Peavey Data Bass

[izzybell2810]

Had a Data Bass combo come in for repair late on today. Diagnosing the problem: Preamp is fine, but nothing from the power amp. I found the speaker is blown. After removing the chassis I tested the output for dc and it checks out ok. I connected to my test speaker and the amp sounds normal for about a second then I'm getting a really distorted noise. It's not mains hum or like any other noise I've heard before. I've scoped the signal and it's showing a square wave. That's as far as I got as it was late. I don't have a great deal of experience with switching amps but understand the principles of them. Could this be a failing low pass filter? Does anyone have any ideas?

[Enzo]

Contact customer service at Peavy for schematics. Too lartge a file to post here. Ask if there are any procedural or service bulletins for this model. For example not powering up with a variac comes to mind.

And a good inside contact at Peavey is Gene Ford - gford@peavey.com

I can;t keep them straight in my head, but if this is what i am thinking, you could have lost your triangle wave carrier. I may be thinking of a different model though. I do see traingle circuits on the schematic.

But before we go too far, isolate the problem.

Plug a clean signal into the power amp in jack. Does it behave the same or is it clear? Likewise, connect the preamp out signal to the input of some other amp. How does the signal sound there?

And verify you are not losing any power supplies. Even the low voltages can be involved in the power amp, as the ICs there run off that.

[izzybell2810]

Thanks for that Enzo! I contacted Peavey for a Schematic earlier today. I checked the output from the preamp, and that sounds fine. I'll check those voltages tomorrow!

Dave.

[izzybell2810]

Well I learned something today! Class D switching amps don't like been powered via a light bulb limiter! It was as simple as that. Luckily I hadn't wasted a great deal of time chasing the prob. It came to me last night about an hour after reading Enzo's reply where he mentioned not powering up with a variac! So this morning I unplugged the limiter and it's working fine!

So thanks goes to you Enzo!