IMSAI 8080 Restoration


The system is in overall good condition. It does not have the original IMSAI backplane installed, and has a second power supply behind the backplane. A wire harness connected to the second power supply looks like it is for an external 8" diskette drive.


We removed all of the boards from the system, reformed the capacitors in the power supplies, and then plugged it in to see if the power supplies would provide a reasonable voltage. The outputs were about +10V, and +/-18V. That should be OK. The console had some LEDs lit, and would respond to key presses.

We installed the Cromemco ZPU and tried the console switches. The LEDs were flickering when the system was powered on, so it looks like there is some life in the CPU. It also looks like some of the Address/Data switches affect the address lines when they should not. We will need to do some debugging on the console and CPU.

We tried unsuccessfully to deposit and examine memory. We should try the RAM boards in one of the Altairs to see if they work, and try some of the other S-100 RAM boards that have.


We reseated all of the ICs on the CPU board. It changed the behavior, but not to the correct behavior.

On the front panel:

  • If you hold RESET, all ADDRESS LEDs are on, DATA bits 0, 1, 6, and 7 are on, WO\ is on.

  • With the CPU cable disconnected and RESET held, all ADDRESS and DATA LEDs are on.

  • With the CPU cable rotated 180 degrees and RESET held, all ADDRESS and DATA bits 0, 1, 6, and 7 are on.

  • With the CPU cable rotated 180 degrees and EXAMINE or EXAMINE NEXT pressed none of the DATA LEDs are on.

ICs 13, 14, 26, & 27 on the CPU board drive the J1 connector. We removed the 4x 74367 ICs, cleaned the black oxide on the leads, and reinstalled them. Now the DATA LEDs look like they are working correctly. This likely means that we need to remove an clean all of the ICs on the CPU board and put some DeOxit on the contacts in the sockets.

We found an STL file for the front panel paddles on Thingverse. The plan is to move the three blue switches from EXAMINE, RESET, and SINGLE STEP to ADDRESS/DATA 7, 6, and 5, and them put the printed handles in the EXAMINE, RESET, and SINGLE STEP locations. We printed two front panel paddles on two different 3D printers. Both handles had problems with the pivot that fits into the metal bracket on the front panel, and the part of the paddle that fits over the stub on the switch. We will look for three molded front panel paddles. Red ones are available from IMSAI, but no blue ones.


We cleaned the leads on all of the socketed ICs on the Cromemco ZPU board. Most of the ICs were silver plated TI parts from 1977. See the image below for the black chip leads.

We found that pin 1 of the SN7474 IC30 was missing. We have not studied the schematic enough to understand what that chip does. We replaced it with a new SN7474. During cleaning pin 6 of the SN7474 IC19 fell off. We replaced it with a new SN7474. We cleaned ICs 17-43. We will do the rest next Saturday.