That's way off and of course not adjustable, it's a fundimental failure but sound fixable. First check the 100 MHz is really 100, which can be done from the ref out without opening anything. Then its documentation time and checking power supplies. It's tuning but not evenly, 23% off, then less so for higher freqs. That fact and as the older units don't have good stabilization it's most likely something analog in the yig control loop. The good news is that the older 492s are simpler than the later models. At some point you may need a counter to dig into the phase gate but as this is so far off I bet you can get results with a scope and basic dvm. Board extenders will be necessary if serious trouble shooting is required.
