Ever since I started doing LCD monitor, and G5 iMac repair, I’ve wanted/needed to pick up an ESR (Equivalent Series Resistance) meter. Electrolytic capacitors are the bain of pretty much all modern power supplies. Pretty much everything else in a modern power supply is invincible compared to the caps… and while I’ve been extremely lucky that every iMac and LCD monitor I’ve repaired had obviously bad capacitors (bulging), that isn’t always the case… which is where an ESR meter comes in.
I won’t get into ESR in general, but you can find info on wikipedia here, or on this site. But basically, as Electrolytic caps age, their ESR increases, and generally once it rises above it set point, the device it’s in will stop working. In many cases, you can tell when this happens due to bulging/leaking. But that isn’t always the case, and having a meter is a good idea. I purchased the Blue ESR kit made by AnaTek, based off a design by Bob Parker for $73 off eBay. The kit arrived in a standard box, and was well organized. In about an hour and a half of soldering, I had the unit up and going. Really all you need is a soldering iron, some flush cutters, and if you want to calibrate the battery low indicator, you’ll want a variable bench power supply. Other than that, it was a piece of cake. While hourly, it’s not worth the $20 saved to buy the kit, it was worth it to get the satisfaction of building a kit and having it work. The instructions are easy to follow, and the unit fired right up when I was done.
I’d highly recommend it if you spend time doing repairs. At some point I’ll buy the Blue Ring Tester kit as well, as it makes diagnosing bad transformers easier, but for now, I’ve probably spent enough on test gear for the time being.
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About a month ago, I purchased a broken Keithley 199 multimeter off ebay for about $70 including shipping. From everything I’d read, it was a pretty accurate 5.5 digit, 310,000 count multimeter with a nice bright LED display (as opposed to the Fluke VFD, or HP non-backlit LCD displays). Having bought sight unseen, I wasn’t sure what exactly was broken. It could just be simple user error, or something more serious, but from the age of the unit, it should be fairly repairable. 
I recently found a Nintendo N64 at a garage sale for $5 with MK4, but no controllers, and the power cord had been chewed off by a dog or something. Knowing it was probably fine, I bought it and went about replacing the cable. Of course the gamebit I had was the 3.8mm variety (for opening games, not consoles), so I had to needle-nose it out. Once that was done, and it was open (there are some clips on the other side of the power supply opposite the screws), I couldn’t get the PCB out. After looking and looking, I just wrestled it out with some force, and found that Nintendo, for some reason, used some double stick foam tape to secure the top of the transformer to the underside of the top of the casing.