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DSO5054A - Channel 4 exhibits vertical instability and noise

Question asked by toploser on Jul 1, 2013
Latest reply on Jul 2, 2013 by toploser
Just pulled a DSO5054A out of its box, it's never been used since it was bought 5 or 6 years ago. It all works fine apart from channel 4 has a big problem... With the input grounded there is a randomly variable offset (+ and -, up to several hundred volts on 50V/div range) and superimposed upon that is higher frequency random noise. To be honest it looks like a floating earth kind of issue.

Pictures attached, two different voltage ranges.

I've had the main board out and given it a good physical examination, can't see any obvious bad solder issues. I've also probed about it with another scope, access to the components is very limited though. Signals coming in to and out of the white 1NB7-8453 'thing' exhibit the offset and noise. I don't have any circuits for this so I'm a bit lost as to what my next step is.

Anybody else got any ideas? 

Or anybody want to buy a 3 channel 500MHz 4GS/s scope, never been used still in the box with probes and stuff.