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MA-2400 Trouble


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#1 ElectRepair

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Posted 21 June 2006 - 10:30 AM

I'm having multiple problems with this amp. Initally, Channel 2 ODEP was always dark & no audio was present. There was no excessive current draw or cycling in & out of protection. Further investigation revealed several cold solder joints on main PCB. I resoldered every joint and, with no load or signal, ramped it up on my variac. I then checked both channels for DC offset which was within tolerance. Bias voltages on both channels were excessive, I adjusted them to specs. I then powered the unit down and left it cool in preperation of the ODEP check\adjustment. When I powered the unit up at line voltage, Channel 1 would cycle 2 or 3 times before showing reserve power. Channel 2 powered up normally. Both channels were out of specs., both -ODEP and +ODEP. No DC offset was present & bias voltages appear to be stable. I have not applied signal or load, as I like my test speakers & don't want to chase the cones across the room. Please give me some input as to where to start looking.

Thank You!

#2 joust

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Posted 08 July 2006 - 01:42 PM

How old is this amp? Probably not new, or else you wouldn't have had to adjust bias... From my experience, try not to adjust ODEP right away... there are other tasks to take care of. Do a proper output refurbishing: remove, clean and reapply a new coat of thermal compound to all devices making contact with the heatsink (output devices, drivers, thermal sensors and bias transistor). Verify the output  transistors' throughholes for overheated thermal compound: I have seen weird things like unbalanced power rails because of the conductive grit. Also, replace the bias trims: they become unreliable with age. Make sure you have good air flow: fan has to be in top shape and wiring out of the way. Finally, test the amp with four ohms load with pink noise at random clipping.