Something strange has happened to our STT-8300M (With selfguiding filter wheel and AO). All of a sudden, images registered only bright stars and the background read zero ADU everywhere. Flats and bias frames were also jet black with nothing on the histogram - reading zero ADU everywhere. I deleted all camera drivers using the DriverChecker. Updated the DriverChecker, updated firmware, reinstalled drivers and still no change. I even loaded CCDOPs and camera drivers on a different machine (Windows 10 vs Windows7) - still no change. Incidentally, I was experiencing the exact same issue on the guiding CCD. I then checked the USB offset. The new values were so different than the previous values and the default values. I reset the EEPROM with the values: CCD Offset = 7 in EE for 940 ADU The CCD was at -10C at the time. This fixed the issue such that there was the usual noise in the bias and dark frames, but there now seems to be a definite gradient in the bias and dark frames. I can't help feeling that I have compensated for an issue by using the wrong tool. I ran the same procedure for the guide CCD. The darks/bias are not reading zero anymore, but I haven't tested the guide CCD any further. I have attached a bias frame from the main CCD where the background is reading zero and a new bias frame after resetting the USB Offset to the above values. This Bias frame shows the gradient - darker at the top and lighter at the bottom. Bias frames taken before this issue started were uniform with no gradient and an average ADU of around 1040. The average seems to be in the low 900's now and gets higher as I move down the frame. Up to this point this unit has been a fantastic ,trouble free camera. Thanks in advance for any suggestions.