Buying an Oscilloscope on ebay
1. It's important to keep in mind that a scope is a highly complex piece of electronic test equipment, and not only has to be functional, but correctly calibrated to actually be useful. Many scopes appear on ebay, and they can be broken down into these rough categories:
A. Known bad or broken (Consider it as parts only, or repairable if you are highly skilled with access to parts)
B. Unknown condition and calibration, not shown with a trace (LEAST Desirable as a usable scope)
C. Unknown condition and calibration, but shown with a working trace (Worth considering, especially if you have lab access)
D. Condition known good, shown working (Very desirable, even for those unable to fix it themselves)
E. Known good, provided with calibration and warranty, shown working (Best possible for all buyers)
2. People sell scopes for many reasons, not all of them good for you. The unit many have problems, especially subtle ones, that can't be fixed, or the unit may be in poor condition, damaged or very old. As general rule, buy NO SCOPE unless it is shown working, unless you only intend to buy it for parts. And certainly buy nothing with no picture at all, the classic tell-tale of a disaster in the making. On the positive side, many excellent instruments get sold off as sites downsize or get new equipment, and there are some excellent buys to be had.
3. We buy a large quantity of scopes off of ebay, both working and broken for overhaul and re-sale, so we have been exposed to every possible level of seller and product, and every possible description. It is fair to say that generally, the unit will be worse than described, largely because many sellers of this type of item are not highly knowledgeable, and may be selling second hand surplus purchases or items they have no real background in selling. Also, pictures (especially small ones) really do not convey how the instrument looks, and it is hard to capture all the defects (especially dirt and scratches) in many photographs. Be prepared to be at least a bit disappointed, as you WILL NOT be getting a brand new, sparkling scope in perfect condition. That does not mean you will be getting junk, just that is will be used, probably a bit dirty, and anything not explicitly stated will be missing, including manuals, probes, and maybe even the line cord.
4. If you are an experienced technician, with good troubelshooting skills, AND YOU CAN GET A MANUAL for your scope, you can seriously consider scopes with faults and problems, but if those things are not true, you need to be very careful about what you buy. Even the skilled have to be careful.. Why?
A. Virtually all analog scopes from Tektronix , Philips/Fluke and HP are "out of support". This means no parts, no CRTs and no manuals can be had from the manufacturer. 2nd tier scope makers like Leader, B+K and many imports may actually have better support for their older analog models, but they are seldom cost effective to fix, as their new price is so low.
B. Used digital scopes that have problems are virtually unrepairable. Parts and support are hard to come by, they tend to be surface mount, fixable by board exchange only, and all older units are "out of support". be forewarned. All Tektronix TDS scopes are "factory support only", but all older units are now "out of support", so your options are very limtied. Older HP digital models are "out of support", and few spares exist, plus the manuals are not for board level repair, only exchange.
5. Many scopes (especially from Tek, Philips/Fluke and HP) cost thousands of dollars when new, some 10's of thousands, so the temptation to scoop up one for $50 can be hard to resist. Just be sure you have run through a simple review. Is it for parts or to use? If for parts, you've already looked to see if it's useful, and if the price seems good, go for it. If it's to use, be sure there is enough functionality visible for that to be possible. Is the CRT good? Do the sweep and vertical sections work? What's the physical condition?
6. But, does it REALLY work? How can I calibrate a scope once I get it? This is not a trivial issue, and the equipment required (plus the manual) is expensive. A known accurate amplitude (p-p) squarewave (from 10mv to 10V) is needed to check the vertical deflection, an accurate time/frequency standard is need to check the timebase/sweep section, and a constant amplitude generator is needed to check bandwidth. These 3 tools can do a pretty good job of checking scope performance. The front panel "calibrator" is just to set probe compensation, and cannot verify operation except in the most simplistic way. You may need to go to an outside lab for calibration if you really need known operation, or buy it already calibrated. This is a costly procedure, usually taking about 2-3 hours, and a lot of equipment. External calibration can easily be $300 or more for a 100Mhz scope, possibly much more than the ebay purchase price, and that assumes no repairs are required.
