Yamaha p35 keyboard low volume

I have a yamaha p35 keyboard the last 5 keys you can parley hear I changed the board contact strip and ribbon no change the volume seems a little low on the keyboard but works fine just the last 5 keys hooled it up to my 100 watt guitar amp and than they worked but a little garbled or just didn,t sound right also tested the volume switch looks fine any ideas 
john