Sunday, 23 of September of 2018

Repair of a Heidolph MR Hei Mix L magnetic stirrer

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This nice magnetig mixer would not start anymore. one of the problems was a small fuse visible in the figure below as the white unit in the red circle. (the other is the broken fuse.

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After repairing this,(placing a good fuse parallel to it)  the small green led at the CPU started flashing continuously, this is common for these kind of mixers and often the repair process stops at this point. I could not find anything about this error, what it means or how to solve it, and probably it can not be solved by hardware or a non-inside person. Resetting does not seem to help.

After looking a bit into the system, it seems to use an MC33035 brushless motor driver (google the number to find the datasheet and pinout) you see this driver IC just to the left of the square CPU unit in the foto above. From there it is quite simple to get this neat machine up and running again. Look for pin 7 on that chip and pry it off the printed circuit board (difficult to see in this picture) and leave it floating. That will enable the motor driver. Next, you can connect the potmeter to the speed input pin of this driver: connect a small wire to the seventh pin from the right on the plastic wire connector (which goes to the control board with dials and leds), place two normal (small) diodes in series, conducting in the direction of the wire (I did this by using two smd-diodes and fixed them with glue to the white connector), to pin 11 of the motor driver chip. That may also be on the small resistor next to it.  Place a resistor of 22 kOhm over that same chip resistor, and your’e set! The only disadvantage is that the rotary button has now inverse working: ‘zero’  is approx 1400 rpm and ‘1400 rpm’ is zero actual speed. You can also modify the led board and short out the transistor driving the power led, but that is trivial. This way, we completely disabled the cpu, but to be honest: i dont see what the task of this unit would be from here on. I think that it is used primarily for the more extravagant types with lighting rotary buttons and screens and for the heating module. This stirrer has neither, so in this case we can do without this  poor useless CPU.

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