C128 'VIC tower' mod board

Hi all,

 

I pulled apart a C128 yesterday that someone sent in with a black screen
complaint. Upon investigating I found a small PCB with a 74LS74 perched over
top of U29 inside the VIC-2 RF shield box. I toned out how it was wired and
discovered it was intercepting /CAS. I posted this on a FB group last night
and someone said, 'that looks like the VIC tower on schematic sheet
310378-4. Sure enough he was correct.

 

http://www.zimmers.net/anonftp/pub/cbm/schematics/computers/c128/310378-4-le
ft.gif

 

The VIC2 in this PCB is a 8654R4. I have another C128 apart with has an
8564R5 without the little 'tower' PCB so I'm guessing they solved the issue
with a new version of the 8564. I have been searching and can't find an
explanation as to why Commodore had to install this little mod board though.
I would be interested if anyone has heard why.

 

Thanks,

Jeff Birt