hideehoo
Experienced Member
Yep, the the Cext pins on the 74LS123's (and their associated caps) were isolated from ground in the video section. When I do a v1.1 validation build I'll try 74LS221's since it also requires a change in the capacitor and resistors values that make up the RC network there.Exciting news!
Did you also make the 74LS123 changes?
Looked thru Otto's 4mhz modification. One line is unclear - "run jumper from junction of R38 & R40 (pin 9 of U77) to U96 pin 5"
I assume it intended it to read the junction of R39 and R40, but even that isn't pin 9 of U77 (unless the schematic is again incorrect). Did you just jumper U77 pin 9 to U96 pin 5?
- Gary
There is no electrical junction between R38 and R40 as you point out, figured that was more of a physical indicator of where you could solder a bodge wire on the board. Here's that mod in schematic form (confirmed with my original board) which is a little easier to follow. With 2Mhz and 4Mhz now coming from U96, you can remove U97 completely. The CAS and MUXC lines are shifted one bit "faster" on U76 due the slower 16Mhz crystal.
Here's what the U77 pin 9 / R40 "junction" change looks like. It's purpose is to move from U96 pin 4 to U96 pin 5 which is only a 4x divisor (vs 8x on pin 4) to get the 4Mhz main clock signal up to the CPU Then the additional traces/jumpers heading south route these 4Mhz and 2Mhz signals down to where U97 used to be to send them to the FD1771 floppy controller.