[vcf-midatlantic] SASI Hard Drive Controllers

Dan Roganti ragooman at gmail.com
Sat Dec 17 16:04:48 EST 2016


On Sat, Dec 17, 2016 at 11:03 AM, Dan Roganti <ragooman at gmail.com> wrote:

>
>
> On Sat, Dec 17, 2016 at 10:06 AM, william degnan via vcf-midatlantic <
> vcf-midatlantic at lists.vintagecomputerfederation.org> wrote:
>
>> Related to my post yesterday, I have a SASI XEBEC 104526 rev 4 that works
>> ok when there is a fan blowing on it but overheats after a while.  It
>> controls a SASI external hard drive used by a Visual 1050 computer.  OK, I
>> will look through the electronics and replace failing components and sure
>> it up.  Seems like the typical course of action for an XEBEC and other
>> (SASI) external controllers.  I am also thinking about the CBM 9090 and
>> related 5-10Mb drives. 35 years in, they all seem to be failed or failing.
>> Similar XEBEC controllers for things like IBM PC's seem to have fared
>> better.  Why?  Or just a coincidence?  Is there a common component that
>> fails more often in SASI controllers but not IDE and others of the era?
>>
>> I know the history of SASI/SCSI and I have some XEBEC docs.    I can
>> research further on my own, but maybe someone who actually supported these
>> back when they were new has some casual knowledge to pass along.  How
>> reliable were these when they were new/current?
>>
>> Bill
>>
>
>
> ​I don't have any repair history of that board, I think that's a long shot
> Unless you find someone who worked at XEBEC, they would have a more
> comprehensive view of failures.
> And most if not all of those components are well beyond their mtbf rating
> already.
> So that basically means, anything can go wrong.
> But the basic repair steps for something like this include several options
> One of which is using a ol' tried and true method of finding the hot
> component, your finger :)
> That's if the slot has room for you to get in there.
> There's other options such as using a temperature probe[thermocouple]
> adapter for your DMM.
> So you can measure the heat from each component.
> Also, for these tricky problems it helps to have a thermal imager -
> they're getting cheaper every year.
> Then you can pinpoint the source of the heat, but first turn off the Fan :)
> Dan
>
>
>
>
​Perhaps it wasn't clear enough but this is a painstaking task
You have to methodically eliminate each potential source,
Until you arrive at the culprit.

Since the computer can still operate when that board overheats,
that means the component is not shorting out the power supply
As it will not be a drastic heat differential, as with any dead short.
So this typically indicates that one of the output signals, as well as,
one of the input signals can be shorted.
That is the most common type of failure with old TTL Logic.

So this will .not. create a large heat differential, as in smoke.
Therefore the reason to measure the temperature of the components
But this is still also a meticulous process, you need patience.
​Unless you have schematics, O'scope and an understanding of the operation,
and know which signal to check on the scope,
it can be a tedious job to uncover the problem.

I'm not sure if I found the right document on your website
But one showed this card has about 30 IC's on there.
So with using one of the options I mentioned before,
you would have to record the temperature of each part
while it's running with Fan on so you know it's operating correctly.
Then again without the Fan as it begins to fail.
Usually there would be enough of a temperature differential, maybe only
5deg,
to identify the culprit.
​Dan​



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