Help with Korg Poly 800 - MDK Mod done but all not well

[Jonaht]

Hello
I had my first go at modding and did the MDK mod on my Korg Poly 800 (mk1) . I was having trouble reloading the sound banks and all the noises that would come out were the same swishy swish noise so decided that there wasn't much to loose on doing the MDK mod and trying to sysex my way to a better place.
So did the track cuts with a dremel  and then soldered on the wires, changed the chip and put it all back together.
Powered up ok. I then went to try and load the midi and set the midi channel on the synth. I then realised that none of the programme setting buttons responded.
It displays programme 1.8, Parameter no 50, value 01 constantly. It also briefly flick ups something that looks like a Y and another character for a millisecond before it displays the error.
I've found a few threads that imply the error is related to the coin battery dying. I have this lithium coin battery mod already and it has been fine for a few years with it. I have change the battery again for a fresh one but its made no difference,
Is it likely I've just not put everything back together quite right or have i like broke something whilst dremeling/soldering?
Any suggestions welcome as I'm not sure where to start of f this is recoverable.
May thanks
J