TDS540 spike problem on all channels

[mwbarth36]

I have been fighting a strange problem with a TDS540 that I recently purchased.  All four channels display a signal, but there is a spike running randomly through the waveform.  I am aware that this is usually a memory problem if it occurs on a given channel.  In my case, however, the spike is present at the same location on all four channels.  The three photos I have attached show this.  Also note that the spike is always in a direction toward the base line (0 Volts).  If the trace is moved to zero volts, the spike is not present.  As far as I can tell this is not likely to be a memory problem as the memory is separate for each channel.  As can be seen from the photos of channel 1 and 4, the spike is displayed in exactly the same place on the trace.  If I had displayed channels 2 and 3, there would have been a spike at the same location, always in a direction toward 0 Volts. 

I know the problem is with the Acq board A10, as I have tried with several known good Main boards (A11), and the spike is still there.  The fact that it is always in the direction of 0 Volts, and that gets more pronounced the further the trace is moved from the base line should give a clue as to the possible problem areas, but I cannot seem to come up with an answer. 

Can anyone help, please!  All comments will be appreciated.

[mwbarth36]

Here is the third photo.

[CaptDon]

I had a similar problem on a TDS540 in the lab. Electrolytic leakage and corrosion on the acquisition board. Had to replace those bad SMD electrolytics but most importantly clean the board with hot soapy water and a very soft brush then rinse thoroughly with hot clear clean water then used a hair dryer to drive all moisture off the board. Had to clean it and dry it Twice to fully resolve the issue!!
 

[mwbarth36]

Just to put the finishing touch on this Thread, I did find the problem and now have a very nice scope.  The problem was with the small, solder in, pc board acting as the filter for the phase lock loop.  The two 33ufd caps had been replaced, but had damaged some of the lands and connections on the board.  I replaced the board, and the scope no longer has the 'spikes', in fact, it works fine and passes all tests.  Thanks for your suggestion, the problem did turn out to be related to the leakage of one of the capacitors.