Repairing a Coffee Machine

Melitta AromaFresh drip coffee machine broke down after around one month of usage – water stopped dripping into the kettle.

Disassembling the unit posed the standard challenges:

1. Not so easy to reach screws – longer screwdriver was needed.
2. Latches in the upper part.
3. One anti-tamper screw – special bit was required.

On the plus side - there were only two types of screws.

The goal was to check for the obvious parts that could fail:

1. Water pump – water was not dripping.
2. Scale build-up – water here is quite hard, maybe the waterway got blocked.
3. Heater – machine was not pumping the water if the heater is dead.

The machine turned out to be much simpler, it doesn't have (and doesn't need) a water pump, water is pumped by heat expansion – point 1 is no longer valid. (This was a very bad, but reasoned, guess.)

After fixing problem no 2 the coffee machine still didn't work.

Focus was on checking why heater didn't heat (point 3).

Each of the elements in the heat circutry was checked:

1. Relay – it does turn on when the machine is on (voltage present, can feel the coil engaging) – OK.
2. Thermostat – resistance/continuity check – OK.
3. Heater – continuity check – OK.
4. Thermal fuse – continuity check – FAIL.
5. One way valve – manual check – OK.

It turned out the thermal fuse was "broken". It took a while to discover that, since it was concealed, for a good reason, in a protective sleeve.

Replacement part was ~7 EUR (including shipping) for 10 pieces.

Repair turned out to be successful – coffee machine was working again.

Below other circuits in the machine, as well as the photo of the PCB board.