Kenwood HB720 repair

Our Kenwood HB720 hand blender suddenly stopped working. The punchline of this post is that inside its excruciatingly-difficult-to-dismantle mechanism (i.e. never intended to be repaired), it includes a fuse, which had blown.  This post will be updated to give the details about how to take the machine apart, and whether I managed to reassemble it after replacing the fuse.

So, if “the motor is broken” on yours, it may just be the fuse that has blown.

One sign that the fuse is probably blown is that a diode test with +ve on phase, -ve on neutral shows open circuit when the Turbo button is pressed (this button bypasses the speed controller, and connects mains power via a bridge rectifier to the DC motor).

Dismantling

Tricky. There are a couple of videos purporting to show how to dismantle it, but I reckon they both are filmed after the unit has been dismantled and reassembled.

First remove the drive-end cover.  This is held in place by three  hooks on the inner plastic part, that engage with square holes on the cover.  One is above the ejector wedge on the side without the number stamped on it. The others are 120 degrees round from that. You might be able to get this cover off just by pulling very hard, but otherwise probe with a small screwdriver(s) to push the hooks inwards and disengage them.

The inner part is glued or welded in to the body.  This thing is a prick to get out. I attacked it with a screwdriver and a scalpel, and still it looks fairly munted.

Carefully prise off the speed control knob at the cord end.  This reveals 4 screws, to be removed. Turn over the plastic circle with the cog and undo the 2 screws to release the cord clamp. Slide the knob and clamp up the cord.

Now push on the plastic tray holding the circuit board, pull on the motor shaft to remove the motor + speed control assembly.

Electronics

So here’s the speed controller board

The fuse is inside black heatshrink “FR-H TUBE 125C”.  It is soldered to the board.

img_20170320_142557468
The blown fuse
img_20170320_142123574
The speed controller

A strange thing is that the fuse is rated at 1.6A, while the machine is nominally 700W.  Unless I’m missing something, 3A at 240V is 720W, so either Kenwood is lying about the power rating, or the fuse rating is too low?

I replaced the fuse with a piece of wire, because I’m not planning to open this up again.

Links

Schematic (polish)

Another set of photos about dismantling and assembling HB720.  Pretty good,  the inner and outer drive-end covers are shown still connected together. I doubt it is possible to remove them in this state.

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