I am getting the 'Battery low' message when I turn on the TD-7. I bought the set used about 10 years ago, so I'm sure it needs a new battery. The manual says 'consult with qualified service personnel', so here I am. The warranty expired long ago and I am comfortable with the risk of opening up the unit. Does anyone know the replacement part number? Also, I would appreciate any tips on replacing the battery. Thank you!