Mark 2 expansion board with reversed diodes!

[conny_g 250]

Recently I was wondering why of my freshly built Mark 2 printer the hotend fan and the model fan of the 2nd hotend won't work.

Today I have taken the time to analyse it. Started by putting the printer to its side, opening the Mark 2 expansion board box and measuring the outputs of the board. Finding that there was no voltage on the fans. I noticed that the expansion board was rather hot on one side. My thermal imaging camera (flair one) said it was actually 80 degrees C hot. I saw it was the 1k resistor near the 6 pole header coming from the control panel board. Weird!

So I unplugged the expansion board and took it to the desk to check what's wrong.

Resistors were ok. One of the transistors had a short, the other at least no proper diode voltages. So it seemed the transistors were dead. But why?

Umm. Shouldn't the diodes be the other way around if they are freewheeling diodes. Checked the schematic. Checked the diodes.

 

Yes, they were mounted the wrong way!!

I bought the board completed, so the manufacturer mounted the diodes the wrong way and that seems to have killed the transistors....

 

So I reverted the diodes, mounted new BC817 transistors, reassembled the printer ... and ... it works, the fans are alive!

@DidierKlein, who is assembling your boards? :-)