Shugart 851 power problem

From: <(>
Date: Tue Apr 25 15:20:41 2000

>#2 Power up the IMSAI box:
>At the drives:
> 5 volt line reads 1.2
> -5 volt line reads -0.9
> 24 volt line reads 2.6
>and (needless to say) the drives don't work. The Cromemco
>ROM monitor gives all sorts of errors trying to access them.
>[And other times when you power it on things are fine]
>What gives? Does this make sense to anybody out there?

It looks to me like one or more of the stepper motor phases are shorted
(or their drivers are shorted) by garbage signals that come over the 50-pin
Shugart interface from the IMSAI when it's powered on. Garbage signals
are a fact of life, and the circuitry on the drives shouldn't respond in
such an awful way to them.

Two things to do:

1. Ohm out the stepper motors looking for shorted phases.

2. Check the driver transistors on the 851's. If any are hot when the
problem occurs, it's almost certainly related. It's not impossible that
the circuitry that drives the driver transistors is screwed up, too.

It wouldn't surprise me if the foldback protection in your floppy power
supply folds all the outputs back simultaneously. This certainly seems
likely based on the voltages you see.

If you can unhook the 24V output from the power supply (this is what runs
the stepper motors) and try again, this might be useful.

 Tim Shoppa                        Email:
 Trailing Edge Technology          WWW:
 7328 Bradley Blvd		   Voice: 301-767-5917
 Bethesda, MD, USA 20817           Fax:   301-767-5927
Received on Tue Apr 25 2000 - 15:20:41 BST

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