I did more experiments with my THS scope and now I can see I have two distinct issues which I have not seen discussed before. Hope someone could give me a clue as to how to go about repairing it.
1-The offset issue on channel 1: I have done the scope calibration twice and numerous SPC and the offset always "comes back" (read below) although it is much smaller than it used to be. Also the offset value changes with range.
However, yesterday I noticed something that must have been there all along. When I plug in the power adapter, the offset gradually, over 2-3 minutes, disappears! and when I unplug it, it takes 2-3 minutes for the offset to slowly come back. The batteries are brand new and fully charged and their voltage is solid. How can that be?
2-"High pass" type of response on channel 1: the bigger issue that I have with the scope is the freq response of channel 1. I connected my function gen with square waveform and a piece of coax with no termination directly to the scope. On channel 2, everything is honky dory on all ranges, the top/bottom of the pulses are flat and the gain is accurate and when I switch to sinusoidal the gain is perfect on all ranges. However, on channel 1 with the same setup, "only" on the two ranges (100mV-500mv and 10V-50V) the top and bottom of square wave is exponential with overshoot. When the wave period is long enough the voltage settles to the correct value, so the DC gain is OK (I have checked with DC voltage and the gain is perfectly ok) but if I increase the frequency it does not have enough time to settle so the square wave appears to have a larger amplitude than it should. And if I inject sine wave the gain is clearly wrong (larger than it must be) on those two ranges.
I tried to measure the input impedance of the scope ports by connecting my LCR meter. On the good channel I get something around 28.05-28.3pF on all ranges (it changes about 0.25-0.3pF when jumping between the ranges) but on the bad channel I get 30.9pF on the two "bad" ranges and 28.08pF on the two good ranges. The scope spec is 25+/-2pF and I had to insert a BNC-SMA adapter in order to connect my LCR meter tweezers, so I know 1-2pF error must be ok but I cannot explain the inconsistency on those two bad ranges on channel 1
One thing to note is that in those two ranges the X10 attenuator kicks in while in other ranges either there is no attenuator or only the X100 attenuator is in the signal path.
Any suggestions?