When I received my (then) THS710A, both scope channels showed some DC offset that changed with the input sensitivity ranges selected. DC path compensation didn't correct this and I read somewhere in the forum that this is relateds to "worn out" analog optocouplers (CNR-201A) that are used to transfer the DC component of the input signal across the isolation barrier. Since these components are not particularly expensive. I decided to get some and replace them.
Guess what the result was: Same picture as before
As a last resort I thought the complete factory calibration of the scope section might possibly help, and fortunately, it did the trick! DC offset vanished completely.
I just wanted to post the information in this thread since it seems to be the most comprehensive and as I understand, many of the THS700 models suffer more or less severely from the DC offset over the time. Calibration is very simple and all that's required for the job is a good (arbitrary) function generator that's capable of outputting a square wave with half-way decent rise time (<20ns) and a DC voltage (the accuracy of the latter should be checked with a reliable multimeter). Fortunately, the scope section and the DMM section of the THS700 is calibrated separately and the DMM section calibration is very unlike to deteriorate over time.
All the information required for the job is available in the service manual, starting at page 51:
http://de.tek.com/oscilloscope/ths720a-manual/ths710a-ths720a-ths730a-ths720p-service-manualMy recommendation in case of DC offset problems is to try this calibration before swapping out any components, especially since removal of the analog isolators is a real PITA....
Cheers,
Thomas
P.S. Some calibration steps really take some time and the instrument may appear like it's crashed -- just get yourself a cup of coffee (or whatever you feel like having at the time...) and wait. The screen contrast is set to 50% during calibration, on my THS700 this results in a rather dim screen (normally it's set at 65%), so don't worry if something like this happens.