As of last night, the gyro has not been locking. I have checked everything I could think of---we are getting transmission peaks, reflection dips, and a reasonable error signal, but the cavity won't lock. I finally checked the PDH servo itself by using an SR560 to lock the cavity. This worked, so I'm thinking there is something wrong with the servo. It must be early in the chain, because I am able to generate signals at the output with the sweep input without a problem.