A little update. In the words of Borat, great success! I was in the right area for the point of failure but as it turned out it wasn't the keyboard controller but in fact the MC146818P RTC. I can't believe I didn't see this sooner but here's a close up of the picture I posted above with the motherboard on the bench. Notice something metal wedged between one of the caps and the MC146818P RTC chip? Click to view full size! Removing it made no difference however I couldn't rule out that it damaged the chip so I set about checking it with my logic probe. Sure enough comparing it's readings to the 5170 logic chart it was not functioning as expected. Not having any spares I decided to desolder the MC146818P from the other dead board I have and then desolder the chip on the new board and put a socket in it's place. After inserting the new chip and firing it up with the original IBM ROM's I'm greeted with this... Click to view full size! Side note, I forgot I brought this little 8" portable LCD TV a few years ago. Really handy to use for testing on the bench as it has Composite/S-Video/VGA and of course Antenna which is pretty much useless now (although come to think of it you could possibly use it on a Atari 2600 for example). A lot easier to move around and takes up a hell of a lot less room than a CRT! It's using the IBM CGA composite output which is why there's "snow" on the screen. Things I plan to do next: Get the RAM back up to 512K. One came up faulty in the initial testing and I have plenty of spares so it shouldn't be too hard to sort out. Once bitten twice shy. I know I was extremely unlucky that a failed tantalum on the serial card caused damage to the first board but I'm not taking any risks. As a general rule I'll be looking to replace the tantalum filter caps on any cards I intend to use starting with the ones for this build. I know tantalums are generally very reliable but working with components which are 30+ years old I've seen my fair share of them pop. Once these are done I will be back to putting everything in the case once again which reminds me I will need to do something about the power supply too which is in need of repair. Also need to get stuck into the 5153's power supply of which it's repair was put on hold. And the hunt for more parts resumes......It's like it's never ending.