Just scored an IMSAI - test-drb@ccmp.vtda.org

24 Jun 1999

I once found a handy gadget for seeking adjacent line shorts on the S-100.
It's essentially a board with the edge connector and little else, except
that the pins on one side are connected.  Since the lines tend to be laid
out alternating from one side to the other, your audible squeaker will allow
you to test one side at a time with the "other" probe tide to the opposite
side.  Finding shorts is really quick and easy with this.  It's easy make,
though several vendors sold them at one time.  Fortunately, it's built
without the part of the board which would prevent you from reversing it, so
you can do just that.  An unaltered board with the connections on one side
shorted wouldn't quite work due to the mechanical offset which prevents you
...
 from reversing a board. 
Dick

-----Original Message-----
From: James Willing &lt;jimw(a)agora.rdrop.com&gt;
To: Discussion re-collecting of classic computers
&lt;classiccmp(a)u.washington.edu&gt;
Date: Thursday, June 24, 1999 2:20 PM
Subject: Re: Just scored an IMSAI

...
 On Thu, 24 Jun 1999, Bill Sudbrink wrote:

  Well, much to my amazement, at lunch today, I
got
 an IMSAI 8080... 
Congradulations!

  Where should I start?

 I figure:

 1) pull all cards. 
Yep...  Then I frequently do a quick 'shorts' check on the S-100 backplane
before proceeding.  Take a ohmmeter on a low resistance setting and step
across the opposing pins with the leads.  Start on pin 3, pin 53, then pin
4, pin 53, then pin 4, pin 54, and onward across the backplane.  This will
quickly catch any adjacent line shorts.  You may also reverse the stepping
sequence in case the backplane is an oddball and the lines are interleaved
differently.

Skip pins 1,2,51,52,50, and 100, since these are the power supply pins and
will give readings back thru the power supply components.

(note: pins 1/51 = +8VDC, pin 2 = +12VDC, pin 52 = -12VDC pins 50/100 =
GND)

  2) attach dummy load and check PS voltage
    and amperage.
    (What is an appropriate dummy load?) 
Well... (and I'll probably catch some heat for this) since the IMSAI
supply is an unregulated linear supply (regulation is done on each
individual S-100 card), a 'dummy load' is not really necessary for a basic
check.  (switching supplies need a load or they get unhappy)

There are only three voltages on the bus, +8VDC, +12VDC, and -12VDC, which
in reality are usually closer to +10~12VDC, +16~18VDC, and -14~18VDC.

With the cards out, you can check these with a voltmeter.  Now, where you
may want to load things up a bit is for a 'ripple' test. (looking for
bad/leaky capacitors &/or bad rectifier stacks.  Although a bad recifier
will often cause the line fuse to blow)

Depending on the amperage rating of the supply, (the IMSAI is a honker!) I
have a bank of automobile lamps (headlights for high amp, taillights for
lower).  I gang these together for the appropriate voltage/amperage
ratings and 'light it up'! (pun intended)  Then you can look for odd sags
in the voltage, and if you have a 'scope look for ripple on the suppliy
lines.

  3) check front panel mechanically.  Use an
    OHMmeter to be sure that all switches are
    opening and closing correctly?  I seem to
    remember that some people said that front
    panel disassembly/assembly is tricky. 
Not THAT complicated, but there are LOTS of screws, spacers, etc...

Usually I just do a 'click' check on the switches, (toggle them and
listen.  Bad ones will often make either a muffled sound or no sound at
all), and then do a function test once the panel and CPU boards are
plugged in.

  4) install CPU and see if it "lights
up"? 
After a final 'visual' check for bad/damaged/improperly installed
components, etc...

Ever onward!     YMMV!!

Regards;
-jim
---
jimw(a)computergarage.org
The Computer Garage - http://www.computergarage.org
Computer Garage Fax - (503) 646-0174

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