Apogee AD-16x Calibration Drift

[gg85]

Hey guys, just wondering if anyone might have any experience with AD-16x calibration and why mine doesn't seem to keep it's cal that well. The DA-16x seems to be holding it cal but over a few days the AD has drifted a little, sometimes .5db. After a while it can be over 1db out on a few channels. When I'm going from DA-16x to a Burl B2 ADC the level is solid and coming back in at -16 but with the Apogee it just doesn't hold near as well. I'm recalibrating a few times a week to keep it as close to -16 as I can.

I know these units are pretty old now, just seeing if someone might know what causes this and if it's something I might be able to fix.

Thanks,
George