Repairing an IKA stirplate RET Basic, magnetic hot plate

[William Beaty]

IKA stir plate RET Basic, Won't stir. Water spill under motor, destroyed SMT resistors and traces.
https://www.ika.com/ika/product_art/spareparts/3188801.pdf
IKA partslist gives the part nos. for this model

The main PCB is: IKA part no. 3185001, price in 2018 is $305.00. The motor is: IKA part no. 2869900 (in 2018 price $312.00) (motor wo/magnet.)
It's an EBM PAPST 24V DC fan motor, (16-28V-)
Ebmpapst part no. M1G055-AI01-01, which I don't find in their PDF catalog.
https://ebmpapst.com/media/content/info-center/downloads_10/catalogs/axial_fans_1/Axialventilator_ESM-iQ_DE.pdf

The four pins for the motor electronics are, top view from left to right:

- +24VDC (more like 33V in the stirplate)
- Tachometer out, Hall chip, TTL O.C. output
- V(inp), 0-5VDC gives analog 0-1000RPM.
- Gnd/common

The motor speed-ctrl electronics is digital, with a hcf4093 quad op amp and
an https://www.st.com/en/power-management/sg3524.html
SG3524 pwm ctrl chip, with 25KHz pwm outputs going to the V(in) motor pin.
Replaced both chips and a couple of SMT resistors. Works now!

Also see my http://staff.washington.edu/wbeaty/chemtrik.html for
more repair articles on lab instruments.