Vintage Synth Repair

Hi, I wonder if anyone could give me some help with this little drum machine. Play mode seems to work fine & it will start & stop ok & everything sounds ok. Problem is with write mode, the LED stays on & pressing either of the start (S5) or stop (S6) buttons wont advance the sequence & so LED stays on & nothing is written. 

I thought id found the problem initially as Q18 was bad but this has been changed now & the problem persists. 4011,C45,C43,C41 have all been replaced & Q15 & Q16 have been checked out of circuit & appear to be working fine. Any hints on what to look for would be great & oh, I have no scope unfortunately.

thanks all