Biamp Advantage 7/250 Mixer/Amp Repair

J

Jeffrey Knorr

Honorary PhD
Jan 11, 2011
172
6
18
Berwick, PA 18603
jrklabs.com
I have a background in Electronics but don't have extensive experience in bench repair. One of my Church clients asked me to check out a problem they're experiencing with a Biamp Advantage 7/250 Mixer/Amplifier. The unit is generating ~24 VDC on the 4Ohm speaker outputs along with a hum. I opened the unit up, and it appears as if at least R23 on the PCB has been cooked. Measuring across R23, I see ~ 27 VDC.



The schematics for this unit can be downloaded here, http://www.biamp.com/downloads/download_redirect.aspx?ID=217 and are shown on Page 4.



I'm capable of replacing the blown resistor but I'm not sure what may have caused the blown resistor or if that's the only problem with the unit. Has anyone seen similar symptoms and have any pointers for me?



Thank you,



Jeff



P.S. Kudos to Biamp for providing the support docs on their products!



 
Re: Biamp Advantage 7/250 Mixer/Amp Repair

If the voltage were higher, I'd say a blown output transistor. In this case perhaps a blown driver transistor. Of course I'm thinking old school - it likely has an amp IC that contains these and more.
 
Re: Biamp Advantage 7/250 Mixer/Amp Repair

I suspect r23 is the least of your problems.



That R is across the power amp pre drivers, and if it is burned up, that means some serious failures have occurred.



I would start by checking all of the output stage power transistors and everything in the general area.



If you have a VOM with a diode scale you can check for base emitter junctions and a similar diode base collector. Also check for open collector-emitter. I suspect you will find one or several power devices with a shorted collector to emitter.



This will not be a simple repair. but perhaps a learning experience if you have some basic test equipment, patience, and perhaps a pile of replacement parts.



JR
 
Re: Biamp Advantage 7/250 Mixer/Amp Repair

This is the kind of project I suspect could be done in the DIY area and help people understand design. It is DIY after all...



You have to understand how stuff works to fix it... so it is edumacational...



JR



edit/ oops, it appears the DIY forum is DIY speakers so this should stay here I guess....



never mind /edit
 
Re: Biamp Advantage 7/250 Mixer/Amp Repair

Put it in the DIY section if you want. Speakers are the main focus there but other stuff can certainly be discussed. It's less likely to be buried there.



I'd move it for you but I'm not a mod for the basement.
 
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