Ashley wrote: Assuming that you want something good but cheap, your best bet is probably to try to find an old Tek 465. It's a good, solid two-channel 100 MHz scope. ISTR that they show up on eBay frequently, but as with anything on eBay, there's no telling what condition it would really be in. Ideally you'd get one with probes, as new scope probes are somewhat pricey. Note that you need a scope that has a rated bandwidth substantially faster than fastest digital signals you expect to probe. For instance, with a 100 MHz scope you're not actually going to get very good results probing a 50 MHz square wave. But 100 MHz bandwidth should be sufficient for anything you'll find it most PDP-11 CPUs. Eric

OK, a little introduction... The basic idea of a 'scope is to have a CRT where the spot is deflected horizontally by the X-plates and vertically by the Y plates. Therefore the picture on the screen is a graph of the voltage applied to X against the volage applied to Y. Now, normally the X-voltage is a ramp signal produced by a circuit called the 'timebase'. The spot on the screen starts at the left side, moves across at a constant speed until it gets to the right hand side, then quickly 'flies back' to the left side and starts again. This means the screen image can be considered to be a graph of the signal on the Y plates against time. There is also normallly an amplifier that takes the signal under test and amplifies it to apply to the Y plates, for obvious reasons. Finally, there's a trigger circuit. It's obviously essential (at least for repetitive waveforms) that the trace starts on the same point of the waveform each time. Otherwise you'll not get a steady display. Same point really means same voltage and sloping in the same direction (either up or down). This then starts the timebase ramp. Notice I said 'normally' when I said the X-axis was the timebase signal. On most 'scopes yuo can apply external signals to both X and Y. This is how you get the lissajou's figure beloved of science fiction film producers, and which do actually have some real use for comparing the phase of 2 sinusoidal signals. I also said 'normally' when I mentioned the Y amplifier. There have been a few 'scopes where the input is applied directly to the CRT plates, normally because no normal amplifier would have a high enough bandwidth. The Tektronix 519 is the clasic example of this. You DO NOT want one of these for general-purpose work. Some 'scopes can display multiple traces -- that is serveral signals at the same time, normally on one common timebase. You can then compare the relative phase or timing of those 2 signals. There are 2 main ways to do this -- double beam scopes have 2 electron guns and display 2 spots simultaneously. The CRT has separate Y plates for the 2 beams and normally a common set of X plates but there have been 'scopes with separate X plates for each beam too. The other way is to electronically switch one set of plates between the 2 signals -- either 'chopping' which involbes swithcing between the 2 signals in much less than the timebase sweep time (so effectively you are displaying them simultaneously) or 'alternate' where you select one input, sweep the timebase (thus displaying it), then select the other input, sweep the timebase again, and so on. Note that some double-beam 'scopes can take multiple trace plug-ins on each input. So you might have one beam switched between 2 signals, the other switched between 4, and thus have 6 traces o nthe screen at once... Storage 'scopes are always useful. This means the image on the screen is stored, either electrostatically in the CRT, or by digitising the input signal, then 'playing it back' to the CRT. This means you just need one sweep of the timebase to capture the signal, you can then look at it for as long as you (sensibly) like. This is obviously useful for non-repetitive signals, which can't be usefully displayed on a repetitive timebase and a non-storage CRT. Delayed timebase is also very useful. What this does is to detect the trigger, then wait a certain (settable) time, then sweep the timebase. You might, for example, waits 10ms, then sween the timebase in 100us. This lets you 'enlarge' a small section of a complex waveform. Differential Inputs. Many 'socpes can display the difference in voltage between 2 input sockets, ignoring the voltage between them and ground (there will be a limit on this voltage, so don't try applying 1000V or anything stupid). You might want to look at the outputs of the differential amplifiers in a disk drive read chain or something Plug-ins. Many of the better older 'socpes had the Y input stages (at least) as plug-in modules that could be replaced with other modules. You might get simple amplifiers, multiple-trace amplifiers, differential amplifiers, curve tracers, etc. Personally, I like this idea, but collecting the plug-ins becomes as addictive as collecting computers... Now, as to user controls : 1) CRT images controls. Birghtness and Focus, basically. Keep the brightness as low as you can, both to protect the CRT phosphor and to give a sharper trace 2) Shift controls (X and Y) move the trace around the screen. Pretty obvious. Multiple-trace 'scopes have a Y shift control for each trace. Multiple timebase 'scopes have an X shift control for each timebase. 3) Y amplifier gain. There'll be a switch calibrated in 'V/cm' or 'V per division' or something like that. The meaning is pretty obvious. If you set it to 2V/cm and the trace is 1.5cm high on the screen, that's 3V. There will also be an adjustable control with a click-stop at one end. This lets you very the gain, at the click-stop position you get the gain marked on the switch. 4) Timebase speed. Similar in concept to the gain controls, but calibrated in s/cm (or ms/cm. us/cm). If a trace on the screen taks 5cm for a complete cycle, and you have the timebase set to 10ms/cm. then that's a period of 50ms, or a frequency of 20Hz. Again there will be a variable control with a click-stop to get you the calibrated times 5) Trigger. There will be a trigger level control that sets the voltage level that starts the ramp, and a slope switch that selects between rising and falling. THere will also normally be a trigger selector, which lets you take the trigger sigal either from the Y amplifier (normal for simple work), another input socket (you might, for example, want to trigger from the index signal of a disk drive, but display the output of the read amplifier -- in fact that's exactly what you do for head alignment), 'line' (trigers from a transformed-down mains signal, useful for seeing if a ripple comes from the mains or not), and maybe some 'TV' modes, useful for locking to a video signal and useless for almost everything else. As to good 'socpes. I would recomend a Tektronix, there are very few bad designs -- but there are some very specialised ones that are not suitable for what you want. Depending on the sort of instrument you want, you might consider : 545, 547, 555 -- Old, heavy, full of valves, and built beautifully. People collect these like we collect PDP11s, but some people also like having a more modern 'scope alongside them, rather as most people who have classic computers have a modern PC too (I don't!. I have my 555 and love it). Plug-in amplifiers, very versatile. The manuals are nothing short of excellent! 561, 564. Smaller, valved, not as high bandwidth IIRC. The 564 is a CRT storage 'scope 465, 466, 468, etc. Smaller, portable. I think the 468 is a storage 'scope, one of that series is, anyway. No plug-ins. Transistorised. 7000 series. Plug-ins for X ad Y. Full of custom ICs that are essentially unobtainium now. I used a 7904 (2 Y plug-ins, 2 X plug-ins, 500MHz bandwidth) at university, and loved it. But due to the custom chips I'd not want one as my only 'scope Avoid the 5000 series. They're low-bandwidth devices, not what you want at all. The manuals have really gone down for more recent 'scopes. You don't even get a schematic now... -tony