Case Studies

Case Studies of Famous Fixes.

Summary

Some of us have a fix we're especially proud of, or can think of one which nicely demonstrates a diagnostic principle, or which taught us something new. This is a page of "Famous Fixes" which hopefully will contain an ever growing collection of gems for our shared education and entertainment.

Computers and Home Office

Windows laptop

The laptop owner (a very expert user) told the Restarter the harddrive needed "debugging" - led to blind-alley.

• Ran a harddrive diagnostic tool (ok)
• Mounted the drive (ok).
• Inspected system log (ok).
• Booted on Linux and noticed there was rubbish coming up on screen at boot - suspected a keyboard or touchpad fault - isolated the keyboard fault.

(Contribution by Ten.)

Laptop USB port

Only took over half of it, but although the owner had bought the right part it didn't make getting the old usb port off any easier. As there were two big solder points holding the case to the board, it took in a lot of heat before solder flowed. And then there were four connections at the back that you ideally needed to heat all at the same time. We accidentally ripped some of the traces off the board but still managed to fixed it by some fine wire soldered from the pins to a nearby surface mount resistor. The owner of the laptop thought she could have done it herself, and even when I did it I myself couldn't believe it worked.

(Contribution by Faraz.)

Macbook

Mysterious intermittent trackpad problem in which the trackpad button became more sensitive and then stopped working altogether.

• Plugging in an external mouse, the mouse button was not working either.
• Noticing that the battery icon showed no battery present when a battery was present gave a clue as to the fault.
• Turns out the battery had gassed and was bulging, affecting the trackpad
• After removing the battery (and safely disposing of it), the trackpad button was working fine.

(Contribution by David.)

Home Entertainment

LCD TV

Bad electrolytic capacitors in a TV which had been found dumped in the street provided a good research opportunity. Great team work, biking off to get spares while Faraz unsoldered the failing components.

(Contribution by Ten.)

Kitchen and Household Items

Kettle

Mysterious kettle owned by the venue - we did not know the circumstances of the fault.

• Check the fuse (ok).
• Visual inspection (ok).
• Tested for continuity from the mains plug to the base (ok).
• Tested base for resistance and continuity (ok).
• By logical process, the only thing left was the physical contact between the kettle jug and the base - turned out it was not making a solid contact and needed to be remade.

(Contribution by Andrew.)

Dehumidifier

This was my own, little used but worked fine for 2 or 3 years but then stopped working. On switching on, the lights seemed to indicate maybe the water collection tank was full, which it wasn't, and it wouldn't stay on.

• Removed covers, visual inspection showed no signs whatsoever of a problem. Reseated connectors with no effect.
• The water level alarm consisted of a float with a magnet which closed a magnetic reed switch as the float approached the full position. No visible faults, and unable to influence the problem with a second magnet close to the reed switch. Gave up for the time being.
• Determined to have one last go before scrapping it. Followed the reed switch wires through to the logic board and hence to a pin on the microcontroller. Verified with a multimeter that the voltage on the microcontroller pin changed as one would expect on lifting the float.
• Noticed that the silk screen printing on the logic board showed a capacitor which was not fitted.
• Followed the copper traces on the board and found that the capacitor (if fitted) would be wired between another microcontroller pin and ground, with a resistor also connecting the pin to the positive supply.
• Theorised that the microcontroller pin was a reset input. The capacitor (if fitted) would hold the pin in a logic "low" state and hence the microcontroller in a reset state while the capacitor charged, for long enough for the power supplies to stabilise. Without the capacitor, the microcontroller might run with unstable power supply and immediately crash.
• Fitted a capacitor calculated to hold the pin "low" for a few tens of milliseconds.
• Powered the dehumidifier on, and it worked like a dream!

Funnily enough, 2 or 3 years later I had a letter from B&Q. A safety issue had been found with this model of dehumidifier, and would I please bring it back for a full refund. Lucky I'd saved it from the scrap heap!

(Contribution by Philip.)