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IMSAI 8080 / Thinkertoys Backplane proj


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  IMSAI 8080 / Thinkertoys Backplane proj by Bill Degnan - 04/20/2008 12:12
Project Update:

1. Replaced tantalum capacitors c15, c16, c17, c18

2. Applied power. MC78M12CT (on the schematic it's IC4) started to smoke.

Updated pics:
New Board pics (have 4/20 file date)

power supply pics (have 4/20 file date)

One of two things is going on...
1) The IC's are dry/brittle and need to be replaced, the 4 new tantalum capacitors did not smoke after being replaced, it was simply the MC78M12CT (IC4) turn.

2) There is an issue with the power, and that's what's blowing out the ic's.

  Final Results - bad 16+V capacitor by Bill Degnan - 04/20/2008 20:10
I have completed the power test of IMSAI 8080 #2 with Thinkertoy wunderbuss mother/backplane board, Dan Roganti sent me useful testing info, it helps my confidence to know for sure that I am doing it correctly.

1. yellow - PCB label: -16V PCB: -17.45v capac: -17.45v output: -17.45v
2. orange - PCB label: +16V PCB: +11 capac: +.0130 output: +.0130
3. black - PCB label: gnd PCB: 0.00 na output: 0.00
4. whilte - PCB label: +8 PCB: +9.2 capac: +9.2 output: +9.2

The original owner may have had the yellow and orange crossed (-16 and +16). That would explain the blow outs. I was careful to preserve the order that the wires were originally set so that when I removed the board I could put it back the way it was:


I have since then swapped the yellow and orange wires so that next time I will not assume the previous owner of a computer has it wired correctly!

How this could have happened, here's my guess:

1. The +16v capacitor unformed (or deformed or whatever you call it)
2. The orig owner thought, hmmm, maybe the +16 and -16 are backwards, let's try the other way...
3. POP
4. Then I buy it for $300 from a guy down the road
5. I replace the 4 tantalum capacitors on c15-c18
6. I turn on the power
7. The MC78M12CT (on the schematic it's IC4) starts to smoke (but the replaced c15-c18 are OK
8. Today - I tested the power (I now will always do this FIRST!....I am learning) with results below.

  Part num for +16v Elecrolytic. Capacitor by Bill Degnan - 04/24/2008 20:11

Picked up replacement 7812.

The writing on the capacitor is:
10000 25
7603 85 DEGREES C

(where "DEGREES" above is the degree sign (superscript "o")

  Progress on Imsai 8080 by Bill Degnan - 01/31/2010 18:02
Progress with the IMSAI 8080 that I have with the Thinkertoy Wunderbus backplane.

What was once a box of parts is now a semi-working computer.

Thanks to Mike and Dan for their help.

1. While probing the power supply board Dan sparked a diode, and somehow that woke it up, now functioning. There may have been something touching it.
2. Replaced a 12V Regulator on the backplane board
3. Re-assembled front panel.
4. Put in a new connector for the on-off switch wire from the front panel to the power supply board.
5. Replaced the 8080 processor chip on the CPU card.

Now the system works, but after a few minutes the front panel starts to flicker and is no longer reliably responsive.

1. Test chips on the front panel board, see if there is a short someplace, etc.



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