I have to agree with mike: The service manuals which can be downloaded
from the usual places (www.tektronix.com, www.teknetelectronics.com)
have NO detailed schematics. The go down to moduel level only. So, I'd
recommend you try to inspect the board and when you are nesr a D/A
converter (AFAIK they used devices from AnaloDevices, marked AD) then
try to find the cps nearby.

To Juerg: I know this is not a perfect way to diagnose. But what
options do you have? The instrument is out of long term support from
Tektronix. No schematic avail for the public.
I guess then you have to do it the hard way. When you are located in
Europe, I might be able to help via phone.


hth,
Andreas