View Full Version : Restoring an IMSAI 8080

February 26th, 2008, 09:53 AM
I thought I saw the preferred power on steps for restoring an IMSAI, but I can't find them.

I know, first document where everything is. Then...
Remove the boards
Put in a resistive load for the power supply (what load though?) and test the power supply first before anything else.

What other general steps should I follow?


February 26th, 2008, 10:54 AM
You got an IMSAI! Oh boy..it'll put Tandy into a new light.

I have learned a lot in the past few years, but I am not really qualified to give advice like some of the others on this board. That said if it was me, you can first check for resistance/ohm issues using the schematic without even powering on the system. Once you have found and fixed all of the bad solder joints I would remove all cards including the front panel board (which is not fun) before I powered anything. I have pics of various stages of the process on my web site.

You can check the 5V / 12V / 8V / ground pins and connectors to be sure they're getting proper power. Once you have confidence that the electric is flowing OK, start by adding the front panel and CPU cards. There's enough RAM on the CPU for testing the toggle switches.

Please consult with more than one source, I may have missed something. Just putting in my 2 cents.

Good luck!


February 26th, 2008, 11:25 AM
Yeah. Probably paid too much. It's an IMSAI 8080 with an 8080 and z80 cpu card, several memory cards and 2 8" drives.

I purchased it from the original owner. It's been on display in a store window for the past 10 years. Should have the original manuals and a visual terminal also. Whoo Hoo!

February 26th, 2008, 11:52 AM
I remember you mentioning it to me a while back. I am glad you finally saved it.

I have a lot of notes going back to 2005 about my IMSAI projects on vintagecomputer.net. I have one that works (described below) and another that I am working to restore the main board which was upgraded to a Thinkertoy Wunderbus.

With a Z80 card and CP/M it should be relatively easy to make a usable system once you get through the electrical problems.


February 26th, 2008, 03:17 PM
Just turn the power on and look for smoke!

I'm kidding of course. Before turning anything on, clean everything. This is particularly important with the front panel PCB and the backplane. If you don't want to take the backplane out completely, which can be a bear, vacuum it good and then turn it over and give it a good shake. Disassemble the front panel assemble so that you can get to the front panel PCB. Look for buildup of crud and clean with some denatured alcohol. If it's really bad, pull all of the IC's and give the board an alcohol bath. The reason for this cleaning exercise is that I've found the front panel extremely sensitive, resulting in stuck address / data lines and flaky timing during examine and deposit operations.

Once all the dust bunnies have been chased out, power it up, with no boards installed, and verify you're getting +8 and +/-16 out to the backplane. Next, put back the front panel and a CPU card. Power it back up and yes, this time, look for smoke. If you have a scope, now is a good time to check each of the DC power lines for ripple. If you see anything significant, you probably have lost one of the filter caps.

Now it's time for the fun part. Press and hold the reset lever up. You should see all of the LED's go on. When you release it, all of the address LED's should go out. If I remember correctly, the data LED's will remain on. You should now be able to set the address LED's by flipping the address switches and toggling the examine switch. Test each address bit looking for ones that are either stuck high or low. You should also be able to toggle the examine-next or deposit-next switches and see the address increment by one each time. Now you can add a memory card and see if you can peek and poke data into memory. Again, look for stuck bits. This time, not only address bits but data bits. If everything is found working up to this point, you're in very good shape. If not, then it's time to pull out the schematics and get to work.

Have fun!

February 26th, 2008, 05:54 PM
With any new machine, I do most of what Mardy says, with the following exception. I spent a few $$$ on a variac (variable transformer) and I built a series resistive load using some power cords and a light bulb.

Connected in series with the power input of the unit under test, the resistive load acts as a current limiter (use a 40w or 60w bulb) and if there's a short, the bulb will burn brightly. I use this coupled with the variac to control the mains voltage going into the machine under test. The slow increase in input voltage using the variac can shake things loose, like dried out caps, etc. There was a thread about this on Classiccmp a while ago. It gives me an element of control. I never liked just flipping the switch and hoping for the best (although I've done it). I've never had anything blow.

Dwight Elvey
February 27th, 2008, 07:50 AM
Automotive lamps make OK loads. Even though they are called
12V, they usually run at higher voltages in the car. For the 8 volt
wire, a 6V brake light is fine. For the 18 volt lines, a 12v brake
or other light is fine.
They'll run at the higher voltage for quite a while. 12 volt
bulbs normally run at 14.5v while the engine is running.
Bulbs are constant current like over their operating range
so the power used doesn't go up as the square of the voltage,
it is more linear with voltage.

