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
