Vintage Synth Repair

Hi
I've got JV880 with old broked encoder
I've replaced it for PEC11-4115K-S0018 ( makes 18 impulses by turn )
but it doesn't works well,i have to turn it wery fast to synth makes changes of any parameter.
Original encoder is RK09710w ,it has 20 sectors on his rotary part.
What type of encoder i have to use to replace it ?

thanx