For a few years now I have been running Ubuntu 14.0.0 on an old Toshiba Satellite A100 laptop (from the Microsoft Windows XP era).  It was sort of as an experiment but it was also because I like to keep the old stuff operational for as long as possible.  Linux operating systems are less resource hungry compared to the offerings from Microsoft and so the good old Toshiba did everything you would expect from a much newer computer.

There were a few issues though.  It would run quite hot for one.  I don’t know if the hardware did not like to be pushed too hard with playing full screen videos, or that there was a problem with the thermal management, lack of Ubuntu drivers, or all three issues.

The lack of integration with Microsoft products also made using a Linux based operating system a little awkward.

For a little while the laptop had developed a fault where the screen would flicker or briefly disappear and then the cursor could be moved but clicking did nothing.  Also, it would sometimes not wake up after going into standby.  It seemed to be a fault that was related to movement of the motherboard. I had stripped it down after it started happening but there was no obvious cause.  As is often the case for these faults it stopped playing up for a while after being checked.

The intermittent fault was getting a little worse but then one evening, quite suddenly, the laptop stopped charging. Oh, and I forgot to mention that the battery was pretty well stuffed and the charger had to be plugged in virtually all the time.  I did all the usual things a technician does.  I checked the charger.  No problems there.  So I stripped down the laptop and started doing some fault tracing.  The two fuses were ok, the low ohm resistors were ok, the coils were ok but one looked like it had been running hot.

And then I noticed this:

Looks like the chip is toast and pretty much so is the whole laptop.  The chip is close to the hard drive connector but the hard drive was still working.  Interesting.  It does not matter if the hard drive is stuffed because everything that is important is on the cloud.

Time to dig another laptop out of the recycling pile.  I found a Toshiba Satellite M300 that looks like it only needed an LCD cable.  Cheap enough on Aliexpress so I ordered one.  Another laptop that was kicking around was a HP G60 that had been running Windows 7.  It had a stuffed hard drive.

Now I know from past experience that you cannot pull a hard drive out of one machine and just bung it into another one and expect it to work.  There are all sorts of driver issues.  To possibly make things easy and as an experiment I thought I might try to drop the hard drive out of the Toshiba straight into the HP.  And guess what! It worked! I am typing this blog post minutes after the transplant!  The only thing I had to do differently is to enter the wireless password. I have struck this before.  It seems that the wireless password is stored in hardware somewhere rather than on the hard drive.

So is this all an indication of how good the generic Linux drivers are?  It is not really a security issue since I still had to enter my login password.  So is this the way of the future where we pull the solid state hard drive out of a stuffed old machine and bung it in a new one?  Sounds like a good idea to me.

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Here is a story for all you electronics techs.  And anyone making assumptions.  Most of us in other words.

This is about a caddy type welder that came in for repair, a 180 amp inverter model. Can’t remember the make or model.   I checked for weld voltage and there was a healthy 80 odd volts coming out but there was a really pathetic spark from the electrode which meant there was next to no current.

So I took it apart to do some fault tracing. Using a light bulb as a load I was getting 20 volts out on the weld terminals, down from the no load voltage of 80 volts.  Ok,  so is there a lot of resistance in the output, or maybe the inverter control circuit is doing something funny. Like a lot of these small caddy welders they have a centre tapped transformer and a full wave bridge rectifier arrangement.

Even with the load in place I measured 80 volts AC on both sides of the transformer secondary.  All the connections were tight with no sign of heating due to contact resistance.  So was it a dodgy diode pack? It was a SOT227 package, which looks like the one pictured here.

I could measure the diode voltage drop in circuit of about 0.3 volts, which is what is expected for these Schottky diodes. Now normally semiconductors go short or blow apart and sometimes go leaky but if this diode pack is faulty it is as if it acting like a one way resistor!  Odd.

Getting the diode pack off the heatsink was no mean feat with a mains filter capacitor, output current sense resistor, and output current bus bars all in the way.  With it out on the bench both diodes measured ok. We didn’t have one in stock and Tony suggested doing a DC check on it.  Putting about 30 volts up the anode and a 100 ohm resistor strung off the cathode I got nothing coming out of it!  I then remeasured the junction and it was now open circuit!  Definitely a dodgy diode (diodes actually).  When I remeasuring the junction voltage a bit later it measured ok.  Probably heat sensitive.

So the moral of the story is that we cannot make any assumptions about how components fail.  Testing at all stages worked in this case in getting to the bottom of the fault pretty quickly.  When fault tracing I am sure we are all guilty of sometimes making assumptions and taking longer than it should to fix something.