A small USB supply, with a tiny switch a dual 8-segment display landed in my hands. No output voltage, nothing on the display, and PC reports overcurrent on a USB port.
A quick look under a glass reveals a part number for the main IC: FP5139 9Ri-83L (datasheet here) which is a boost converter. It also provides a sample application circuit. This is always useful.
After inspecting the board visually I could find any problems at first. I measured resistance between both input and output rails and found a short on the input side. I thought this could be a broken capacitor. They are quite easy to desolder so I took some of them out one by one (the ones that had a short in-circuit). It turned out they were all fine.
After that came the time for transistors. Quite tricky to get off the board with a soldering iron, so I used a hot air with a small nozzle. I started with the one signed Q1, and quite luckily a hit. A simple component tester revelas the damage:
So the transistor has turned into a resistor 😉 This cannot be. The sad thing now is, that you can barely squeeze any markings on a packege this small (looks like SOT23 package type) so the “N6FH” did not help me with identifing the part. Lukckily (and this is a small twist) I actually got a second converter as well (and the second one works) so I simply verified what this part was. Again, using the component tester I got the result:
Nice – a MOSFET with N channel in Enhancement mode (‘normally open’) – should be relatively easy to find attached to something in the parts bin… and a few minutes later I salvaged a similar part frome an old motherboard. There it is:
I know – it ain’t perfect but lets just shove it in and see what happens. In the meantime take a look at the broken transistor. What u can see here is what i failed to notice at the beginning:
So this ‘huge crater’ on the side… it must be like 0,5mm.
After soldering the new-old transistor the converter started functioning. I plugged it into a USB port and the display came to life. Output voltages switchable and present.
This was a successful little adventure. I know the replacement part is not a perfect match, but it will do in this case – quick fix of a small, cheap USB converter module.
CONCLUSIONS?
- Do a visual inspections with more attention to details.
- Get a proper third-hand or a ‘holder’ of some sort so u can be precise and not melt the display casing xD