Electronic load repair

I wanted my first post to be interesting, but it was a facepalm instead.

While using my aliexpress cheap electronic load to test a 12V lead-acid battery, I smelled some burnt smell magic smoke and it showed 0A of current regardless of the position of the pot meters.

Original Aliexpress photo about the electronic load

I removed the big heatsink and before measuring / inspecting anything, I quickly replaced the IRFP260N power mosfet, because I had one in the drawer.

Electronic load without heatsink (and with the replaced MOSFET)

After that it showed the same symptoms, 0A of current.

I went through some systematic analysis steps:

1. measuring the old and new power MOSFET (old was bad, new was good), so the replacement was neccessary, but not sufficient.

2. inspecting the whole board under a microscope checking for burnt parts: found nothing.

3. measuring a few components for damage, such as shorted caps and shorted/open resistors, but found nothing

4. taking photos of both sides of the PCB, merging them as semi-transparent images and try to follow the traces to reconstruct the schematics

Reconstructing the traces

During the reconstruction it looked like a standard electronic load design using a power MOSFET and a comparator op-amp, but I had issues drawing the schematics, because the MOSFET pins were weird...

My wrong schematics reconstruction vs a good simple electronic load reference circuit... Check for the MOSFET pins

It took me half a minute to understand I could fix the whole issue under 2 minutes, if I was aware of the orientation of the power MOSFET...

After flipping it, everything was fine...