- Tektronix 575, the vacuum-tube monster
- Tektronix 576, also a monster, built out of transistors, but still enormous
- Tektronix 577, the little "desktop" one
You occasionally see a 575 at flea markets (I have several; I bought my last one for $40). However, they are big and bulky, and if they need repair, you've got to find the right vacuum tubes (a replacement CRT is also hard to find). If you're cool with that, it's a great bargain.
I think the 576, despite its size, is the most desirable one. I've never seen one at a flea market. They still command $1000 or more. This one was Jim Williams' favorite; in his second book he wrote:
No analog lab is complete without one of these. The Tektronix 575 is an excellent choice. It is the same size as older Tektronix lab 'scopes and is indispensable for device characterization. The more modern 576 is fully solid state, and has extended capabilities and more features. A 576 is still reasonably expensive (>$1500.00). I winced when I finally bought one, but the pain fades quickly with use. A 575 is adequate; the 576 is the one you really want.
The 577 is the smallest one. I saw one at a flea market once. They go for around $500 (I think). They're pretty nice, although they don't have all the features of a 576.
Where to buy? If you want a 575, you might try the flea market, although you won't find one every month. Your best bet (if you're not in a hurry) is probably to watch Craigslist. Prices on eBay are just crazy.
Did I steer him right?
(Photographs stolen from Barrytech's "Tektronix Older Vintage Oscilloscopes" page.)