March 23rd, 2008, 10:20 PM
For any of the high wattage power supplies in my machines, I use a dummy load which is wired to dissipate up to 200w for the +/-16V power rail and 200W for the 8V power rail. I added switches so I can increase the wattage one step at a time (turning off the power between each step). This really stresses the power supply so you can check the voltage ripple under a high load with a scope to know if the caps are functioning properly.

However some machines may not specify what the maximum allowable ripple voltage should be at a high power load. For my Imsai, the manual only states that it's capable of 28A, 500W without providing the individual output specs of each voltage. So you have to use your judgment when considering the voltage ripple. The 7805 minimum input can be as low as 7v and the 7812 min. input can be as low as 15v -IIRC-. So if the volt.ripple dips significantly below this under high power load, then you should look into getting the caps replaced. I've been running my Ismai since '88 and my buddy had it before that.

When finding any machine, you may want to consider reforming the caps too, since they may have been sitting idle for some long period of time. I always fire-up my machines a couple of times per month to keep them 'juiced' to avoid having to reform the caps. But if you replace any of them you want to consider reforming them too since they mostly likely have been sitting at the surplus parts supplier for a long time--unless they're brand new parts--.

The dummy loads are made using 8ohm,50W power resistors and they get hot, so you need a metal enclosure. I just used a thick gauge metal screen that I folded into a case and mounted them inside with a sets of bolts. This gave it enough ventilation when running for several minutes.


April 12th, 2008, 04:40 PM
Well, the IMSAI is finally here. It is a really clean (physically) unit.

It has a 5 slot backplane with the 8080 CPU board, 64k dram board, tarbell floppy controller, and an SIO 2 card. One open slot.

I also got the dual 8" Lobo drives and a Visual 200 terminal. I probably paid more than I should, but it is here. I've been looking at this machine in the window for many years now. I can't wait to fire her up, but I want to clean it out and do it right.

The original owner also has a 16k sram board somewhere. When he finds that he'll drop it off for me.

It also included 2 Imsai manuals on setup, the manual for the SIO board and the visual terminal, and the original 1978 receipt.

April 12th, 2008, 09:03 PM
I have never seen a Lobo twin 8" drive with a Tarbell controller, in action.

Terry Yager
April 12th, 2008, 10:27 PM
Yeah, c'mon...where's the pix? That terminal is very kewl...


April 13th, 2008, 09:34 AM
OK. Here are some initial pictures. Click for bigger version:
http://www.catcorner.org/IMSAI/Images/IMSAI-Lobo-Drives_THM.jpg (http://www.catcorner.org/IMSAI/Images/IMSAI-Lobo-Drives.jpg)

http://www.catcorner.org/IMSAI/Images/Lobo-Drived-Logo_THM.jpg (http://www.catcorner.org/IMSAI/Images/Lobo-Drived-Logo.jpg)

http://www.catcorner.org/IMSAI/Images/IMSAI-Lobo-Drives-From-Back_THM.jpg (http://www.catcorner.org/IMSAI/Images/IMSAI-Lobo-Drives-From-Back.jpg)

http://www.catcorner.org/IMSAI/Images/Manuals-CPM-Visual-Terminal_THM.jpg (http://www.catcorner.org/IMSAI/Images/Manuals-CPM-Visual-Terminal.jpg)

http://www.catcorner.org/IMSAI/Images/Manuals-IMSAI-IO4_THM.jpg (http://www.catcorner.org/IMSAI/Images/Manuals-IMSAI-IO4.jpg)

http://www.catcorner.org/IMSAI/Images/OriginalInvoice_THM.jpg (http://www.catcorner.org/IMSAI/Images/OriginalInvoice.jpg)

http://www.catcorner.org/IMSAI/Images/Visual200Terminal_THM.jpg (http://www.catcorner.org/IMSAI/Images/Visual200Terminal.jpg)

Terry Yager
April 13th, 2008, 11:42 AM
Love that terminal, it's just so vintage!


Dwight Elvey
April 13th, 2008, 08:10 PM
The Tarbel was a common controller. You should be able to find
CP/M disk images quite easily.
I can't help much, mine uses a Digital Systems controller. See: