Now, as to why it didn't do that at the show last Thurday and just started working - who knows?

What's surprising is that the cap is part of the low pass filter, not the high pass. We were all examining the top end.

I need to order one of those.......just as an aside ....... what would happen on the low pass if you took that cap out totally?

[Bill Fitzmaurice]

Without the cap the low pass is only 1st order, and at the wrong frequency.

[Bruce Weldy]

Without the cap the low pass is only 1st order, and at the wrong frequency.

Thanks. I just ordered two of 'em. Will be here tomorrow and I'll get this thing buttoned up. I'm glad it finally burned through - not knowing is worse than having to fix it.

I appreciate the help.

[Tom Smit]

Glad you found it, Bruce!

[Seth]

Indeed. Good catch Bruce... and impeccable luck having it appear fine for one photo and show damage shortly after, yet prior to buttoning it up. Happy ending.

[Bruce Weldy]

Well, you ain't gonna' believe this, but I installed the new cap......and the screeching is back. I guess the cap was the weak link and burned up. But, the problem is still there. Could the inductors or the other caps be the culprits?

I'm at a loss.

[Bruce Weldy]

Okay, I did what Bill suggested in the beginning, I took out the highpass along with the array. Had just the low pass and the woofer hooked up......squealed like a pig.

Examined the inductor closely and saw some seepage on the inside of the coil. In the pic you can make out a little of it up by the yellow zip tie.

I guess I need to find one of those now....

[Bruce Weldy]

Digging back through the plans becasue I seem to remember something about wire size of the coils....but, can't remember if it's to get the largest gauge or the smallest. And I can't find it - even looking at the old plans and the newer ones.

[Bill Fitzmaurice]

It's not the gauge size that matters, it's that the DCR of the 0.6 mH coil no more than 0.4 ohms. A solid core may be less expensive than an air core.

[Bruce Weldy]

It's not the gauge size that matters, it's that the DCR of the 0.6 mH coil no more than 0.4 ohms. A solid core may be less expensive than an air core.

This one is just under .4 DCR.......does it look ok? It looks just like the one that's in the cab now.

https://www.parts-express.com/Dayton-Au ... quantity=1

[Seth]

That seems so weird. Not that I'm a electronics buff at all, but I've really gotta stretch my imagination to even begin to hypothesize how something like a short in the coil could cause the noise. Wow. Better call in Agents Mulder and Skully from The X-Files to investigate.

[Tom Smit]

That seems so weird. Not that I'm a electronics buff at all, but I've really gotta stretch my imagination to even begin to hypothesize how something like a short in the coil could cause the noise. Wow. Better call in Agents Mulder and Skully from The X-Files to investigate.

I am wondering if the coil was an on again/off again short, which reacted with the cap, causing an oscillation at the right frequency, while reacting with the amp. Just throwing darts.