All 5xx series spectrodensitometers posess a host of powerful functions. Among them, it is possible to set a configuration lockout which in turn prevents idle hands from accidentally modifying the unit's setup. To re-enable the configuration menu proceed as follows:
Turn the unit off (power switch is on the bottom of the unit near the hinge).
While manually depressing the read switch (near aperture), turn the unit back on.
That's it! The "CONFIGURATION" menu is now restored.