Superboard II - Rev B repair

[Steve Gray]

While repairing my new C4P I thought I'd pull out my Superboard, which also was not working. I thought I'd make a new post to keep things separate.

Anyway, I'm not sure how I overlooked this before but the machine is missing the two 2T28 chips beside the CPU (UC6 and 7 if memory is correct). I understand these chips are impossible to find now. I can borrow some temporarily from one of the other machines to see if the board will run, but eventually they will need replacing..or not. I understand by reading the web, some Williams arcade machines use this chip as well and apparently they can be removed and pins jumpered. Apparently the 8T28 chips are simple buffer/amplifiers. So, does anyone know if this will work on the Superboard? Or, anyone have some spare chips?

Steve

[dave]

American Microsemiconductor has 8T26's and 8T28's in stock for a reaonable price.

If you get a bunch of one chip or the other, there are some places you can replace 8T28's with 8T26's and vice versa. For example, you can replace all the bus drivers in a C2/4/8 system with the same type, either th '26 or '28. Also, a RAM board can use either one, or a combination of the two. But not video RAM. The 600 board video RAM feeds the chargen ROM, which expects uninverted data. A 540 video board can use the same chip as the CPU board, since any inversion on the CPU board (by a '26') or not ('28) is re-verted (or not) on the video board, so the logic is positive.

Dave

[Steve Gray]

American Microsemiconductor has 8T26's and 8T28's in stock for a reaonable price.

If you get a bunch of one chip or the other, there are some places you can replace 8T28's with 8T26's and vice versa. For example, you can replace all the bus drivers in a C2/4/8 system with the same type, either th '26 or '28. Also, a RAM board can use either one, or a combination of the two. But not video RAM. The 600 board video RAM feeds the chargen ROM, which expects uninverted data. A 540 video board can use the same chip as the CPU board, since any inversion on the CPU board (by a '26') or not ('28) is re-verted (or not) on the video board, so the logic is positive.

Dave

That's great! Thanks. I didn't realize that the chips were inverters.

Steve

[dave]

Yes, both chips are bidirectional buffers, and the only difference is that the 8T26 is an inverting part, and the 8T28 is a non-inverting part.