I just got a unit (a netMCA spinoff) which was basically short circuit at its 12V power input, 0.1 to 0.2 Ohm. The third part I unsoldered turned out to be it. An SCR, a C106. Its job is to short the input power (past the fuse) if any of the other 4 similar protections for the lower voltages (5, 3.3, 2.5 and 1.5) got triggered. Once I replaced the shorted SCR everything worked, no other victims No artifacts on the lower voltages, these never trigger except during development when I short something with the probe or something (I am quite impatient so they have come to rescue more than once).
Any idea what could have caused this SCR to get short? After nearly
5 years of service that one. I have never had this and we have netMCA-s (standard, this one is repurposed somewhat but not in a way relevant to what is discussed) and they typically run 24/7 for years, the first ones for over a decade now.I can't think of another reason than someone tampering with the unit (I had two units sabotaged at this customer some 2 years ago, but it was mechanical, someone had put some aspirin or something on the heaters (260C) and they got corroded - with the remnants of the stuff on them for me to find). So foul play is my first thought. But could it be something else? Part defect showing after 5 years...? I am just wondering, whatever the conclusion my course of action will remain the same (give them the repaired unit, that is; anything else could be reputation damaging).
------------------------------------------------------ Dimiter Popoff, TGI