Akai MPC500 experiences digital noise

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Picked it up cheap (reason more obvious now...). Seems this model can suffer from digital noise . My example does (high pitch sample and hold variable frequency style). Level varies with volume, decreasing from zero towards around 2/3 level then increasing again up to full. Power source is an AC adapter or batteries, seems worse with batteries. A decent laptop supply I have seems better than the supplied cheapo wall wart. Would like to eliminate the noise, even on batteries. As the cheap MPC, clearly Akai cut some corners somewhere, what components should I be inspecting/replacing to locate issue? Note, from research this seems to be a design issue not a result of damage. Noise affects line and headphone outputs. Any background/posts on digital noise I should check out, or other forums I should/could request help on? Apologies if I'm overlooking obvious resources.

Capable with a soldering iron and a basic (arduino, school technology class) understanding of electronics. MPC innards less smt more lego than I expected.

Thanks for listening...
 
Further info...

Noise character/volume also changes in concert with operational functions, although not in an entirely predictable manner. You can be pretty certain if it seems relatively quiet and then you navigate to a different operational aspect of the instrument it'll get noisier again.
 
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