I bought a “secondhand” extruder and I found two problems:
- Heating not controllable, start to heat directly and becomes 270 degr and shutdown;
- Step-motor does not change in direction (retract filament).
After many hours searching, measuring and testing I found the causes of these problems. I order new components, put them on the PCB and tested it, and now it’s working fine. I suspect that all these problems are related to cable/connection exchanges between step-motor and heater element. Hope that others can take advantage of these solutions. See descriptions and photo’s below.
The cause of this problem was a defective U3 (AM4953P) dual MOSFET, it has a shortcut between source and drain for the connected heating element. Replacement of SMD chip fixed the problem. See made electrical scheme and PCB picture of U3.
This was a bit more difficult, because the DIR (direction) input was defective (shorten) on the driver board 4988. A driver board swap was the only option, but I discovered different amount of pins (18) between the original and a brand new driver board (like Pololu 4988, 16 pin). The conclusion…2 pins where not used and the rest was exact the same as a Pololu board, so a swap looks feasible. Removing the driver board was like hell, glue underneath and a very tiny workspace. Finally cut the driver board in pieces, unfortunately no hot air gun for making it loose. After soldering an IC-socket onto the PCB, its ready for mounting the new driver board. So no soldering anymore needed for any replacement of the driver board. I set the current limit (80% of 1.5A step-motor = 1.2A as the original board) of the new driver board by changing the Vref. It should be 0.96v, but with VDD=3.3v it comes only to approx. 0.825v. This means the current is limited to 1 amp and 66% below the of max current step-motor. Gives this problems, then I suggest to change the fixed 30K with a 20K resistor on the driver board. Then it’s adjustable to max Vref=1.1v with a maximum current limit=1.375 amp. For now it works fine again. See scheme and pictures.