When the rail is wrong
A rail that is dead, low, the wrong voltage or keeps cutting out has one of a few causes, and a meter finds them in order: the socket side of the fuse, after the fuse, after the diode, the rail. Stop at the first reading that is wrong.
Work along the path
Every board puts the same things in the same order: the USB-C socket, the fuse, the diode, then the switch or regulator that makes the voltage, then the rails. A meter on each in turn finds the first place the voltage is wrong, and the fault is just before it.
The black probe goes on any − pin: all six are the same ground. The red probe goes on the point. On every board the fuse is the small pale rectangular part just below the USB-C socket, and the diode is the larger black one beside it. Their metal ends are the "before" and "after" points: one end of the fuse always reads what the socket is getting, tripped or not.
The four symptoms
Dead. 0 V at both ends of the fuse is the cable or the charger. 5 V at one end and 0 V on the rail is the fuse, tripped: there is a short, or there was one a moment ago. Unplug, find it, wait a minute, try again.
Low. A few tenths of a volt under the socket is the diode and fuse, and it is normal. More than half a volt under means more current than you think, or a breadboard contact with resistance in it: measure at the adapter's own pins, then at the far end of the rail. If the far end is much lower, the breadboard is the problem.
Wrong. On the standard, look at the LED: blue is 3.3 V. On the step-down and booster, measure while turning the trimmer. A step-down that will not go above about 3.5 V is at its ceiling; a booster that will not go below about 4.3 V is at its floor. Both are how those circuits work.
Cutting out. A rail that dies under load and comes back a minute later is the fuse reaching half an amp. The fix is less current, not a different fuse.
Before any of it
Look at which way round the adapter is. A board that is the wrong way round reads perfectly on a meter — the voltage is fine, it is just on the rail marked for ground — and every part wired by the printing sees it reversed.
When it does not work
Start with the cable. Some USB-C cables are damaged or wired for charging one particular device, and a cable that works on a phone is the quickest test. Then the charger: plug a phone into it. If both are good and there is still nothing, measure the fuse's two ends; 5 V on one and 0 V on the other means it is tripped, and there is a short somewhere on the rails.
Everything before the switch or regulator is shared, so the input is fine. On the standard, check that side's switch is pushed fully to one end. On the step-down or booster, measure that side's rail while turning its trimmer: no change at all means that side's regulator, or the trimmer, is the question.
Something else is powering it: a dev board plugged into your computer with its 5V pin wired to the rail, usually. The adapter's diode stops that reaching your charger, but it is still two supplies on one breadboard. Decide which one powers the rail and disconnect the other's supply wire.
Unplug at once. Check which way round the adapter is against the breadboard's printing, then check the voltage on that side against what the part needs — a 3.3 V part on a 5 V side, or on a booster left at 12 V, is the usual cause. Then look for a wire across + and −.
The circuit draws more than that board can give at that voltage. On the standard that shows as the 5 V rail sagging under about 4.5 V, or the 3.3 V sagging past 200 mA. On the step-down it is heat, on the booster it is the half-amp fuse. Measure the current, and see that board's chapter for its limit.
Every article, and the reference: pins, ranges and what is in the box.
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