S100-Manuals - Repairs and bringing up a System

It is impossible to give details for repairing every component for every system.  This is not an electronics repair course but does give some some clues for simple repairs.

Much of the board specific board testing can sometimes be found in the manuals for the boards, especially if the board was originally offered as a kit.  Many manuals can be found by following my mirror page, many other manuals can be found elsewhere including Herb Johnson's site listed in my links page.

It can not be overstated how helpful having the manuals are.

Do not assume everything on this page is from my mind, everything I know I learned from someone else, some over the past 50 years some picked up during the writing of this page.  Some things on this page are paraphrases others may include exact text from others.  I thank everyone I've gotten this information from.  If anyone has anything to add or has a correction to make you can email me at randy at s100-manuals dot com.

Basic tools:

Don't use cheap tools, these tools can do more damage than good.  Cheap tools often use soft metal instead of hard "tool steel", these soft metal tools do not "grab tight" screws & nuts which will damage the heads when they slip.

Don't use your grand-fathers soldering iron/gun that generates too much heat.

Do not use pliers when a wrench or nut driver fits, pliers will be more likely to damage the parts.

Hammers have little use when working on computers.

I buy most of my hand tools from Sears but there are plenty of quality tool manufacturers but even Sears sells cheap tools as well as decent tools.

Must have:  Screw drivers of different sizes/types, nut-drivers, small crescent wrench, small & medium needle nose pliers, small & medium wire cutters, small flush cutting wire cutters, small channel lock pliers, magnifying glass, flash light, DMM (digital multi-meter), soldering iron (either a quality soldering station or one of no more than 25 watts), and non-conductive tools (wood, plastic, whatever).  Non-conductive tools can be something as simple as a small wooden dowel to push/tap areas you are testing.

Desired and cheap tools:  Logic probe, logic pulser, and S100 extender card.  Please note ebay often has S100 extender cards listed also they often list wire-wrap cards that you can use as test cards creating front panel like cards (led's on important lines, using processor hold a simple circuit to stop, run, single step, slowly step).

Desired tools but a little more $:  Oscilloscope, frequency counter, soldering station, de-soldering station, and variac.

All of the desired tools can be found on ebay but remember Caveat Emptor (Latin for buyer beware), if you buy something used don't expect a warranty.


Remember the old joke "I am not saying that stupidity should be a capital crime but if we remove the warning labels from everything the problem will work itself out".

There are many safety issues when dealing with S100 systems:

General information:

PC boards - Printed Circuit boards are generally fiber-glass cloth with epoxy resins that are made into sheets.  Copper traces are plated on these sheets to do the wiring that otherwise would be done by point-to-point wiring.

PC boards can have any number of layers that have copper plated on.  Commonly S100 PC boards use two layers, one on each side.  Normally all the components are mounted on one side referred to as the top.  The different layers usually have places where one layer electrically connects to another layer, this is done by drilling a hole through the board and plating the hole with copper (called plated through hole).

Heat is the enemy of PC boards, often when you look at the board you will see discolored areas that are evidence of heat damage (brown to black).

To avoid damaging PC boards use as little heat as possible when soldering/de-soldering.  Over heating can delaminate boards or lift the copper traces.

Damaged copper traces and pads can be replaced with either 30 gauge wire possibly held in place with glue or fingernail polish or the traces and pads can be replaced with professional trace repair kits that use adhesive backed strips of copper etc.

Capacitors have a high failure rate on older equipment.  Capacitors can easily be tested before power is applied, electrolytic & tantalum capacitors are the biggest problems:

Resistors can burn-out and can be grossly discolored (dark brown to black).  Resistors can also be tested in or out of circuit but have a low failure rate.  An in circuit test may show either low resistance one way and normal to a little lower the other or normal to a little lower both ways.

