Our old Black & Decker coffee maker packed it in this morning -- it made the coffee, but then the heater went off and wouldn't come back on. While it's probably futile to pursue a repair attempt on the thing, I figured I'd at least take a look to see what failed.
When I went to unscrew the bottom cover, I encountered tamper-proof screw heads of a style I'd not seen before. They're basically a plain slot, but with a 'bump' in the centre of the slot. Here's a view of them, they're the two screws at the left.
The two screws at the right are combination Phillips/slot -- those were at the rear screw positions. At the right side of the photograph is the crude screwdriver I made from a length of 6mm diameter steel rod. The screws are small (No. 4 x 1/2"), and they weren't in very tight, so my mild steel screwdriver was quite adequate for the job.
With the bottom cover off, here's a view of the innards.
Note the length of white insulating tubing at about the centre of the photograph that's held in place by two clips. Inside that tubing are two thermal fuses connected in series with the heater element. If either one goes open, the heater won't operate.[1]
And sure enough, one of the two is open. Here's a view of the culprit.
The curious thing about this failure is that the fuse that's gone open is marked 240C, while its series mate (still concealed in this photo by the tubing) is marked 216C. That strongly suggests that the fuse failure here was not a legitimate one from an overheat condition, but an inadvertant nuisance failure of a marginal fuse.
What I'll do is I'll get a suitable splice crimp terminal,[2] and delete the open fuse. Unless I'm missing something here, that should get the coffee maker working again, with still the one thermal fuse in the heater circuit.
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Working Again -- SUNDAY, APRIL 21, 2013
I finally obtained a proper, seamless crimp barrel, and effected the repair I had in mind. Here's a view of that. (I Loosened off the heater assembly to facilitate the crimping job.)
I deleted the open thermal fuse, shortened the length of insulating tubing a bit and installed a crimp barrel to restore the heater circuit.
When I reattached the heater and tried it out, it worked normally. The thermostat cycles on and off as it should, so that thermal fuse failure was a fluke -- it wasn't in response to an overheat condition. The only shortcoming to this repair is that the coffee maker no longer has redundant thermal fusing;[3] it only has the remaining 216C fuse. From what I've seen here, that fuse is more likely to fail needlessly than it is to fail to open if ever it ought to.
I'll saw away those occlusion 'bumps' in the slots of the two tamper-proof screws, and button up the coffee maker. It can go aside as a spare, since my wife had a replacement unit on hand already.
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Notes:
[1] Thermal fuses are widely used in all manner of appliances and electrical/electronic gear, as a last-ditch fire-prevention safety feature. In a coffee maker, the fuses cover for the possibility of a thermostat failing 'closed', and letting the heater just go on heating indefinitely.
[2] Needless to say, soldering is not on for making connections to thermal fuses. Only crimp connectors or screw-terminal connections can be used.
[3] Redundant thermal fusing does seem to be the norm in coffee makers. I happen to have an old, retired Proctor-Silex unit on hand, so I took a look inside. It has effectively the same thermal fusing arrangement as the Black & Decker appliance.
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