Chemistry Electronics Services
ARCHIVE OF REPAIR TRICKS, USEFUL INFO, ETC.

REPAIR IKA-WORKS "RCT BASIC" MAGNETIC STIR PLATE

The quick cheap fix is: replace the entire guts, two PCBs with permanent ribbon between,
PCB Assy for stirplate number RCT-B-S1, Part No. 5038001, $208.00 (2016 price)
[If you wanted main pcb for CMAG HS7 hotplate/stirrer, that's pn 3599001, nearly $300]
Order from:

    IKA Works
    2635 North Chase Pkwy SE
    Wilmington, NC 28405-7419
    Phone: +1 910 452 7059

When re-installing, remember to plug in the thermistors and the motor, and don't get their
connectors offset (off by one.)

TYPICAL FAILS, check first

If no power, check power switch; half of the DPST sometimes fails from corrosion. Also look for large filter capacitor dead/leaked, sometimes with open fuse (adjacent to capacitor; small rectangular ceramic "picofuse.") If everything works but no heater, check if heater wire tips corroded or unplugged, or safety pot turned all the way down (screwdriver adj, next to the power switch.)

If no heater and no stir, check power transformer on PCB, sometimes fails open, or sometimes overheats w/low output volts (from shorted internal turns.) If it's none of the above and no power, suspect a dead CPU killed by failed power supply (that's a Cypress PSoC cy8c27643)

If 3-phase motor is weak, stalled, check for dead FET drivers in SO8 package, IRF7343 dual N/P channel half-bridge drivers (one or more dead of the three.) Note three Littlefuse TVS diodes in that area, check for shorts. More rare is a dead SOT23 relay-drive transistor going to the heater relay or safety relay.

TEMP. SENSORS
Internal temperature sensors are 1.1K ohms at room temp, so probably platinum RTD pt1000. The external IKA probe (pt1000.60) has the same 1,100-ohm value on pin-1 (pin-5 is common.) This probe also shorts pin-3 to pin-5.

NOTES: safety shutdown

The LM358 dual op-amp compares the setting of the safety-temperature pot with the output of the main sensor thermistor. If the two values approach too closely, or if the safety pot is set too low, the op-amp will shut down heater by driving the base of the SOT23 BC817 (NPN transistor) low to shut off the safety relay; removing 120VAC from heater.

A second BC817 NPN must pull the above mentioned NPN's emitter to less than ~5V, in order to drive the safety relay on. The second BC817 is driven by the Q-output pin5 of the 74123 retrig 1-shot, which is kept constantly high-triggered by the LED scan signal from the CPU. And, its own emitter in turn is pulled low by being connected to the 5pin DIN's pin3. The pin3 must be grounded in order for the rest of the circuit to supply 120VAC to heater via the safety relay (you must add a jumper between pin3 and pin5 on the rear DIN.)

ERROR-4: Motor speed wrong. Stalled, bearing fail, conn is disconnected, or MOS driver fail (usually water corrosion.)

ERROR-5: Temperature probe not responding. Probe temperature stays constant, probe is not inserted in heated liquid (or not placed on hotplate surface.)

ERROR-6: Missing temperature probe, unplugged, or corroded DIN connector. Enable the stirplate by installing the supplied gray jumper-plug. Or, ground pin-3 on the rear 5-pin DIN circular connector by using a bent piece of solid wire #16ga to jumper from pin-3 to pin-5. See pin diagram on the PCB photo below. The DIN connector on the non-IKA VWR corp stirplate has the same pinout.

ERROR-25: Can't turn triac on/off, or missing heater-drive AC. Caused by: heater wires unplugged (or tips corroded,) or the safety-relay is off (because of 1K thermistors wrong or unplugged, or safety-temperature pot set wrong, or the rear probe connector's shorting-plug missing.) Or perhaps the triac or heater-relay has failed. Triac is T1635-600G, 16amp 600V, with gate driven by MOC3063 opto isolator. More rare: failed heating element, it should measure well under 30ohms (24 ohms when hot = 600 watt.)

ERROR-31: Heater-control relay fails power-up test. Bad relay contacts, or failed transistor on the 24-volt coil-driver, or failed optoisolator on 120VAC-sense circuit.

ERROR-44: Safety thermistor unplugged or failed.

IKA: User manuals for magnetic stir plates

IKA: Catalog page, IKA magnetic stir plates