Keithley 2304A_error limit current (LIM)

[anupong1983]

Does anyone can help or who has repaired the PSU KEITHLEY 2304A with an error limit current "LIM"?
I set the any voltages and current then press operate to "on" or "off" the unit's display always stated 1.8-2.3V and 0.000A current and when I increase voltage more then 2V the LIM will showed on.
The problem found on Analog board (top board), but I do not have circuit diagram to check which part have problems.

Thank you so much.
Anupong