Cuisinart DBM-8 Coffee Grinder acting funny?

If your Cuisinart DBM-8 coffee grinder is acting funny, read on. This unit is a fairly inexpensive burr type coffee grinder and is quite popular due to it being available at a nationwide big-box warehouse style retailer. The motor is controlled with an electronic timer and this is where things go wrong.

First, there are two safety switches wired in series with a third “power” switch. Failure of any of these will result in an inoperative unit. Note that the grind time control lever, when fully to the left, opens the “power” switch.

The electronic timer is based on a CD4541BE timer chip (Texas Instruments) driving a set of relays that switch the 120VAC to the motor. The motor itself is a universal type (with brushes). Note that over time, carbon dust from the brushes can buildup on the circuit board which could eventually wreak havoc (bad design point #1).

Power for the electronics comes from a half-wave rectifier, a flameproof resistor and a Zener diode. Yes, very cheap and non-isolated. This is where my unit eventually failed. Symptoms were erratic behavior — move the grind time lever and the unit would stop operating for a while. Let it “rest” and it would be fine. Unplugging and then replugging would sometimes help. Eventually, total failure.

Apart from evidence of really shoddy assembly (borderline cold joints, etc), the failed device was the Zener diode. Evidence of overheating and discoloration of the phenolic PCB marked the device as suspect. Testing revealed that the diode was “leaky” and only sustained about 2.5V which is insufficient for the circuit. Alas, I could not determine the actual design voltage of the device but I replaced with a value that made sense after reading the datasheet of the CD4541BE. (Yes, I’m being deliberately vague here since this is a mains connected circuit and it can be dangerous if you do not know what you are doing. If what is written is “Greek” to you, then that is an indication that you should not attempt this repair.)

Disassembly of the unit is simple — remove the four rubber feet to expose 4 screws. Remove. The bottom plate will come off — you may have to help feed the power cord. The PCB can be slid out of its carrier at this point and the first safety switch can be seen. Examining the other switches and the start/timer slider assembly requires removing another 4 screws accessible using a long Phillips screwdriver at the corners PLUS one at the upper lip of the unit (need to remove the grinds container).

Good luck. Yes, this is not a question, but more of an answer, but I couldn’t figure out how to document this any other way.

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