Integrated circuits (IC's) can sometimes have obvious gross damage.  Tarnish can eat away the leads, chemicals like Tarn-X can remove the tarnish.  An IC can explode or have a bulge that is grayed and shows cracks.  IC's are normally tested in circuit but usually requires a logic probe or oscilloscope to track or even a logic analyzer.  Much testing can be done with a fairly inexpensive logic analyzer.  When replacing IC's it is common to have a pin fold under and have poor connection, I've found bent IC pins that suddenly cause problems after years of use.  Using a magnifying glass you should examine the IC's in their sockets.

Voltage regulators are specialty IC's and are easy to test with a DMM.  You should use heatsink compound on all voltage regulators or transistors attached to heatsinks.


Fans not working or working poorly should be replaced, as previously stated heat is the enemy.

In the past many people worried about ventilation on their S100 systems.  I learned long ago not to waste the chance to get advice from experts.  I asked Thomas "Todd" Fischer's advice on ventilation on Imsai's:

My advice is to leave the ventilation alone. As originally designed, Joe Killian's approach to convection cooling (fans were an option) worked pretty well, as long as the 6 x 4 column of vents were to the front and right of the front panel (in order to allow cooling of the front panel heat sink. Much alchemy was practiced over the years as many "experts" preached their wares to overcome the perceived evils of "pizza oven syndrome". Nothing  ever proffered by these efforts really stuck, and the stock IMSAI lives on.
Best regards,

-Thomas "Todd" Fischer

Both the boards and the power-supply need proper ventilation.  If in doubt get a thermometer and measure the temperatures in different areas.  It may sound strange but the more boards in an S100 system the cooler the boards generally run:  This is because of two issues: 

Many people blocked off some of their ventilation holes to force air along a path of their choosing.  Keeping Todd's advice in mind think long and hard before changing much.  With a fan (an option on Imsai's that I recommend) you may consider blocking the right side of the chassis to help force air across the S100 boards.  I use a CompuPro chassis that is configured like an Imsai, the power-supply is on the right side and it includes a fan.  CompuPro only has vent holes along the left side which forces the air across the boards before exiting through the power-supply.  A higher CFM fan never hurts but can be noisier.

The real problem was not the chassis the problem was usually the cards, many cards just plain generated too much heat.  Changing the fan and ventilation paths were just compensation for some boards.  I had a 32K memory card (when most cards were 16K) that got way too hot.  I ended up getting a second identical card with 0K and put 16K on each.

I do not recommend blocking any air vents but if you decide to:  Never block air from flowing past the heatsinks and do not block off so much that you restrict air flow!

Check out checklist:

Bringing up an S100 system that has set a while requires careful work:

A very handy and easily constructed tool is a light bulb socket and switch in series with a power outlet. This will current limit the source supply and can prevent a lot of damage - different wattage bulbs will give you different levels of current limiting.

A linear supply (like the IMSAI) should draw very little current after it charges the filter caps if all of the cards are out, - start with say a 40 or 60w bulb and power it up - you should see the bulk glow briefly and then go out (or nearly out) as the supply reaches operating voltages.

If the bulb lights and stays lit brightly, then there is likely a short somewhere (this is where you would probably have seen smoke had you not used the light bulb tool) - check rectifier diodes and filter capacitors etc. and on to usual troubleshooting.

Many smaller devices will happily run in series with a 100w bulb, but be aware that the tool does cause a voltage drop if the target is drawing power (the more power drawn, the higher the voltage drop). Even if the target won't run, I usually power "everything" the first time through the bulb as well - Unless it's a very big (power-wise) item, you can usually tell the difference between "normal" and a "excessive" draw by he brightness of the lamp (takes a bit of experience). Generally, you do not want to power larger items in this "brownout" state for lengthy periods of time, as some devices don't really like it much!

One final note - variac's are very useful, but with switching supplies you need to be sure to get it up to operating voltage - I generally use a combination of the variac and bulb tool depending on the circumstances.

