sounds either like a bad earth some where,or a boards not screwed down inside,if it mainly does it while playing a cassette,could be static..i`m sure i`ve had a box or 2 that have done it in the past,and i cant remember if i got to the bottom of the problem or not..
Just like my super jumbo,when i plug the mains lead in,it humms and its still there even when i switch it on,its only quiet but its there,its always done it,but i just live with it...

its coming from the voltage regulater it goes away if i tape the unit or angles it backwards pau;l

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@ghettoman. Well, I happen to know that hums are usually the fault of drying out electrolytic caps in the power supply. They take wall power and clean out the 60hz alternating buzz. If you've ever heard an old tube radio in need of restoration, may I ask if you've got a touch of that grotty tone going on? Is this by any chance an offbrand Taiwanese or Hong Kong boomer? That would likely mean off-brand caps, explaining why they're possibly dried up already. Surprised we don't see more of these problems in the funky plastic wonders since it's been twenty+ years since they left the factories, presumably full of cheap capacitors.

There is 2 kinds of buzzing. The kind that comes from the speakers, and the kind that comes from vibration of the unit itself.

If it's buzzing with the unit plugged in, but not when it's on batteries, it's a very good possibility that the transformer is vibrating and shifting the box will cause the transformer to change position, thereby minimizing the vibrations. Best thing to do if that is the case, is to dampen the vibrations. Some people do it by applying foam weatherstrip around the transformer, glue, silicone, whatever. It's worth looking into and easy enough to fix.

If the buzzing is coming from the speakers, then it could indeed be dried out electrolytic capacitors or a loose wire inside draping across some sensitive electronics such as the ceramic caps or transistors, or a poor ground, or continuity issues with the circuit board.

I'd guess the former rather than the latter.

Please follow up if you find the problem and let us know what it was.