The third piece of equipment I will talk about is Oscilloscopes. I do not intend to explain how to use one, or what to use it for. That is outside the range of this article, and available elsewhere. I do think you should consider adding one to your arsenal of test equipment. Used scopes from the Big names are readily available on Ebay, at surplus stores, and in swap meets. They go for relatively small amounts of money. The scopes that are on the market from the Big manufacturers (HP and Tektronix) are on the order of Lab scopes, and a very short time ago were state of the art. What I am going to cover here are Analog models. Stand alone Digital Oscilloscopes are priced out of reach for most hobbyists. There are PC based scopes, and they work well for their use, but a true Analog scope is much more versatile for most of our needs.
Buying a used analog scope can be a great deal, but there are some pitfalls to watch for. Many people who sell them do not know how to test them properly, so you may be left with a vague description. Trying to do serious repair on one may be hard, if not impossible. And if the CRT is bad. It is truly junk. Make sure you buy it from a reputable seller, and that you have some legitimate assurance that it has a useable trace, and will display a waveform from at least the test terminal. The one you buy may very well need a good contact cleaning.
In the below, I am assuming you will buy a used lab type scope made by a Major manufacturer (Hewlett Packard or Tektronix). One other note. For most of these the service and operation manuals are also still readily available if you should need to service one.
Any Major brand Lab scope will have basic accuracy that is far beyond what you need for hobby use or experimenting. If you are just a bit lucky, and, careful in your selection - buying one in decent shape - the calibration will most likely also be close enough. You are looking at waveforms and relationships, not doing precise measurements! Actually doing a full calibration can be extensive and time consuming, and require specialized equipment, but you most likely don't have to worry. A basic calibration of say the voltage scales is relatively easy, and it will likely not be necessary. My point, I guess, is, pick a Big Name Lab Scope, in decent condition from a reputable seller, and make sure the basics (Trace, displays a waveform) are OK, and you will most likely be fine.
Most every scope available will have dual channels. Do not get a single channel model. Two channels are necessary for many measurements. More than two channels is harder to find, and I do not think it is worth the extra cost.
This is a feature that allows you to delay the start of the trace a fixed amount of time after the triggering event. Very handy for finding Glitches, and troubleshooting digital circuits where the trigger comes from the clock. A worthwhile feature to look for even if it is used only occasionally.
Just like the Accuracy, most any Name brand scope will have more than enough ranges for virtually any application. Pay some attention to the Bandwidth. At least 100mhz. Any lower will not serve the experimenter very well. Many are available with 250 mhz bandwidth, and this is worth looking for. You will not likely need anything higher for general purpose use, or troubleshooting. BE aware also that the "calibration" bandwidth is usually much lower than the "usable" bandwidth. A 250 mhz scope should serve well for testing and troubleshooting well into much higher frequencies. The calibration will be off, but you can still check for signals, and get a good idea of what they are. The biggest limitation is normally the maximum sweep speed.
These are scopes that use a special mesh CRT to increase the persistence. This allows you to temporarily "store" the display. In my experience, this is not a very useable feature (some will dis-agree), but it adds a lot of complexity to the scope, and requires a special very expensive CRT. Additionally, the operator has to be very careful with beam intensity, and measurement procedures. Not using these correctly will destroy the CRT, and many I have seen are burnt, or have a (heat) distorted screen. I would avoid these like the plague!
Any name brand scope you are likely to get will have triggered sweep, but I wanted to mention it. A scope without triggered sweep is virtually useless. These are commonly called Service scopes and many inexpensive ones are available. Do not bother! Caution! Many, Many, B&K scopes are of this breed! Be careful!
Most name brand scopes have some provision for an external Y-Axis (Horizontal) input. This is a very useful feature for experimenters, and you should make sure it is on your prospect. Some use the second channel of the Dual channel for the input (desirable, if you can find it), but most just have a separate BNC Connector, often on the back.
