So you’ve got a Broken Thing…
What’s your time worth? Is it worth 4 hours of your time to repair a £10 commodity item? Clearly there are “sentimentality” and “convenience” modifiers that can be applied.
Everything is a time/money trade off. Modern equipment, with some honourable exceptions, is not really meant to be repaired. You need to be prepared to improvise. See bottom of page for notes about tools and test gear (spoiler: you don’t need as much as you think you do).
Bricolage, as well as being an great album by Amon Tobin, is the French word for a worthy improvisation or creative workaround. Jugaad is an Indian word with similar meaning. Its sad to note that the English ‘bodge’ has pejorative connotations. But we’re post-industrial now, with our Ballardian landscapes of flyovers and endless callcentres and cheap conference centres. But that’s a rant for another time.
Stop and Think Step:
Probably half of all ‘faults are caused by operator error. It is essential that before you roll up your sleeves and operate you should take a photo with your phone of any switch or dial settings, power off, then set them all to their defaults. If you haven’t found a manual use your intuition. Then restart.
Most things with a microcontroller inside (which is most things these days) have lots of exception handling code and the classic method of yanking the power and leaving it off till the caps have discharged before powering up again will often force it to reinitialise itself and clear whatever pathalogical condition it has found itself in.
Once you’re inside (see Third Step) you may find a jumper to clear internal settings. Be careful with this, without a (service) manual you might brick it.
There are many easy wins to be had here, thanks to the unappreciated firmware coders.
See Sixth Step below for more on dealing with embedded systems.
Check the fuse or external PSU. Check any mechanical connection ie leads, connectors, etc.
Gain access to the internals.
It’s natural to want to get stuck in but it is so easy to snap off little plastic tabs or gouge edges levering with a screwdriver. It’ll never look right or fit together properly again; OK for your own stuff but incompatible with scoring brownie points by reviving yer gal’s favourite curlers.
Check for hidden screws under stickers. Some cunning manufacturers use 3 normal screws and one tri-lobe or similar. If you don’t have a security bit set its sometimes possible to jam in a smaller flat bladed screwdriver and turn the security screw. Failing that just drill it’s head out with a small carbide burr or stub drill. Rigidity is important to avoid drifting off course. Secure the work and use a drill press if you have one.
For cases that use those horrid little click tabs I’ve found that guitar picks are an excellent way to avoid leaving gouge marks like you would if using a flat blade screwdriver. Some picks have a very thin edge. I’m sure there’s a whole vocabulary to describe them if you’re into that sort of thing. They’re cheap, ‘used’ ones can possibly can be got for free from music shops and studios.
Some power bricks are joined by ultrasonic welding or glue. In these cases you have no choice but to use a hacksaw or mini-tool cutting disc and glue the case back together afterwards. It can be done neatly but it takes patience and a steady hand.
Clean it. Use a 1″ paintbrush to remove dust and clean fan blades. Sometimes hoovers can develop a large static charge from the air movement past the nozzle. This is bad. PC World had to rethink their ‘PC Health Check’ after zapping quite a few motherboards this way.
Check EVERYTHING mechanical. That means anything that isn’t glued or soldered together permanently. Unplug and replug all internal wiring looms, check for obvious corrosion or weak pins/sockets. If it has linkages or other moving stuff check for broken parts or foreign objects jamming movement. Try and sequence them by hand, does it operate smoothly? Beware – some mechanisms use a worm gear on the motor as the first reduction so if you don’t apply the test movement to the motor shaft you may well break the worm.
Be careful when lubricating. Some lubes will attack plastics or mix with dust and form a gummy paste that makes things worse. A lot of plastics are self lubricating anyway and most bronze sleeve bearings are made of sintered metal and are already impregnated with enough oil.
‘Mechanical’ is also electronic, reseat any chips that are socketed. These days its rarer to find but in slightly older gear its often the case that system firmware is stored in a DIP or PLCC socketed EEPROM. Pins corrode but often just reseating the chip is enough to scrape through the muck, just like with connectors.
Electronics. First thing to check is electrolytic capacitors. If you’re lucky they’ll look like this:
and it will be obvious. Not always though. When they’re in a reasonably high power PSU and are no-name caps I would replace them all anyway.
Look for broken power devices and any sign of excess heat.
In some switch mode PSUs there can be slight discolouration on the board below some power resistors, this is a red herring and fairly normal. The above is not that.
There are often unpopulated TTL level serial headers or JTAG headers that can be very useful. If you have JTAG stuff this post is probably too basic for you.
Serial headers are usually a group of 4 unpopulated holes of 0.1″ pitch. You can solder small patch wires to them but its easier and more reliable to solder in a section of pin strip or 0.1″ pitch male connector. The Vcc connection is not required, just Rx, Tx, Gnd.
