Thursday, September 27, 2012

Some interesting domestic electronic fixes

Fridge/Freezer earthing conundrum

I've had my Miele fridge for maybe six years and it's one of those frost-free models; I assume an extractor fan maintains negative pressure inside so that moist air that has entered when you open it gets drawn out. About three years ago (after I'd re-arranged some of the mains in the kitchen) it started icing up every few months like an old-style freezer. Of course it was outside it's warrantee so we lived with it. Around eighteen months ago whilst cleaning behind it I noticed the mains cord had been tugged and the earth was disconnected. Of course I re-ended the cable but since then the freezer has stopped icing up.
I've searched online for a schematic for this model to see if I can figure out what is going on. I'm reluctant to disconnect the earth to prove it, but I'm left wondering (and no other engineer I've mentioned it to can give me an answer) why the anti-frost mechanism didn't work without the safety earth? I stuck the freezer on my trusty Martindale 2100 PAT tester and it was fine; no excessive residual earth current and all isolation good; even at 8A.

When is a micro-switch not a switch?

My Valiant combination boiler has the usual configuration of a butterfly valve that routes hot water to either the taps or the heating pump. There is a micro-switch on the valve such that when you open the hot water tap the boiler is forced on (it's unlikely it was on heating the radiators exactly when you needed to wash your hands). The boiler is fifteen years old and I've replaced that micro-switch two or three times before. Earlier this year it seemed to have gone again yet removing the connections and testing for continuity when the valve opened showed the switch (seemed!) to be working. In the end I admitted defeat and got a heating guy in. He concluded the same as me and he 'phone the Valiant tech support line.

"Replace the micro-switch"

"No, no - it's not the switch, it buzzes out correctly when the valve closes"

"Never mind that, replace the switch"

Because it's a common model of boiler and the micro-switch goes often he had the kit in his van. Sure enough, he replaced the switch and the boiler starting working correctly. I was now beginning to doubt a lifetime of electronics knowledge! I got him the call the helpline back and the chap at the other end explained that it's not just a DC voltage that gets switched but the i2c data link! Yes, I was amazed; I can only assume it's part of a wider control system (maybe common across much more complicated boilers); but if there is any electrical noise or impedance on the switch data doesn't get back to the control board and the boiler CPU doesn't fire up the gas.

XBox 360 and failed DVD drive.

What would imagine would be the part most likely to fail in any kind of consumer device? The mechanical part that has a 50p laser diode AKA the DVD drive. Given that it has two connection - SATA and power it should be a user-serviceable part and given you can buy them from eBay and spare parts suppliers home tinkerers like me should be able to fix their XBoxes. But no, the XBox OS is keyed to the firmware serial number of the drive that is installed at the factory and if it sees another drive it not only refuses to read the disk, but it informs the XBox Live! (why the exclamation?) mothership and your account is banned for being a game-stealing pirate.
In the end you have to get the same model of DVD drive (there have been four revisions over the lifetime of the '360) and swap the board between drives so that your new mechanics have an old firmware. It worked for me. 

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