I recently received this question from a fellow retro-computer hobbyist. Since it was such a good question, I figured I’d post my reply here on this blog. Though I can’t make a specific recommendation that is right for everyone, here is how I responded to this inquiry.
I use an old 100MHZ Tek 465 that I picked up off of ebay for about $80 (shipped). I was lucky, and it was almost completely functional, when it arrived. I spent another 50 bucks on probes and some minor repairs (replacing broken knobs). Total cost was less than $130.
Prior to landing this unit, I would occasionally borrow a more modern HP 20MHZ 4 channel digital storage scope from work. The user interface was nice and storage was great to have, but the bandwidth was a bit limited, considering early Apple computers have a base 14MHZ clock.
At times, it wouldn’t hurt to have more than 2 channels and occasionally a little more bandwidth would help. So I guess I think that 100 MHZ, 2 channels is minimum for my purposes. 100MHZ sounds fast, but remember that you want to see glitches and signal slope, not just digital highs and lows. The real neat thing about using this old scope is that this same tool was in use in the 70′s and 80′s by developers of the first personal computers. The price was right, too. At the time I bought it, I don’t think that you could touch the bandwidth at that price any other way.
A modern digital storage scope would be easier to use, especially on transient signals, as the scope stores the capture and you can view it at your leisure. On a traditional analog scope, a transient just flashes on the screen and is only present as long as the screen phosphor glows (one shot).
There are also digital storage scopes that plug into PC’s USB port and use the PC as the brains and display. Some of them have logic analyzer functions, which would be nice to have. Many of these units have limited bandwidth, compared to my old 100MHZ Tek.
I guess the bottom line is you need to decide much you want to spend and what kind of features are important to you.