Hi Fraser,
Thank you for your answer.
I contacted Digimess and they replied very fast. Your are right, they sell the service manual in a DOC format file for GBP 40.
Meanwhile, I found that the capacitor-meter still "works" but only for capacitors with values below 200 nF. Above this value it just shows the "Out of Range" message, despite the instrument being rated to measure up to 1.999 mF (milliFarad).
Inside each instrument (I have access to several units), there is an OpAmp (AD845) that is the only IC mounted on a socket. I had to exchange OpAmps between two units, just to check, but no luck, the OpAmp of the faulty instrument is good.
I guess that either I'll have to reverse engineer the schematics of part of the unit or buy the service manual.
I'll keep this topic updated if I get some evolution (in either direction).
Best regards,
Emmanuel.