Dear Steve,
I do thank you for your answer! I have spent my whole morning looking for that component, and actually, it seems this comes from Hitachi... However, 4 different chips could be the one you are talking about.
These are:
HD-U050 BP
HD-14051 BP
HD-14053BP
HD-63B50P
I found this in the service manual, but I don't know which one is the right one to replace... I suppose it's the HD14053BP as it's closer to the midi implementation. Moreover, there is one more mistery... On the chip inside my synth, something is written right under the name of the chip (HD1 4053BP) : 5K4T. I do not know what that means and this is quite annoying, indeed, because on some new 4053BP chips I can buy on the net, all of those hav different marks from this one! There are a lot of those, but no sign of the "5K4T" mark... This is strange.
Would you mind telling me what to do?
Thank you in advance,
Gloubi