I was asked to have a look at this digital scale. The LED in the bottom left corner came on green when power was connected, but nothing happened when the ON/OFF button was pressed.
After some measuring, I found there was no 5 V rail. The power to the two 5 V regulators is enabled through Q2 and Q1, an “inverted Darlington” pair. Q2, a S9013 NPN transistor is grounding the base of Q1, a S9012 PNP transistor that in turn connects 8 V to the input of the 5 V regulators.
I replaced the S9013 with a 2N5551 from the parts bin and the S9012 with a C557C (which has the pins reversed!).
And now it works again! 🎉
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