Worked with fellow MidAtlantic Retro Computing Hobbyist member Bill Sudbrink to try to complete restoration of IMSAI 8080. I could offer little technical assistance, and I learned a lot about S-100 systems and Oscilloscopes.
SUMMARY OF ISSUES:
-Screen is outputting "C" characters over and over - bus noise causes us to loose prompt.
-No keyboard cable to attach IMSAI to George Risk model 756A ASCII keyboard.
-Unsure if processor to front panel cable OK.
-Cannot boot diskette
Splice processor to front panel cable and repair small chip cut in cable.
1. Remove cards except processor and Cuter.
2. Checked switches and lights. Looking for CPU & Cutter OK when set databus switches to 7F.
Cuter OK but Cutter RAM is phantoming out the RAM on the RAM card so that for example when asking for address C000 - The ROM card responds with what's on the Cuter chip, and tells RAM to go PHANTOM.
Determined that the reset switch disables the ROM card, (so leave it alone.)
3. Inserted video card. Prompt appears intermittently. System bus not stable. (The connection cable to the monitor is co-ax jack-->RCA-->mono display)
4. Inserted bus terminator card into back bus slot to prevent ring back on the bus.
5. Insert serial parallel card (3P + S) for keyboard.
6. We need a 5v+ -12v so used schematics to locate on card. We made our own 30 pin connector out of a 44 pin connector, using a hack saw. We made the cable to the keyboard and connector.
Solder points for wire from user manual. We chose the following wire assignments:
brown: bit 1
white: bit 2
grey: bit 3
purple: bit 4
blue: bit 5
green: bit 6
orange: bit 7
2nd blue: strobe
7. Modify 3P + S card for -12 (J2) to pin 1 (See figure 4-1 in 3P+S manual, J1 & J2 pin connectors)
8. Solder the wires to pins on connector attached to right connector when facing card from front of computer.
strobe to pin 3
ground wires (black wires) to 12,11, 10, 9 on card connector. Because there were more than 4 black grounds, we had to bundle the grounds. This was tough.
11. Press a key on the keyboard..getting three keys for every one keystroke. Seems that Cuter and the keyboard are not communicating correctly, or there is a strobe issue.
Consistently getting 3 characters for one keystroke.
Tried "not strobe"...from pulse...move to 3. Did not work. Caused many characters to appear on screen for one keystroke. You need the strobe.
Got out the oscilloscope to see if Cutter is sending "ack" to keyboard that it received the keystroke.
The strobe is 1 millisecond, and Cutter can handle 3 keystrokes per millisecond. Determined that yes, Cutter is sending ack.
12. Check the JK flip flop on the 3P+S. Is there a bad chip?
IC 15 pin 11 is supposed to connect to IC 20 pin 11. We could not get a confirmed connection with oscilloscope.
We next tested oscilloscope pin 11 of IC 20 to see if we can get a signal, nothing again.
Is IC 20 bad?
We re-seated 15 and 20.
On the oscilloscope we had two probes. We saw that when the strobe was coming in, the JK flip flop did nothing.
Next Check IC 21 pin 6
We determined that the 74109 chip (IC 15) is bad. I happened to have another one, so we were able to replace and move on. The spare came from my cassette controller card, so I will need another one!
13. We now can try to use the strobe latch circuit to get proper communication with the keyboard. NOET: Used a strobe latch patch, to card pin 3
14. Put keyboard strobe signal back on 3...not good, causes crazy chars to appear on screen. never mind
15. Change strobe on keyboard side from pin 1 to pin 2. Caused crazy characters on screen. never mind again.
16. Theory 3
IC Pin 9 missing back to latch, needs to find a 7404, see page IV 21...but there is no 7404 chip on the 3P+S card.
* IC 15 provides both the ack and not the ack, to stop the key input to just one key per keystroke.
..this did not work...
17. We then studied the patched wiring of the 3p+S card. We found a bad solder.
Problem solved, keyboard works.