The owner's manual states:

Source: https://usa.yamaha.com/files/download/other_assets/4/333564/S30E1.pdf on Page 8:

DC IN terminal (Page 9)
For connecting an appropriate AC power adaptor
(PA-5C or an equivalent recommended by Yamaha)
to supply power to the S30.

And on page 9:

1Make sure that the instrument’s STANDBY/ON switch is at the STANDBY(off) position.
2Connect the PA-5C’s DC plug to the S30’s DC IN terminal on the instrument’s rear panel.
3Connect the adaptor’s AC plug to the nearest electrical outlet.
Do not attempt to use an AC adaptor other than the Yamaha PA-5C or an equivalent recommended by Yamaha.
The use of an incompatible adaptor may cause irreparable damage to the S30, and may even pose a serious shock
hazard! ALWAYS UNPLUG THE AC ADAPTOR FROM THE AC POWER OUTLET WHEN THE S30 IS NOT
IN USE.

The specs for the PA-5C:

12 V DC, 1.5 amps with the center of barrel plug as positive.

Thanks Jason. I had the manual but could not find any reference to the voltage of the adaptor. Even a search on the PA-5C didn't show me the voltage. But, now I know. Thanks again. It looks like I'll have to source a new adaptor for the keyboard before I start using it.

Just purchased a used Yamaha S30 an I'm not sure the seller supplied the correct AC adaptor.

Now might be the time to check with the seller and ask if they actually used the adapter they gave you with that keyboard.

If they did it could have caused damage.

If they did NOT then they may still have the correct adapter and could swap it with you.

Just so you know - the power supply for the S-30 is powering 5V and 9V regulators. These regulator specs will determine the voltage ranges accepted. The 9V regulator accepts input voltages of 10-18V and the 5V regulator accepts input voltages of 7-40V. The 9V regulator determines the input range as 10V-18V. Beyond this and the power solution on the S-30 is out of spec. The impact of going above the 12V voltage may translate into more heat and then thermals start to determine longterm reliability. I wouldn't be concerned about using that power supply for short durations as nothing, per spec, is going to "blow up". I wouldn't leave it plugged in and turned on for months at a time out of an abundance of caution.

I wouldn't be concerned about using that power supply for short durations

Not even a close call for me.

Guitar Center lists a used S-30 for $400 and the correct power adapter for $20.

Even if you use that 'NEW MATH' that is going around it's pretty easy to conclude that you don't
risk a $400 instrument to try to save $20.

I have expertise in electrical design including power supply design. I'm not recommending using the "wrong" power supply but wanted to counter that the sky is falling by using a 15V supply.

The proper way to verify this would be to call your regional Yamaha distributor and ask to speak to Service or Support... They will have the information on the official Yamaha (recommended) replacement... (which is likely the Yamaha PA-150).