Since three work and three don't, it's look like you're heading in the right direction.
However, you should also check for corrosion of the lines that run from the 82C55 all the way to the displays because the corrosion may have simply corroded away the lines themselves.
What did you use to "clean up" the board? The battery acid won't go away unless you use a strong solvent.
/Mike
From: "andrewsamoyed@... [korgpolyex]" <korgpolyex@yahoogroups.com>
To: korgpolyex@yahoogroups.com
Sent: Sunday, January 8, 2017 6:44 AM
Subject: [korgpolyex] Dead displays
To: korgpolyex@yahoogroups.com
Sent: Sunday, January 8, 2017 6:44 AM
Subject: [korgpolyex] Dead displays
Hi Folks, I'm a new member but I've gratefully read your advice in the past when fiddling with a couple of polys.
I have a problem with an 'as-is' MkI that I bought on ebay - three of the 7-segment digits don't work at all. Reading L-R, It's the 3rd, 5th and 6th.
I've replaced the 7-segment displays themselves, and checked the cables. There was extensive corrosion from a battery leak on several of the boards and it was very erratic when i first turned it on, but thankfully after a clean-up everything else seems to be working fine
There's some weird crosstalk going on between the digits, with kind of 'ghosting' of the non-working 3rd digit on the 4th display. Before I cleaned the board, there was even weirder stuff going on with crosstalk.
I've ordered a replacement IC30 (BA618) LED driver and IC31 (M54513) darlington array. My basic electronics ability is ok, but honestly my troubleshooting and testing isn't the best. So I just wanted to check whether any of you folks had any suggestions or advice on sources of the problem that I might have missed, or whether I'm totally on the wrong track.
Any advice very much appreciated!
Andrew