As I started devoting more time to hobby electronics, as opposed to that which I did for work, I decided I needed some better test equipment at home. I had gotten a good taste of high-quality and expensive equipment at my various jobs, including oscilloscopes, logic analyzers, frequency counters, and more, and my single multimeter at home just wasn't cutting it.
So I got on ebay, and found to my surprise tons of older test equipment for dirt-cheap prices. Most of this equipment is more than sufficient for the type of work I do at home, even though it's decades old. Technology simply hasn't changed that much, except in the area of high-frequency (> 100MHz) digital electronics where newer equipment is a must. I doubt I'll be doing anything at home faster than maybe 20MHz (certain microcontrollers run this fast) for quite some time.
One of the more useful pieces of equipment you can have, which usually isn't terribly expensive, is a DC power supply. When creating new circuits, there's many times you want to just apply a specific supply voltage, without having to create a custom power supply to do so. Power supplies like these HP models I have allow you to simply dial a voltage or a current, and the supply will maintain that voltage or current regardless of load. The analog meter allows you to see the voltage or current output; even though it isn't as high-tech looking as a digital display, it looks attractive and serves the purpose, and unlike a digital display allows you to see rapid changes.
These HP power supplies are all part of the popular 6200 series which was made in the 70s and 80s after HP purchased Harrison. They are all-analog, linear supplies. They're not terribly efficient, but they have excellent ripple and regulation characteristics, and being constructed with all discrete components, are easy to repair.
The first supply is the 6206B, which supplies up to 60V at up to 0.5A. This is a good general purpose supply since most analog circuits use less than 60V. The second supply is the 6207B, which supplies up to 160V at 0.2A. This is good for higher-voltage work. So far, I've only used it for lighting up Nixie tubes. The final supply, which is quite large, is the 6286A, which supplies up to 20V at 10A. This is good for high-current needs, and is great when doing any automotive (12V) work, such as powering stereos, HID headlights, or any other circuits meant for use in a car.
I'd really like to find either a dual or triple supply next, such as the 6236B/C, so that I can test analog circuits using positive and negative supplies, like many op-amp circuits.
Other than calibration of the meters, these power supplies have required little or no work, except for the 6286A. It was wired for 240V operation with a British power cord, requiring me to change the strapping on the main transformer and add a new power cord. It also had very poor regulation until I replaced a bad electrolytic capacitor, a common failure mode on equipment this age. They all cost less than $75 on Ebay.
The most useful piece of test equipment, in my opinion, is definitely the oscilloscope. It allows you to probe a working circuit and see exactly what's happening in it, basically showing voltage versus time. There are two basic types of scopes, digital and analog. Most digital scopes these days are "storage" scopes, meaning they can store waveforms for later viewing and analysis. Analog storage scopes such as the Tektronix 466 are a rarity, and their usefulness is debatable.
While digital scopes certainly are quite useful and have their place, for hobbyist usage an analog scope is usually sufficient, and has several big advantages over a digital scope: