We’ve all been there. [Kasyan TV] had a universal adapter for a used laptop, and though it worked for a long time, it finally failed. Can it be fixed? Of course, it can, but it is up to you if it is worth it or not. You can find [Kasyan’s] teardown and repair in the video below.
Inside the unit, there were a surprising number of components crammed into a small area. The brick also had power factor correction. The first step, of course, was to map out the actual circuit topology.
The unit contains quite a bit of heat sinking. [Kasyan] noted that the capacitors in place were possibly operated very near their operating limit. Since the power supply burned, there was an obvious place to start looking for problems.
One of the two synchronous rectifier FETs was a dead short. Everything else seemed to be good. The original FETs were not available, but better ones were put in their place. A snubber diode, though, appeared to be the root cause of the failure. Testing with a programmable load showed the repair to be a success.
Of course, you aren’t likely to have this exact failure, but the detailed analysis of what the circuit is doing might help you troubleshoot your own power supply one day.
We were surprised none of the traces burned out, but that can be fixed, too. Oddly, this cheap supply looked to be better than some of the inexpensive bench supplies we’ve seen. Go figure.
at least 20 years ago I repaired an original laptop PSU.
Just measuring its output voltage everything looked fine but charging the laptop just wouldn’t work – not to mention running it only from the PSU.
But measuring the output voltage with an analog(!) meter while it was connected to the laptop immediately showed something “wrong”: The voltage was dipping a lot below the specced 19V in a constant repeating equitimed(?) “pattern”.
(AFAIKR)
Turns out a massive choke(?) (copper wire wound ferrite donut core) had mechanically “broken loose” and ripped out one of the copper wires/connectors from the PCB.
Re-soldered the choke/impedance/inductance, glued it down and the PSU worked fine again.
Repaired and old projector the “same” way – the light bulb driver PCB (ballast?) had a loose, disconnected choke.
Loose chokes are the most common failure I see in hand held tools in my workshop, torches and so on. Seems the weight of the choke + abuse doesn’t go to well together. Thankfully as you found hot glue and solder make an easy fix.
PFC? Synchronous rectifier? On a laptop power brick of all things?
What is the world coming to? Back in my day they had a UC3843 and a flyback transformer that was wound by a child somewhere.
Synchronous rectifier? Nope, just undersized rectifier diodes with white goop on them (should take care of the thermals right?).
The problem usually is opening them so that they can be safely closed again. You have to either saw the edges or break it open by unreasonable force. A squeeze with a vise can sometimes work. But you have to glue it together if it still fits back together or find a new enclosure for it that’s impact resistant (the rigours of portable computing).
I did repair quite a few monitor and LED strip PSUs though. Not worth it time/cost wise but if it’s fun for you, it can be quite rewarding pastime.
Try this next time:
A 1” wide wood chisel put into the seam.
A light and fast whack to the back of the chisel handle with another tool.
You may need to work your way around the seam in a few places.
Cracks the seam super easily, then just glue back together with CA glue.
Nice fix. But I agree, probably worth only the educational value.
I once converted (read: completely rebuilt the secondary of) an old PC power supply to provide 13.8V at some large number of amps, to be used as a power supply for an old mobile 50MHz transceiver. It worked, and I learned a lot about how those supplies are designed. Enough to preserve the OVP and OCP circuits.
If it was the standard 18-20v supply I might agree, but this one usefully seems to cover 16-24v in 2v increments (although the 25v caps on the output would be better as 35 or 50v).
I generally keep a large supply of these power adapters on hand, and I have noticed laptops tend to be OK with +/- 20%. Have run the 16v ones off of 14v, and the “18v” ones don’t mind 20v and vice-versa.
Voice of the video seems generated from text, isn’t it ?
And it’s very annoying when (pause)
it seems to pause for line breaks.