How old is too old to buy an analog oscilloscope?

[Mint.]

How old does an analog oscilloscope have to be that it becomes less reliable, what to look out for when buying used one?

[Rerouter]

well aging is relative, it mainly comes down to how well they where looked after, and how good they where to begin with, and more often than not you hear of a story of someone reclaiming an old scope from a backyard shed of hell for the thing, doing a good job cleaning it and calibrate it and done,

biggest issue that you may face is postage, not the repair, because the CRT tubes in older scopes can be very fragile in comparison to modern day fragile goods handling, (e.g. if you want something to survive auspost, wrap it to survive being thrown out of the second story window, or whatever different regions post may be like)

if that's not a concern, then you also want to make sure you can get documentation, both user and service on the scope on hand, say on BAMA (boat anchor manual archive) or from various groups or sites, as you want to know what your getting can be serviced easily, ironically, this is generally easier to get for older scopes,

as for how old is too old, well that depends on what your getting it for, and what functionality your need,

far too many tech-heads go chasing prized tek curve tracers because back then they where designed in a way that is more useful than today's products can compete with, similar things can happen with scopes, there are some odd and rare models of various makers, designed to fill niche's that never really took off, but on there own are great products, including almost handheld dual channel CRO's, to 4 channel separate time base monsters,

[saturation]

IMHO the best scopes to own second hand use as few custom ICs as possible, do not use vacuum tubes [ except the CRT naturally] and are made by Tektronix.  The reason for it is Tek has great support among the rebuilders, 'solid state' devices are more reliable and easy to service, and custom ICs are harder to replace.

There are many old good brands, but Tek was a leap above others.   Roughly these are from the 1970-1990s.  Avoid scopes under 20 MHz, definitely the 5 and 10 MHz, because they do not have many of the trigger options found in better scopes.

As rerouter said, the next item is how the actual one you are looking at was treated or used; the best are single original owner such as by an EE, or those rebuilt by collectors who have passed away and whose estates are being liquidated.

[alm]

Not sure if reliability is the major concern, some of the Tek 5xx series tube scope still appear to work quite well, though I wouldn't recommend one based on size, weight and power draw.

[bullet308]

According to the folks I lean on for information on 'scopes (mostly the Yahoo Tek Scopes list), with Tek there is a sweet spot where there the gear is not too old, is reasonably easy to work on and get parts for and it does not use many/any proprietary ICs (and not many ICs at all). These are scopes were made in early to mid 1970s. On that basis, I was lead to buy a Tek 454A, which is a really nice 150 MHz scope. I bought two of them, both of which were complete duds (my own dumb fault), but I look forward to restoring them to functionality one day, and am reasonably confidant that I actually will be able to, owing to their being made with fairly common discrete components.

[JoeyP]

I repaired and calibrated scopes and a wide range of other test equipment for a living during the period from the mid 1980's to the late 1990's. A lot of the equipment was much older, some dating back to vacuum tubes and nixie tube readouts. In my experience, the Tek scopes were always easier to maintain. The HP scopes of that era were not so great (except for the older 180 series, which were excellent solid-state scopes at that time, but are probably too old now to be serviceable). I still have a couple of Tek 7904's that I keep around for the bandwidth. Though they do have their share of ASICs, they are so cheap now on ebay that I bought spares just for parts. These scopes seem to go and go and go, and can't be beat now that you can buy them for around 1% of their original prices. The 454 and 485 were less common, and in my opinion not as serviceable as the 465 and 475. The 465's were great in their time, but are getting tired now. There are mechanical parts and switch contacts which do wear out/break. I personally wouldn't buy anything older than a later production Tek 475 at this point. There isn't any HP scope from that era that I could recommend. Above all else, be sure to get one with a strong/well-focused CRT.