@fiziks, the Yamaha YC88 has both L/R 1/4" Unbalanced (TS or mono jacks) and L/R XLR Balanced outputs. The ouput nominal level isn't specified but I think it's safe to assume it's either -10dB (consumer) or +4dB (pro).
Independently from the output type (1/4" TS or XLR) you decide to use, you must connect them to the Model 12 inputs using an identical pair of audio cables.
In general, mixer inputs use a sort of dimensional coding in order to inform the user about their different use case: XLR are usually microphone inputs with higher gain range than 1/4" TRS which are usually line inputs (very generally speaking here, it's the designer choice if 1/4" jacks are balanced mono, unbalanced stereo, normalized mono/stereo...).
On the Model 12, stereo channels 7/8 and 9/10 have both a pair of XLR Combo (coaxial XLR + 1/4" Balanced jack) + 1/4" TRS Balanced jack.
Without going too deep, you just have to connect your keyboard to the Model 12 1/4" sockets using a pair of:
- unbalanced TS (Tip/Sleeve, mono) to unbalanced TS (mono) when using the YC88 1/4" outputs;
- balanced XLR to balanced TRS (Tip/Ring/Sleeve, stereo) when using the YC88 XLR outputs.
As a side note, when a unbalanced cable is plugged into a balanced input the signal may be attenuated due to internal auto-balance circuitry (I guess in the -3/-6 dB range).
Let us know if it solves your issue.