0.1″ Pin Strip (usually comes as strip of 40 very cheaply)
TP-Link TL-WR2543ND Router with header added. Note no V+
TTL serial headers can be very useful, all you need are one of the £2 USB to TTL serial dongles from ebay or Amazon; just hook the 2 data lines and the ground, ignore the power line. Then get PuTTY, and assuming the device has a linux core like so many things do these days, when you power up your device the PuTTY window will show you the boot data and may well let you change the firmware. Most embedded systems have, to their shame, default logins. If not have a search around, its likely someone else has had to face this before.
If you get nothing over serial first check your PuTTY settings, then your connections, then try swapping Rx and Tx. Or maybe it isnt a serial header, or maybe the MCU is fried. If you have a scope try probing the Rx and Tx lines as you power the device up, you should see some random looking square waves. You might have to switch the scope to DC coupled mode as the serial bitrate might not be high enough to masquerade as an AC signal and make it through the DC blocking caps. Even if it does the signal edges will be very distorted.
I had some drama with a TP-Link TL-WR2543ND Router whilst upgrading its factory firmware to DDWRT that involves most of the above, there’ll be a post on it soon with details on how to set up PuTTY and TFTP etc. Far too much data for a general purpose page like this. Once its up I’ll link to it from here.
I will also make a page on converting ATX PSUs into general purpose PSUs, including how to wire them in series without an explosion! ATX PSUs are essentially free these days from scrap PCs; even buying new ones costs about £12 for 500W.
Power => connectors => moving things => capacitors => power devices => main silicon => firmware
Component Testing and Tools:
The minimum you will need is:
Assortment of screwdrivers, mains tester screwdriver, pliers, cheap multimeter, soldering iron, solder sucker, croc clip jumper wires, ATX PSU, 1″ paint brush (unused).
Nice to have:
Security bit set, ‘lab’ adjustable PSU, soldering station ie temp controlled, fluxed solder wick (I like Chemtronics Chem-Wik), a real oscilloscope (NOT a crappy audio bandwidth USB or soundcard based one), wrist grounding strap, LCR meter or AVR based component tester.
If you don’t recognise any of these things just select the text, right click, do Google Image search.
The AVR based component tester needs a mention by itself, see below. I happened to have a pretty posh digital LCR meter already, LCR400 LCR Meter.
The AVR version does everything it does plus an enormous amount more. Maybe not as accurately but its cheap, £8.20 delivered. Needs 20 mins soldering up but no SMD. Based on the brilliant Open Source work of a German group headed by Karl-Heinz Kübbeler at www.mikrocontroller.net (hence the cheap Chinese kits). The firmware is updated fairly often. Depending on if you have the reference hardware version or not you may need to fiddle with it a bit. There is usually a how-to somewhere for most of the Chinese clones when a firmware update comes out.
Apart from having a semi decent multimeter its the most useful bit of test gear you can own. Within 2 days of making one I had tested, categorised, and stored ALL of my unknown or reclaimed component tubs. That was a lot of caps, diodes, FETs and inductors. It will tell you the function, pinout, and characteristics of almost any component that isn’t an IC. Like a magic trick. Many more functions than the Peak analysers, also <10% of the price. Accuracy caveats apply but it’s easily good enough. You zero out the probes or whatever when you set it up. ESR and other cap measurements are accurate and quick. Just hook the unknown up any pin to any pin, it works it all out its own self and tells you. Don’t connect a charged cap!
As copy’n’pasted from it’s bumpf the AVR one can do:
Automatic detection PNP and NPN type bipolar transistors , N, P -channel MOSFET, JFET FET , diodes , two diodes, thyristors , resistors , capacitors , inductors. Automatic detection pin definitions.
Measurement of the bipolar transistor current amplification factor (B) and conduction voltage emitter junction (Uf). Darlington transistor may be identified by the amplification factor of the high threshold voltage and high current.
Can be detected inside the bipolar transistor and MOSFET protection diodes and displayed on the screen.
The threshold voltage and the MOSFET gate capacitance measurement.
Support two measuring resistors , the potential can also be measured . If the potentiometer is adjusted to its end , the tester can not distinguish the two ends of the pin and the middle.
Resistance measurement resolution is 0.1Ohm , the highest measurement value of 50MOhm.
Capacitance measurement range from 25pf to 100mF (100000UF). Resolution up to 1PF.
Can detect more than 2UF capacitor equivalent series resistance (ESR), with a resolution of 0.01Ohm. This feature is very important for the detection capacitor performance.
You can display symbols of the two diodes in the right direction , also shows the forward voltage drop.
LED detection of diode forward voltage drop higher than normal . Dual LED detected as double diodes. Simultaneous detection of light-emitting diodes will flash.
Each test time is about two seconds , only large capacitance and inductance measurements will take a long time.