Tuesday, October 11, 2011

Repair (aka. bypass) safety switches on "dead" Krups GVX2 burr coffee grinder

Every geek needs coffee, but lately my Krups GVX2 burr coffee grinder started acting up.

Sometimes it would stop grinding prematurely, other times it just would not start. Sometimes simply pressing the top lid would do the trick, other times it had to be taken off, put back on and then pressed down. Sometimes I would just give up.

The grinder won't start unless both the top lid is on and the ground coffee container is in place. It seemed that the safety switch for the top lid was being a little over-cautious (aka. broken).

I turns out the two micro switches are held in place by thin strips of cheap plastic, which can evidently break from repeated use.

While perhaps not entirely impossible, I don't see how you can stick your fingers in the blades, unless you're trying hard. So instead of throwing out the grinder, I decided to bypass the switches entirely, so the grinder would work, regardless of the top lid and removable container.

Another option would be to fix the mount for the switches with bits of plastic and/or glue - you decide which option suits you best. In either case, do so entirely at your own risk. It is a device connected to mains electricity after all.

Here is a description of what I did (forgot to take pictures)...

Tools Needed

  • 2 small flat head screwdriver
  • Wire cutter
  • Soldering iron and solder
  • Electrical tape or heat shrink
  • (Butter knife) 

Opening the Machine

  • Remove bean hopper, ground coffee container and any beans left in the machine.
Then remove the dials. The side dial is held in place by three latches, the front dial by friction only.
  • Gently pry off the front dial, using a butter knife (or flat head screwdriver).
  • Use a small flat head screwdriver to pry the side dial a bit to the side, then use another small screwdriver one of the exposed latches. Repeat until the dial comes off.
The large outer shell is held in place by four latches in the bottom. It takes a bit of convincing, before it comes off.
  • Turn the machine upside down.
  • Pull out the four rubber feet.
  • Using a butter knife or small flat head screwdriver, release one of the latches, while pulling down on the shell. It should give a millimetre or two. Repeat for the other three latches.
  • Slide the shell off the machine and turn it again.
In my case, the front metal plate also fell off. Notice the small plastic "hook" holding the wires to the right of the circuit board.
  • Remove the two screws holding the funnel in place and remove it.
  • Unscrew and remove the micro switch from the funnel. Discard the screws if you're going to bypass the switches.
  • Carefully cut the plastic strip on the bottom left of the printed circuit board.
  • Pull up the circuit board to expose the micro switch for ground coffee container.
  • Unscrew and remove the micro switch above the container. Discard the screws if you're going to bypass the switches.
  • Turn the machine upside down, to get the small plastic pins and spring out. Discard them if you're going to bypass the switches.
Correction: It turns out the micro switch just above the container serves an additional purpose: It helps keep the container in place, when the machines spits out the ground coffee.

While it may be necessary to unscrew the switch to work with the wires, I do recommend assembling switch, plastic pins and spring again when done. Else you'll probably need to put a rubber band around the machine, like I have now c",).

Now for the fun part...

Bypassing the Switches

The two switches are serially connected and sits between the power supply and circuit board (PCB). The short wire between the PCB and container micro switch has an extra layer of insulation, to protect it from the heat from the hot resistors. The wire is short as it is, so be careful not to make it shorter, when removing insulation from the tip.
  • Following the wire from the power supply at the bottom, cut it as close to the funnel switch as possible.
  • Following the (short) wire from the PCB to the container switch, also cut this as close the switch as possible.
  • Remove the insulation from both wire ends.
  • If using heat shrink, place it on the long wire.
  • Solder the two ends together. The easiest way to do this, is to heat the wire ends separately and apply just enough solder, to make a nice shiny "coat" on the wire. Then hold the wire ends together and apply heat. Remove the soldering iron as soon as the solder melt and blow lightly.
  • Apply electrical tape or cover the wire ends with the heat shrink and use the soldering iron to shrink it.

Putting it Back Together

  • Put the plastic pin and spring for the container micro switch back in place and screw the switch back on.
  • Put the circuit board back in place, carefully aligning the newly soldered wire in front of the PCB, along the bottom.
  • Press wires back in the small  plastic "hook".
  • Put the funnel and secure it with the two screws.
  • Attach the front metal plate. There are pins that fits in the small holes on either side of the bean hopper.
  • Slide on the shell. A bit of pressure may be needed before it clicks in place.
  • Turn the machine upside down and check that the shell fits snugly.
  • Re-attach rubber feet.
  • Rejoice!

6 comments:

  1. I had similar symptoms, but different cause. The timer was dysfunctional so no matter what setting I had it on it just ground in short pulses. Solution was to by-pass the timer entirely. Easily done by removing TR1 (left hand side of PCB) and putting a link between where middle on right hand pins were (viewed in assembled view - PCB marking are upside down) these are connections with thick PCB conductors. Now the grinder is simply switched on and off at wall socket. Shame a mechanically sound machine is ruined by shoddy controls and electronics.

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    Replies
    1. Hi Richard, my grinder has the exact same fault (i.e. only grinding in short pulses). I've followed the above guide to disassemble, however could you give a bit more detail about how you bypassed the timer? I can see the TR1 on the PCB but not sure where to go from there.

      Thanks!

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    2. HI Richard, my grinder has the exact same fault (i.e. only grinding in short bursts). I've followed the guide above to disassemble, however can you give a bit more detail about how you bypassed the timer? I can see the TR1 on the PCB, however not sure where to go from there.

      Thanks!

      Delete
  2. Great; thanks. Mine had two problems, bad switch and thermal fuse (burned out by rapid repeated grind loads by family member). I cut out the thermal fuse (looks like a diode at the motor base); replaced with wire. Then, removed the switch beneath the circuit board. To avoid rubber band problem, keep the spring, coffee bin retainer pin (black), holder (little black lid) and screw. Put those all back, without the switch, and the receiver snaps in place (no rubber band).

    Use caution when removing grind fine-ness knob (on side of unit); Release clips with slim knife, pushed straight in, not by prying. Plastic clips (part of knob) can break off.

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    Replies
    1. Thanks for the great blog and thanks Laird for the thermal fuse advice. I am replacing the thermal fuse in my GVX2 and was hoping you could confirm I have correctly located the component please? Is the thermal fuse the black insulted component cable tied to the motor's field coil?

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  3. When this fault happens the grinder usually runs if the receptacle is removed and the dislodged and misbehaving microswitch pressed in manually - even if not powered, you can hear the click. Obviously this is not in itself very useful (unless you want a handful of ground coffee), but has anyone thought of holding down the microswitch, putting a drop of superglue or epoxy adhesive on it, and holding it in place until the glue cures? This could be a quick and dirty way to achieve the same result as the rather fiddly disassembly. I haven't dared to try it yet. It would depend, among other things, on a glue which would stick the plastic used, and would probably also harmlessly glue the switch actuator to the hole it protrudes through. If I had a spare microswitch to sacrifice I would experiment on it first.

    In spite of my misgivings, if gluing went wrong it would probably still be possible to disassemble etc., so nothing lost if the microswitch is going to be disabled anyway.

    (I've posted elsewhere as pol098)

    ReplyDelete