You will need probes. And, in my experience they go bad regularly. So avoid Used, unless you know where they are coming from. Many places such as MCM, JDR, Digi-Key and Newark sell good probes for reasonable prices. Get a kit, as the tips don't often fit on another brand of probe. And contrary to some beliefs, operationally they are all the same. You don't need a Tek probe to work with a Tek scope. Quality wise though, they are all over the board.
Many manufacturers (Especially HP) have add-on adapters that increase the usefulness of the scope in some way. (TV Sync separator, Interval Meter, DMM, etc)I do not have experience with many of them. These are fine if they come with a particular scope, but, don't use it as part of your decision. I have made use of HPs DMM on rare occasions as it is on one of my scopes, but, not a deal breaker.
The subject of Military surplus could rate a book, just for test gear. There are a large number of ex-military scopes out there. The main things they have going for them is they are normally built much more ruggedly than civilian grade, they use better Mil-Spec parts, and they MEY have had better maintenance. Most Mil spec scopes are derivatives of civilian designs, and often exactly the same. One that is very popular, that I am aware of is the USM-281. This is basically a Tektronix 4000 series. I do not know much first hand about the myriad of choices, so I cannot comment, but my take is that they are just as viable as civilian equipment if the deal is right, and, if you value ruggedness, may be a better choice. One note. Military equipment is usually built under contract by a number of different suppliers, so even though it may be a Tektronix design, and number of companies may have actually built it. Fortunately however, they are all held to the same Mil-Spec standard.
This is an HP1710B scope. It is 100mhz with dual trace, and delayed sweep. I bought this at the Dayton Hamfest about 15 years ago for about $150.00. It worked great for most of that time, and served my needs perfectly. It's accuracy when I bought it was just about spot on, far more than I required. About 4 years ago it developed a problem with the intensity of the sweep (could not adjust, and way too bright). So, along came the one just below.
This is an HP 1715A scope. 250 Mhz, also delayed sweep, and an add-on on top to measure period and voltage (using on screen cursors). I bought this on Ebay 4 yrs ago for about $100.00. I picked this model because it is much the same as my previous scope, so I could use it for comparison troubleshooting. IT also came with a service manual. Between the two, I found a bad transistor in the intensity drive of my other scope. This one needed a SERIOUS cleaning of all the switch decks and pots. A can of WD40 later, and it works just fine. It is not as accurate as the other one, but close enough for my uses.
There are many models of these HP scopes in this series that were made in the 70s and early 80s. I have used a number of the same scopes professionally, and I think very highly of them. They are relatively compact, and perform very well with a lot of features. The biggest difference is in additional features, as the basic scopes are about the same. Some are 100 mhz, some are 250 or 400 Mhz. Again, stay Away from the storage versions!
This is a picture of my very old and classic Tektronix 545. Believe it or not there are still some of these in service. You can also find them as props in Many movies (Usually labeled as 'rocket launcher controller'. This was the standard Lab scope through the 60s. I still used it through the early eighties. It is big, heavy, has a big noisy fan, but works very well (and keeps the shop warm). It is full of tubes. Many of them are special Matched sets for use in Push Pull circuits. It accepts Plug in modules for the vertical. as below. I apologize for the picture. This is now "buried" at the bottom of my storeroom, and hard to take a picture of.
|This is a picture of a plug in module for the Tek 545 above. Note the tubes. This is a comparitor module that allowed you to compare an input signal with an internal variable precision reference voltage.|
Finally, This is a picture of a couple of scope probes. The one on the left is a 'modern' aftermarket x10 probe. On the right is an older 'official' Tektronix probe. It has a very stiff, and relatively large cable, just the thing for those tight spots.
The major maintenance you can do is to keep them clean, repair and replace probes, and clean the switch contacts occasionally. A genuine calibration is rarely justified for hobby use. I would suggest if you get a good example of one of these older scopes, that you pursue getting a service and/or operation manual for it. It may pay off.