There are lots of people and resources available to help.  Some people will want to be paid for some services.  As I see it a collectable computer or a collectable car pay the professional and accept help from the amateur!

www.imsai.net is a great site, Herb Johnson is a pro and a great person to go to for help, www.classiccmp.org has two great mailing lists, on Usenet comp.os.cpm is a good place to ask questions.

The links page has many pointers to other sites.

The following is a Horizon specific repair guide from Dave Dunfield, most everything applies to most S100 systems:

I recently gave this advice to a guy who wanted to power up an out-of-service Horizon - This is also an S-100 system, but a bit different from the IMSAI in that is has built-in drives, and no front panel - For the IMSAI, once I got to the "power on with boards" test, I would begin with only the CPU, one memory card, and the front panel, and see if I can operate the front panel (deposit/examine memory etc.) before proceeding to test the other cards and complete system.

As always, first comes a good visual inspection - remove all the boards, look for physical damage and corrosion. Check that the fans turn smoothly, if you can get at them, check that the drives motors/hubs and head stepper all operate smoothly. Manually position the steppers at least 1/2 way "in" - ie: a good distance from track 0 (you will see why later).

Next, checkout the power supply while the cards are still out. Before you proceed, disconnect the power connectors from the disk drives.

I believe the Horizon is a linear supply - I would normally use a Variac to bring the power up slowly, allowing the big filter capacitors to reform. Another good tool is a light-bulb wired in series with the hot line of an AC power receptacle. This limits current and prevents major damage in the event that something is shorted. Start with a 40w bulb, which should be OK with no cards in. the the bulb glows brightly when power is applied, try a 100w bulb. If the 100w bulb comes on when no cards/drives are installed, there may be a power supply problem.

At this point, the fan should be running, and the front panel light should be on. Using a voltmeter, check the voltages on the drive power connectors. The two center pins are ground (an some systems only one of them is connected), and the two outer pins should be at +5v and +12v - these are regulated so the voltages should be within .25v of those values. Let it run for a few minutes and then check it again - to insure that something is not overheating and causing damage. (Always trust your nose - if you smell something "hot" or see smoke, always shut down immediately and find the cause).

It would be worth checking the +8v and +/-16v lines on the S-100 bus at this point (with the cards out) if you can - but this can be tricky to do without an extender card (I have a card made up with readily accessible taps for the power supplies). Sometimes you can access this easily from the filter capacitors. If you decide to measure on the S-100 connectors, be VERY careful not to short to the neighboring pins. I usually have several multi-meters going so that I can monitor all the supplies at once during the complete initial checkout.

An annoying thing about the Horizon is that it basically doesn't do anything unless everything is working. There is no ROM startup, so you pretty much have to put everything back in and boot it before you can see what is working.

Power off, wait a few minutes, reconnect the drives and install the CPU, Memory and Disk Controller cards. It probably won't run through the light bulb, however power it up through a 100w bulb the first time - if you hear any "pops" or smell/see smoke, power down immediately and perform another visual inspection to locate the damaged component. if all looks well, remove the light bulb and repeat the test with "full power".

During the above tests, pay attention the the drives - At some point, you should see drive 1 activate, and hear it stepping back to track zero. It will only step back once - it may happen during the "light bulb" test, but if not, it should have done so during the "full power" test. - If you missed it, you can visually check the position of the drive head to see if it did move.

Carefully check all power supply voltages.

If you get this far, you are ready for the "big test".

Make sure you WRITE PROTECT your system disk.

Connect a terminal, I believe the normal Horizon settings are 9600 bps, 8 bits. If you have one, connect a light box in between the terminal and the computer - this will let you see if the computer prompts even if the terminal is set "wrong".

Let the terminal warm up until you see the cursor. Get the disk ready, inserted about 1/2 way in the drive. Turn on the power, and immediately insert the disk the rest of the way and close the door. If all goes well, the system will boot up and you should get the OS prompt.

Herb Johnson (Dr. S100) has a great website supporting S100 systems and more.