This is what the Yamaha Music India Pvt. Ltd. references for AV & Hi-Fi Service Centers give,

NAMAN ELECTRONICS
418, GANESH PETH,
NEAR GURUDWAR,
PUNE - 411002, MAHARASHTRA
+91-20-2605 1200

NATIONAL ELECTRONICS
SHOP 6, NATIONAL HEIGHTS,
418, GANESH PETH,
NEAR GANESH PETH GURUDWARA,
PUNE - 411002, MAHARASHTRA
+91-20-2605 1300,
+91-20-2605 1400 (I think this is a Fax number)

PS: If other numbers do not work, you can try talking with Mr Deepak directly at +(91)-(20)-66821099.

Here is a map reference for the National Electronics place.
I could not reach their number at the moment; maybe they're closed Tuesdays.
Am also trying to get a Yamaha Synth repaired...
(thanks for reminding me to look this up :-)

We may be too late for the day if this JustDial reference is to be believed,

Monday    : 09:30 am - 06:30 pm  
Tuesday   : 09:30 am - 06:30 pm  
Wednesday : 09:30 am - 06:30 pm  
Thursday  : 09:30 am - 06:30 pm  
Friday    : 09:30 am - 06:30 pm  
Saturday  : 09:30 am - 06:30 pm  
Sunday    : Closed - Closed  

PS: Mr Deepak confirms that they are open till 7pm on working days.

Ok! from that JustDial page I got another number for a 'Mr Deepak',
This is +(91)-(20)-66821099.
Got to talk with Mr Deepak himself, who says he will call back shortly.
There may be hope here...

Update: I was able to talk with Mr Deepak just now.
He confirms they are an authorized Yamaha service center and they can fix music systems as well as keyboards. This should be useful for both of us to start with.