原创 A drill that won't do its job

I was brought up Zimbabwe, which used to be Rhodesia until 1980. Named after Cecil John Rhodes, who had the ambition to paint the map of Africa red—the colour of British colonies. However, in 1965, the then newly elected prime minister, Ian Smith, gave the finger to Britain and declared independence. So I have something in common with Americans—I also come from a country that gave Britain the heave-ho.

One result of our independence was that Britain—and most of the rest of the world—put sanctions on us, which resulted in a culture of invention, re-use, and repair. It only lasted for 15 years (you've beaten us by a bit there!), but I digress. Suffice to say that it instilled in me a great reluctance to throw away anything that could be fixed or used for something.

Fast forward 33 years and I found myself in Australia, a throwaway society of note, rooting around in a skip (a garbage dumpster in American English) on a work site I was visiting. (This habit of mine arouses great amusement in my Australian colleagues, but it is amazing what perfectly good or easily fixable things you find in skips here.) In this skip, I saw a nice, black, moulded-plastic case, so I grabbed it. It was heavy and had the Black and Decker logo on it. I opened it and found a perfectly fine looking cordless drill, battery, and charger.

Now, even Aussies won't throw away a good drill very often, so I immediately assumed that the drill, the battery, or the charger was not working. I pulled the trigger on the drill and it gave a weak half turn. So hopefully the drill was OK. When I got it home I plugged in the charger and put the battery in it. No lights. I charged the battery—a 14.4V NiCd—on my workshop power supply overnight and tried it in the drill in the morning. It seemed to work fine. So all I had to do was fix the charger. This was reassuringly heavy, but getting into it gave me my next problem.

The case was held together with security Torx screws, which have a little pin up the middle that has to be matched by a hole in the driver bit. I had the right bit, but it was too short to get into the hole in the charger case. A visit to my favourite tool store produced a set of longer bits for $10 (and I never need an excuse to add to my collection of screwdrivers!). When I opened the charger I found a nice hunk of iron—no namby-pamby switching power supplies here—and both windings on the transformer measured low resistance. There was a main board with five or six transistors on it and a connector board with two thermistors, one of which contacted the top NiCd cell through a hole in the battery case.

So, it was obviously a thermally limited charger. The label on the top said it was a one-hour charger. Even more reassuring, since so many chargers advertise themselves as fast chargers but don't have any limiting at all, and usually have the ominous warning in the handbook "Do not charge battery for more than five hours." Forget this a couple of times and your batteries are cactus.

I couldn't get a schematic for the charger but it was simple enough to trace. This is the diagram I arrived at:

Do you notice anything strange about this diagram? The control circuitry, particularly the emitters of Q3/4/5/6, do not have any connection to the negative of the battery or power supply. And looks like it should! I re-examined the PCB and could not find anything—the negative of the bridge rectifier only had connections to the smoothing capacitor C1 and the battery connector.

Then I remembered the thermistors. There appeared to be two of them in parallel, and one leg was connected to the emitters of Q3-6. That leg also went to a nice shiny bit of metal that, when the battery was inserted in the charger, contacted the case of the bottom battery and made the connection from the control circuit, through the battery negative outer case to the negative rail of the charger.

While I was looking at this I noticed that this connection on the small, separate PCB that held the battery connectors and thermistors did not look too healthy. I gave the wire a tug and could see the end of it moving in the blob of solder on the small PCB. Bingo! One of the dreaded dry joints. I never just resolder these. I sucked off the solder that was there—as usual, an insufficient amount due to the wave soldering of the board—cleaned up the wire and then resoldered the joint.

I was able to place the connector board onto the battery without putting it back in the charger case and lo and behold: the "Charging" light came on and the battery voltage went up. Being an impatient sort of fellow, I touched my soldering iron tip to one of the thermistors and the "Full charge" light came on. Job done! Or so I thought. When I tried to actually USE the drill, I found the chuck had seized. Maybe that's why it was chucked out. Pardon the bad pun...).

But I managed to sort that out with judicious use of WD-40, a pair of slip-joint pliers and some of what my dad used to call "brute force and ordinary ignorance." Result: it's now fully working.

For $10 and a fair bit of time, I got myself a nice drill, a useful set of screwdriver bits, and the satisfaction of bringing something back from the dead!

Don Tavidash submitted this article as part of Frankenstein's Fix, a design contest hosted by EE Times (US).