Shugart 800-8 media centering problem

[Jay Jaeger]

On 8/15/2015 3:13 PM, Jay Jaeger wrote:

> 
> To make matters worse, that test with the first SA 801 managed to smoke
> the 24V power supply on this system, so now I have to pull it all apart,
> and pull out the regulator transistor which is mounted from the reverse
> side through to the board and soldered on the top side of the board.
> (possibly 3 of them, if that transistor isn't obviously the problem).  A
> royal pain in the arse.
> 
> So, there won't be any progress for quite a while.  Maybe months.
> 
> Sigh.
> 
> 

Got the power supply series pass transistor out from the backside
without all *that* much difficulty.  Soldering irons with temperature
sensors are "the bomb", though I may have been able to manage it with
the same 45W de-soldering iron with built-in bulb I used to clean out
the holes.

The power supply was definitely a victim of over-current: the series
pass transistor is blown open (2N3055, fortunately and oddly, available
from the local Radio Shack), and 4 overheated resistors, one of which is
in series with the output - that one is actually cracked.  I have a
spare of the same resistance and probably greater wattage as that one
that I can pull from a different dead/unused power supply. The other
resistors are all ordinary 1/2 W.  The transistor driving the series
pass transistor seems fine based on a simple ohmmeter test.  So sometime
tomorrow or so, I ought to be able to fix it.  (It fooled me for a
little while to - even with an open series pass transistor, there is
still 24V output - it only dropped out (to about 2V) when the head
loaded on a drive.)

I am still not 100% sure if I shorted something while I was testing -
the test drive was placed on top of the system, or I did some other
silly thing with the jumpers such that two steppers ended up active at
the same time (I don't think so), or if the SA-801 is just bad.  I'll
have to rig up a test where I power the 24V to the SA-801 from a current
limited bench supply I have before I try it again.

JRJ