Most manufacturers eliminate backup batteries many years ago. I would surprised if the 900 had one. No need for a backup battery when there are chips that do the same thing indefinitely and never need replacement. This technology changed with PCs more than a decade ago, when bios batteries were eliminated by a new form of chip.
Gary
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PSR-S950, TC Helicon Harmony-M, Digitech VR, Samson Q7, Sennheiser E855, Custom Console, and lots of other silly stuff!
K+E=W (Knowledge Plus Experience = Wisdom.)