View Full Version : Booting an IMSAI 8080

June 16th, 2014, 10:49 PM
A couple of years ago I picked up an IMSAI 8080 system complete with manuals and documentation, peripherals such as disk drives, tape drives, boards, 8" disks, and paper tape. After I bought it, I tested it out by turning it on, and the power works and I even managed to program in kill the bit and tested it out, but I remember it seeming to have some difficulty retaining code at times. I haven't tested it in several years but I assume it's still in this condition. I'm now trying to figure out how to boot it up again. I will probably go and buy an old terminal to use. Does anyone know what process I need to go through in order to test the memory and load a disk loader program in the computer? I would greatly appreciate any tutorial or information.

June 18th, 2014, 05:05 AM
Hi All;
Wiltors, Congratulations on the big haul !!
First, I wouldn't assume that just because, when you turned it on and nothing "blew up" that the Power Supply is OK..
At the very least, I would check ALL of the voltages with a meter, and at best check them all with a meter and a scope for ripple and such.. Also, there are Regulators and Zener Diodes and Caps on each Board, that needs to be checked, for correct Voltage(s).. Just, because one Board is good, Doesn't mean that they are all good..
This machine is getting Old enough that the Caps are possibly going Bad.. And needing replacement.. Especially the Big filter Caps on the 18 Volt line..
Then with all of Your Documentation, You need to see where you have any Proms and at what addresses, You might have more than one Prom, with more than one loader or program..
You don't have to get a Terminal to find out what is going on with the Machine, any PC with Tera-Term will do, as a Terminal for Your Imsai 8800..
One of the Programs that You might have in Prom is a Memory Test program.. Otherwise there are plenty of Testing programs out there.. And I have some laying around here somewhere..
Also, all the Boards need to be checked and possibly cleaned, the fingers would need to be cleaned, I use an Ink Erasure myself..
Before even thinking of trying to get it to boot from a drive..
Such as Reading the Paper Tapes first.. And / or running small Test programs, to verify that things are working..


June 21st, 2014, 06:07 AM
Yes, first do the voltage checks and such as recommended by Marty. We also need to know which boards your IMSAI has in order to properly instruct you on booting. Can you list all the boards you have and perhaps any disks or paper tapes you you would like to try?

June 28th, 2014, 04:40 PM
I'd suggest you start by verifying the RAM is OK before you try to boot anything. See if you can get "chase the lights" to work, and toggle in a RAM test if you can find one. This means booting with just the CPU card and a single RAM board. You need to read the RAM board manual, see how to set RAM (or at least know what RAM each board has and why it's set that way). Make a lot of notes about all of the jumper settings for all of the cards you have installed, assuming the cards at one point worked together with the current jumper settings.

You're asking for all of the planets to align just right to expect an IMSAI to boot up, without some quantity of troubleshooting, the first time.

But this is fun IMHO to learn how it all works, take your time, get it right.


July 30th, 2014, 05:44 AM
Thanks for all the input. I have powered this beast up when I got several years ago. Only one shorted Tantalum cap. (yea it exploded). After replacing it I was able to enter instructions from the front panel. I have a box full of S-100 boards I'll photograph and post in a few days. I also have a working Lear-Seigler terminal I could use with it as well as a variety of 5.25 and 8 inch floppy drives.