7. What models are really usable, and easy to support? Are any models especially troubelsome? This calls for some opinion, and reference to a bit of history, but for those looking to get the best value from an ebay purchase, it is possible to make some generalizations. Keep in mind, this is not carved in stone, and opinions and your results may very. However, stay away from anything with a bad CRT, spares are now very hard to find, and quite expensive. If you don't see a working CRT (the heart of the instrument), walk on by. My discusion here is limited to later solid state models, but early tube units like the Tektronix 535, 545, etc. are still excellent scopes, but higher maintenace. They remain very popular, and are covered in other on line guides.
Most popular, and easiest to support are the 465 (serial numbers above 250,00 are best), the 465B, and later serial number 475 and 475A scopes. These are real workhorses, and as long as the vertical and horizontal switches are in good shape, and not intermittent, can give good service at an attractive price. Keep in mind, early serial numbers date back to the late 60's so it can be a very old scope, so look for the highest serial numbers, and the best physical condition. The 465M modular scope (used by the military) can be good, but most have been badly supported, and can have serious physical damage if screws are left out. Buy those only if they are working. Avoid the orphan models in this 400 series, 464, 466, etc, as they have little support, and no real redeeming features.
The next generation 2200 series are a bit flimsier, but much lighter, and use switching power supplies. The best of the series is the 2235/A 100Mhz dual trace (the replacement for the 465B). Stay away from the 2205, 2225 they were made in the orient, and have serious quality issues as used scopes. The digital models 2230, 2232 have many critical parts (CCD's especially), so they must be working, and PASS ALL the on screen tests, or run the other way. There are many 2213/2215 units out there, a good low end 60Mhz scope, but watch out for bad and gassy CRTs, there are none for spares. If the unit does not have a focused, and bright CRT, skip it.
There is an analog TAS series (also imports) they have zero support, so be careful. TAS465, 475, etc. A good deal if working and cheap but otherwise, no.
The 2335, 2336 and 2337 ruggedized portables are quite nice, but wicked to repair. Again, see it working, or pass on it. Most of these have broken case latches or hinges, so be sure you get one with with all the lid and case parts intact. They are very small and good for field work if in good shape.
The higher end 2400 series have many critical 155 series hybrid parts, many of which are now totally unobtainable. 2445/A and 2465/A/B models are quite powerful, but they must be fully working, and pass all on-screen tests to be considered. This was the last CRT based analog series from Tek, and is loaded with features. Watch out for any heat restrictions (fan blockage or failure, etc.), as this makes the hybrids fail, and then all you have a nice looking doorstop.
There are also big bench scopes like the 7000 series (quite nice), which take horizontal and vertical plug ins. This is an older series, and while of exceptional quality, you need to be careful to avoid the very early frame types. Best in this series, are the large screen 100Mhz 7603, the 200Mhz+ 7704A, and the 500Mhz+ 7904A and 7903 (rackmount). These are the best designs from the series, and can be supported. There are also some good dual beam models, like the 400Mhz 7844 and storage 7934. the 74XX and 75XX series are not good units, nor is the digital frame 7854, these are older, service nightmares. the 1Ghz 7104 is very specialized, and unless the CRT is still in very good condtion, not too useful. Good verticals are the 7A26 225Mhz dual, the 7A22 differential and the 7A24 50 ohm 400Mhz dual. The 7603 generally uses a 7B53A timebase, the faster frames the 7B80/85 time base pair, the 7903 uses the 7B92A timebase.
The 5000 series plug in scopes were a low end series, and other than some use in the biomedical fiels (the storage frames) not popular. These are now very old, and they are not good used choices unless you get lucky. The quality here is not really up to the usual Tek standards.
There are some plug in TM500/5000 series scopes, the SC502 (15Mhz), the SC503 (10Mhz storage) an SC504 (80Mhz), these are all excellent scopes, and a very worthy addition to any TM500 series rack. In a TM515 travelling frame, they make a great field unit. There are quite difficult to fix, and require special tools and extenders to service, so steer clear of them unless you know they are working. The blue CRT contrst filter is often missing from these for some unknown reason.
