I need to buy a replace battery for a yamaha sp900

It could just be a glitch, or a low battery, or a power supply problem.

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Contact Yamaha customer service for your country directly to source this part.

If the new battery tests good on a voltmeter ( you'd be surprised how often they don't) I would suspect that the battery holder's contacts need to be cleaned.

Due to the nature of the board connections and design; you need to use the exact replacement made for your model. Yamaha made the original and any other clone (if available) may not function properly. You could end up wasting your money. Either get the Yamaha replacement, or look for a used model in eBay or the internet. Other than that, use the $477 as a down payment on a new system.

It doesn't use a USB to MIDI adapter (such as the UX-16.) It has an onboard MIDI controller and uses a standard USB A-B cable. You just need to install the driver from the CD-ROM or here, and plug it in. You can get the manual from the same page or here. Setup is on page 99.

I am going to make a wild guess that they may have a battery backup to maintain previous settings and the battery may be depleted. They would LIKELY do a checksum on the data in the memory and would find it corrupted. I have searched and found that the unit indeed does have a battery.
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Replace it as a first step. It is likely a Lithium coin cell.

Then the problem is to do a "FACTORY RESET" and the manual doesn't tell how. USUALLY one holds down one or more buttons while turning on the power. You might experiment and get lucky doing this.

If you can't find the reset combination by luck and trying, contact Yamaha and ask... they are USUALLY cooperative.

http://www.yamaha.com/yamahavgn/CDA/ContentDetail/WrappedTextDetail.html?CNTID=544117

This is the only thing I could find unfortunately:http://www.yamaha.com/yamahavgn/CDA/ContentDetail/Text_WithCatMenu_XC.html?CNTID=554234# Have you tried contacting them?

I would do a factory reset. Search Google for: factory reset PSR 2100.

It will be holding certain buttons or keys down when powering on.

Sorry Dude
If you can not replace a broken string, then you need a few lessons from a friend.Find a pal to jam with.
It sounds like you broke your E string. Take it down to the local music shop. Buy a new string and ask the guy to put it on for you. Offer them a tip, its worth it.I could tell you. But you need the string so ask for assistance and see it in 3D. Keep on playing DFD