From neurotica.com

This article is a work in progress.

This article is intended as a general guide for selecting a used oscilloscope for basic electronic repair use.

Market saturation works in our favor here. With less and less actual engineering work being done in the USA, more and more high-grade equipment hits the surplus market when companies or departments close up shop.

Contents 1 Analog vs. Digital Oscilloscopes 2 Brands 3 Features To Look For 3.1 Multiple-Trace Capability 3.2 Triggered Sweep 3.3 Bandwidth 4 Specific Models 4.1 Tektronix 4.2 Hewlett-Packard (now Agilent) 4.2.1 Digitizing Oscilloscopes

Analog vs. Digital Oscilloscopes

First, a word about digital oscilloscopes: contrary to marketing, they aren't simply a "modern replacement" for analog oscilloscopes as many (usually inexperienced) people believ. They are different tools that are suited to different jobs. There is a lot of overlap in the jobs that analog and digital scopes are suitable for, but it's far from 100% coverage. And most importantly, information displayed on a digital scope can be just plain misleading if you don't know exactly what you're doing, which comes from a thorough understanding of measurement using an analog scope.

Brands

Many companies make oscilloscopes. Brands you'll see frequently are Tektronix, Hewlett-Packard (now Agilent), LeCroy, Nicolet, Hitachi, Philips, BK Precision, and others. Tektronix and Hewlett-Packard are generally the most respected and well-known. LeCroy makes very high-end digitizing oscilloscopes. Nicolet used to make high-end digitizing oscilloscopes for laboratory applications, but primarily very high-resolution units for low-frequency work. BK Precision makes much lower-end (but still reasonably solid and serviceable) analog units.

Features To Look For

All oscilloscopes perform the same basic function: They present an input signal as a graph of voltage in the Y axis against time in the X axis. Wrapped around that basic function, however, is a highly variable feature set. As with everything else, some of those features are things you'll need most all the time, while others are less important.

Multiple-Trace Capability

Dual trace is pretty important for many applications. Having multiple traces just means that the oscilloscope can display multiple signals simultaneously. Generally this is done by either displaying the signals one after the other on alternating sweeps, or by chopping rapidly between them, drawing a segment of one signal, then a segment of the next, etc. The former is most often used for higher sweep speeds while the latter is used at slower sweep speeds. There are some quad-trace oscilloscopes out there, but dual trace has been the defacto standard for many years.

Dual-beam oscilloscopes aren't seen much anymore, but they do have one impressive capability: They can display two signals simultaneously but with different sweep speeds for each. One example of a dual-beam oscilloscope is the Tektronix model 555.

Triggered Sweep

Some units (the BK Precision model 1403 comes to mind) don't have triggered sweep. You really want triggered sweep. Sweep triggering holds the start of the horizontal sweep until the voltage of the input signal crosses a set voltage threshold. Trigger circuits can usually be set to trigger on the positive or negative slope, so the sweep starts when the input voltage exceeds or drops below the trigger threshold. The trigger threshold itself is usually set with a knob.

Some oscilloscopes will have trigger threshold presets, like "TTL" for example, which will set the trigger threshold to a value that is appropriate for TTL logic circuits. Such features can be convenient, but are generally not all that important. It doesn't take much effort to set the trigger threshold with a knob.

Bandwidth

The "megahertz myth" from the Intel PC world works in favor of knowledgeable people in the used oscilloscope market. Nontechnical folks have been taught by marketing people for many years that more megahertz means more powerful and newer...and not new means old, and old means bad. Since a 150MHz PC would obviously be useless for most applications, then a 150MHz oscilloscope must be useless too...despite the fact that there's no correlation whatsoever. More vertical amplifier bandwidth does not mean a better oscilloscope, it just means you can look at faster signals in more detail. Sometimes that can actually be a hindrance by showing too much high-frequency information when that's not what you're looking for. Some oscilloscopes even have a "bandwidth limit" setting which limits the vertical amplifier bandwidth to around 20MHz to address this problem.

