Unlike the Bally MPU’s, the Williams Level 3, 4, and 6 MPU board have two LED’s that indicate a limited level of diagnostics. Williams added more sophisticated start-up diagnostics in the Level 7 and later MPU’s. There are only four possible conditions of the LED’s.

Both LED’s ON: CMOS RAM (IC-19) bad

Both LED’s OFF: If they don’t even blink, check the Power Supply

Top LED ON, Bottom LED OFF: NMOS RAM (IC-13 or 16) bad

Bottom LED ON, Top LED OFF: ROM or PROM (IC-17, 20,21,or 22) failure

Re-seat all IC’s before replacing them. Be especially careful with IC-19, the 5101 CMOS RAM, it is extremely static sensitive.

Examine the board carefully for IC sockets with “scanbe” written on them. They are poor sockets (at best) and should be replaced with machine pin strip sockets. These sockets (machine pin strip sockets) are designed for re-work and can be soldered on both the top and bottom of the board. The best ones (the ones I use) are gold plated. They are NOT available at Radio Shack!

Good Luck! Allan