TESTING TRANSISTORS WITH CURVE TRACER
I unsoldered all the transistors from the board so that I could put them on the curve tracer to verify whether the transistor was working properly. There are two 026, an 086, a 123 and a 108 transistor that I removed. I have ready replacements for some but I don't have a spare 123.
Originally the failure occurred after the regulator has been operating for a while, was powered down, and then power applied again within a minute or two. That may mean that when the transistor heats up its behavior changes, so I will apply some heat and cold air while testing the transistors.
I tested the 123 and 086, which seemed to behave properly and were not so sensitive to heat or cold that they misbehaved. I unmounted the 108 transistor from the heat sink and found it to be quite flakey on the curve tracer.
REPLACING SUSPECT TRANSISTOR
I used a spare 108 that I owned to replace the bad one. In addition to the spare I used, I had bought a large set of 108 transistors from eBay years ago. When I began testing them, I found wildly different amplification ratios (beta) among them, no two the same. I wonder if these were simply printed with 108 and sold as rare transistors, but are actually various PNP parts of a different type. Counterfeit integrated circuits are a widespread problem, but perhaps I blundered into some counterfeit transistors.
RESULTS OF POWER CYCLING
The machine was much better, so much so that I had begun to believe the problem had been fully resolved. However, I did manage to get it to fail in the same way - oscillations around 3V - it just took more to force the problem. Drat.
POSSIBLE NEXT STEPS
I can test the two 026 transistors, since I do have spares to use. I did pull the two diodes and test them, which also let me determine that an IBM DY diode is a 3V Zener.
I could also test the six 108 transistors in the rest of the regulator, which might be the failing component somehow.
No comments:
Post a Comment