Used test equipment reaches eBay from two very different kinds of seller. Engineers and dealers pull it out of working labs and usually know what it does. Estate clearers and surplus sellers often have no way to test it at all. Either way, the listing tells you part of the story, and the rest is in the photos and in what you ask. Here's what to check, roughly in the order things go wrong.
What "powers on" and a self-test tell you
"Powers on" in a title means someone plugged it in and something lit up. It doesn't mean the instrument measures correctly, and on a scope it doesn't even mean there's a trace. Ask for a photo of the display after the power-on sequence has finished. For a scope, ask for a probe on the front-panel calibrator or probe-adjust output. A clean square wave on the screen tells you far more than a lit power lamp.
Many instruments built from the 1980s on run a self-test when they're switched on, and an error code is useful information. It points at a circuit. On an HP 34401A, ERROR 604 is an A/D converter fault, and one well-documented repair on xdevs.com traced it to a single failed logic chip, with shorted and dried-out capacitors found along the way. Ask the seller to photograph any message, then look the code up in the service manual before you decide. A seller who says it "shows an error but works fine" hasn't told you which error, so ask.
Input protection damage
Meters and scopes get hurt at the front end, when somebody connects them to something they shouldn't. On a handheld or bench DMM the usual casualty is a current fuse, blown by measuring voltage with a lead still in an amps jack. The Fluke 87V has a fuse check built in. Plug a test lead into the A or mA/µA jack and turn the dial to a non-current function. If that jack's fuse is good, the meter chirps and flashes LEAd. If it stays quiet, the fuse is gone. Fluke says to fit only the exact fuse rating listed in the manual, so a meter with a random fuse in it deserves a question.
A blown fuse is the easy outcome. Look closely at the input jacks in the photos. Brown or melted plastic around a jack, or a scorch mark on the case near it, means current went somewhere it shouldn't have. The 34401A has an externally accessible 3 A fuse on its current input and a 7 A fuse inside, so a meter that won't read current may only need a fuse. A burnt jack usually means more damage behind it.
On a scope, an overloaded input can show up as a dead channel or one that reads differently from the others. Ask the seller to put the same signal into every channel and photograph each trace.
The line-voltage selector
A lot of older bench gear has to be told what mains voltage it's plugged into. The Tektronix 465 runs on anything from 110 to 240 V, chosen with a primary voltage selector and a regulating-range selector, and the 34401A is specified for 100, 120, 220 or 240 V. An instrument shipped from a 230 V country may arrive set for 230 V. Plugged into 120 V, it usually just won't run properly. The dangerous case runs the other way. A unit set for 115 V and plugged into 230 V can blow its fuse or damage its power supply.
Before the first power-on, look at the selector and pull the line fuse. The selector should match your mains, and the fuse should match the rating the rear panel or the manual gives for that setting. If a listing mentions a blown fuse, ask what it was plugged into when it went.
Fans and aging capacitors
Capacitors are the slow clock running on old gear. Electrolytics dry out or leak. TekWiki says tantalum capacitors in Tektronix instruments over 20 years old often fail short-circuited, which can pull a supply rail down or burn a resistor or a board trace, and the 465's TekWiki page lists shorted tantalums among its faults. Later designs have their own version. On later-production 2465B scopes, surface-mount electrolytics on the digital control board leak after a decade or so and corrode the board around them. TekWiki calls leaking surface-mount capacitors a common failure on early TDS scopes like the TDS 540.
Ask whether the capacitors have been replaced, and by whom. A recap with photos of the work is a good sign. A noisy, slow or seized fan is a simple part to swap, but an instrument that ran hot with a dead fan may have cooked something else. Ask the seller whether the fan spins up at power-on and how it sounds.
Probes, options and license keys
A digital scope listing can leave out half of what made the scope useful. The TDS 3054B came with four P6139A probes. Its GPIB, RS-232, VGA and Ethernet ports live on plug-in communication modules, and its extra functions came on plug-in application modules, all of which are easy to lose. TekWiki notes the FFT and advanced-trigger functions were permanently enabled at firmware version 3.21, so the firmware version matters too. On a TDS 540, options like video trigger (05), hardcopy ports (13) and advanced DSP math (2F) were ordered with the scope. Ask for proof of what's fitted, such as a photo of the option label or the scope's own configuration screen.
On an analog 2465B, Option 06 adds the counter/timer/trigger (CTT), Option 09 adds the CTT plus a word recognizer probe, Option 10 adds GPIB and Option 11 adds two probe-power connectors on the rear panel. If probes aren't in the photos, assume they aren't in the box.
Shipping damage on heavy bench gear
Heavy instruments break in transit in predictable ways. Knobs and BNC connectors stick out and snap off. A scope's CRT is a long glass tube held at the front and the back, and a hard drop can crack its neck. Big analyzers and rackmount gear can arrive with a bent frame or bent rack ears after a trip loose in a single box. Ask how it will be packed before you buy. Good answers are a double box with foam or thick padding between the boxes, the front panel protected so the knobs can't take the load, and, for an analog scope, padding around the CRT end. A seller who plans to wrap it in bubble wrap and drop it into one box is setting you up for a damage claim.
Ex-lab, ex-school or surplus
Where an instrument has been tells you how it was treated. Ex-lab units, often with an asset tag and a row of cal stickers, usually lived on a bench, got calibrated on schedule and were retired when the lab moved on. Ex-school units tend to show more cosmetic wear and more abuse at the inputs, since students blow fuses as part of learning. Ex-military and government surplus can be very good, and some models were built around military needs (TekWiki describes the 465M as the commercial version of the AN/USM-425 military scope). Surplus lots are also where you find units missing handles, covers and manuals after years in storage.
Old cal stickers and asset tags show the instrument had a working life. They say little about how accurate it is today. The calibration guide covers what a sticker does and doesn't prove.
"Untested" and "as is"
Treat "untested" as "doesn't work" until the seller shows you otherwise. Sometimes it really means they have no way to test a spectrum analyzer. Sometimes it means they tried and didn't like what they saw. "As is" means the seller is telling you not to count on it. Buy one only if you can repair it, you want it for parts, or the photos show it working anyway. Either way, ask whether they powered it on at all and what happened when they did.
Starting a bench from scratch? The bench build-order guide covers what to buy first, or you can browse the catalog.