Later Tek TDS digital scopes are factory support only, and early ones have serious issues with leaking caps on the surface mount boards. Be very careful about these as a used purchase, see it work, and see the self -test screen with everything PASSED, if everything is working, great, if not, walk away.
HP scopes:The real workhorse models are the 1740A (100Mhz) and 1741A (100Mhz storage), these are very nice scopes, and on a par with the Tektronix 465 series. There is also a later 1745A with a bigger CRT is that is even nicer. These are easier to fix than the Tek version, but parts and CRTs are not very common. Again, see it working if possible. Storage modesl are not too useful for most people, but some have a normal/storage switch on the rear, iso the storage can be disabled, to make it work like a regular scope, often a useful trick.
There is an excellent fast scope, the 1725A, a 275Mhz analog scope, with selectable 50ohm/1M inputs. This is a great unit, but has some service issues, so you need to see a full sceeen waveform at high speed before buying it.
There are other 1700 seriess models, but these are the best, and easiest to keep working.
There are many older HP digital scopes, and these are a bit problematic. Some are based on the 1630 series logic analyzer base, and are a bit clunky, but can be useful when both types of display are needed. this led to the 54200A scope, with a poor user interface, but if cheap and working, can be useful. No board level service info available, so fixing them is hard.
Be very careful with all these early 54xxx models, as they may say 100Mhz bandwidth but have only 10Ms or 20Ms sampling, so they are real artifact generators, and work only with repetitive waveforms. Not a great deal unless they are very cheap, and you see them work.
Philips/Fluke:Philips had a very extensive test equipment line in europe, and in later life teamed with Fluke to sell their scopes in the the USA.
Early analog scopes from Philips are Ok, but are now very old, and not ideal for a used purchase. Their later series with the LCD display next to the CRT or with on screen display are excellent, and still a great value as a used scope. They also pioneered the "combiscope" with both analog and digital capability; these were very expensive, and now show up surplus at good prices.
Good analog models to consider, PM 3050/52, 3055, 3060, 3070, 3072
Good mixed mode combiscope models to consider: PM 3350-90
avoid the PM 3311/12 series older digital units, they are just not a good value, and very hard to keep running.
Virtually all the LCD display units next to the CRT will lose their backlighting, but it is just a lamp, and can be repaired with a bit of effort. It can also be replaced with a few LEDs, a bit of an improvement.
Manuals and parts are just hard to find, so support is difficult, and other than the latest combiscopes, they are "out of support" at fluke.
Fluke has a series of LCD based handheld scopes, the 90 series (which came from Philips), and many later models like the 123, etc. developed here. These are good if you get the types with the white backlighitng, but hard to read if you get the original green ones. This whole series is factory support only, and surface mount construction, so it is hard to fix. the battery packs can also be trouble, and are quite expensive. If looking at a used one, you need to see it working, and working from the batteries, to be sure it's worth having.
Summary:So, there's some scope guidance, based on about 30 years of experience. Hopefully it will help you zero in on a good choice, or avoid a bad one. If you need service parts for these units, you can google Sphere Research Corporation, they do carry a wide assortment of internal parts for oth HP, Fluke and Tek, including CRTs. There are also service info pages for these brands, and many sources for service and manuals, some completely free.
Use a bit of care picking a scope, and ask the seller to send pics of it working if they aren't in the listing. You can avoid a lot of trouble by being a smart consumer, and taking a bit of time to analyze and compare what is really being offered. Most of the scopes discussed above cost US$3,500-15,000 new, so a perfect one is not likely to be had for US$25 , with a full cal. Don't forget that shipping and packing are very important to protect the scope, and this cost can easily be up to US$100. Finally, your scope is worthless without good probes, so don't forget they are part of the measuring system, and can be pricy at the 100Mhz and up range. In addition, Tek scopes and Philips/Fluke need "indicating" style probes to correctly activate the scale factor readouts, and these can easily be US$100 each. These costs, plus calibration are all important to consider, especially when comparing different offerings.
All for now,