For example, I frequently look at digital logic signals with my Tektronix 2465A. When I want to pay attention to signal transition times or patterns, all the power supply hash (switching noise, etc) and other garbage picked up by the probe just gets in the way. It all goes away when I hit the "20MHz bandwidth limit" button, and I see only the signal that I was looking for.

The minimum bandwidth you'll want for general work (say, audio and low-speed digital troubleshooting or exploring) is 30MHz or so. Nearly all oscilloscopes will handle this.

Specific Models

Tektronix

If you want the best bang for the buck, look for a Tek 453 or 454. Expect to pay $50-125 for one of those. Here's some info on these models:

• http://www.reprise.com/host/scopes/default.asp
• http://www.diyguitarist.com/TestEquipment/Tektronix453.htm

Larry Snyder, a Tektronix employee from 1975 to 1986, points out that CRTs and HV transformers for the 453/454 are difficult to find. I agree, but this shouldn't discourage you from obtaining one, because the failure rate is very low, and getting an entire spare oscilloscope is easy and cheap.

The 465 (not the "TAS 465", that's a different instrument) came after that, and was the "gold standard" of oscilloscopes for many, many years, with many still in service in labs all over the world even today...they do everything an oscilloscope needs to do, and they just don't die. The 475 is a faster follow-on model. Expect to pay $100-250 for a 465 or 475 in good condition.

More notes from Larry Snyder: "465 -- well over a million of them out there. That in itself says support won't be an issue. The only common problem was the contacts on the trigger view switch tended to get crapped up from the fan, due to the normal airflow in the box. The 465B addressed that and several other potential issues. I have a B. It's great. 475 is essentially a 465 on steroids. Later serial numbers are better. ... For almost anyone, at least today, I'd say 465B."

Now for more modern designs. Avoid the 22xx series; they were Tek's attempt at "low cost" scopes and are generally not up to their usual standard. Note that they're still better than many of the cheap import scopes that are flooding the market today.

If you can afford to spend a bit more money, the 2445 is a solid instrument with a complete feature set. It is a much more modern scope with neat and useful features like on-screen cursors to aid in measurement. It is also a bit lighter than its predecessors, so it's easily portable, either on an airplane or moving between the closet and the test bench. Around the time these were built, Tektronix made the transition from pure analog to analog with a digital user interface. All the controls are digital (tap-up/tap-down buttons, shaft encoders instead of plain rotary switches and variable resistors, etc) to facilitate configuration storage and presets, as well as on-screen textual display of scale parameters, trigger threshold, etc. This also opens the door for remote control of settings, and some of these later models have GPIB (IEEE-488) interfaces to do just that. I have a soft spot for the 2445 in particular because I used one for a long time when I worked at Princeton. Despite the excessive number of oscilloscopes in my lab (sometimes I think I have a problem) I would still like to have one if I can get it cheaply.

Expect to pay $125-300 for a 2445. They occasionally go below $100, but not often. This is a tremendous amount of functionality for the money, and a very solid and well-built instrument.

The 2465 is "more of the same". I have a 2465A; it's a superb instrument. They can be had for anywhere between $200 and $600 or so nowadays.

Starting with the 24xx models, Tek started using custom chips and hybrids in their scopes, making them difficult and more expensive to repair. However, in practice, they so rarely break that it's really not a big issue...just don't buy a broken one. I bought my 2465A used in 1999 for about $1200, it sees medium-duty use here, and it has never neede repair of any kind.

All of the above-mentioned oscilloscopes are practically indestructible and very easy to use, and are/were top-end, lab-grade models, not toy "hobbyist" scopes. (if you're going to do something, do it right, or don't bother at all) I would say the "sweet spot" in the balance between cost, functionality, and modernness is probably at the 2445/2445A now, and will be at the 2465/2465A/2465B in 2-3 years.

Hewlett-Packard (now Agilent)

Digitizing Oscilloscopes

(this section is incomplete)