Just scored an IMSAI

From: John Lawson <jpl15_at_netcom.com>
Date: Thu Jun 24 15:18:45 1999

On Thu, 24 Jun 1999, Bill Sudbrink wrote:

> Well, much to my amazement, at lunch today, I got
> an IMSAI 8080. In beautiful condition, a few small
> scratches, almost no dust. Hasn't been up in years.
> I know we've done this before on this list, but...
>
> Where should I start?
>
> I figure:
>
> 1) pull all cards.
>



> 2) attach dummy load and check PS voltage
> and amperage.
> (What is an appropriate dummy load?)

   I might be a good idea not to do this step at this time... (see
below): the IMSAI power supply is a brute-force, very simple design
that puts out loads of current at slightly higher than needed
voltages, and then relies on regulators on the individual cards to
do the final power steps. I emphasize Lots of Amps available... it
is not a modern supply (like is used in most small computers today)
and does not require a dummy load to function. It is a Neanderthal
power supply.


  The most hazardous part of brining up an old brute-force power
supply like the IMSAI (and many other S-100 machines) is that the
large electrolytic capacitors have dried out or become 'unformed'
with age, thus presenting a dead short to the rectifiers thay are
hooked to. Thus you can get fireworks if you just slam the power to
it. If you can get the use of a Variac or similar variable
transformer of about 5 amps capacity, then place it in the mains
circuit and use it to s-l-o-w-l-y bring up the voltage on the power
cord, watching for signs of overload... bad humming, things getting
warm, etc. Do this over a period of maybe an hour or so... from )
vols, turn it up a few percent, leave it for a while, then repeat
until you have full mains on the power supply and you can measure
around 8 volts DC on the main power terminals. It is recommended
that you get copies of the docs for this before going much farther.
  
  Also there are many folk on The LIst who have these machines (as
do I) who can help you with bringing it up after it's long sleep.

 There is a way to do this with a bank of 100W incandescent lamps,
but that requires a lot of wiring stuff.

>
> 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.
>

  If power is good, I'd put in the CPU and see if the panel can
examine and load and address data...


> 4) install CPU and see if it "lights up"?
>

  Then the rest of it, too... did you get any disk drives? Other
peripherals? Documentation? Software??

  Nosy minds want to know...



  Cheers

John
Received on Thu Jun 24 1999 - 15:18:45 BST

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