> Recently it started having
> problems with irratic measurements.
Assuming the device is all solid state (no vacuum tube), I would do the next basic thing - check voltage stability of the power supply, and check ESR of all capacitors (especially electrolytic caps)
In these electronics, the caps are the most obvious thing that goes bad (assuming no corrosion on the contacts, etc.) - and maybe the only things that might go bad after 15 years.
As for the rest, I'm in the same boat - this is where my "expertise" starts and ends.