16 Aug
2015
16 Aug
'15
3:35 a.m.
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