Eventually, I will describe how to "hobby" an AS/400. That is, to use it completely out of its native environment (a business) at home for learning and experimenting.
The folks at Midrange.com offered me some useful advice. I am currently trying to find a machine and will eventually combine it into this website.
The key here is to get one with the software. OS/400 is quite difficult to come by otherwise.
Steve Richter says:My suggestion would be to contact a local IBM i BP or IBM i ISV. They'd probably able to give you an old machine from the 150/170/250/270 range for a few crates of beer. I would offer something, but shipping over the big pond is ridiculously expensive. There are a few machine models that may be interesting for you - almost all of them have "bigger brothers", but the smallest models are probably most interesting to you. Model 150: The oldest one, extremely small and not noisy. Supports Ethernet with a #2838 card, not all of the models had them. Console is Twinax. I think the max OS without keys was V4R5, and newer OS were possible but uncommon. Model 170: Probably the most easy to find, as these machines have become completely impossible to use due to their extravagant slowness. Latest OS on it is V5R3, but if you get such a machine with V5R3 it will be even slooooower. Also uses #2838 Ethernet cards, which were pretty common. Console is Twinax. Model 250/270: Also quite old, but it a bit newer. The 250 maxes out at V5R3, the 270 at V5R4. Ethernet is standard, and they also support Operations Console with Ethernet or a Serial cable. I believe the serial cable version was more common, but i may be wrong. Model 800: They run up to V6R1, Ethernet is standard, and OpsCon with both Ethernet and Serial Cable is possible. They offer decent speed for a single user. It's possible to get one of these for free, but the likelyhood will be low. Model 520: The previous generation machines. Most of them are currently nearing their 3 year mark and they're up for replacement in shops that keep current. Can run V6R1 and will be able to run post that. Can use OpsCon (comes with dual ethernet standard, so you can always convert one to OpsCon LAN), HMC and Twinax. It would probably be very difficult to get such a machine for free, unless you're a very good bargainer. Model 515: Hardware-wise pretty much the same as a second-generation 520, it has a fully unlocked processor and is instead priced per user. Licensing is not enforced, so it would probably the best machine to have fun with. But the model was launched later, their 3 year mark is up in 1.5 years or so. My Advice: Get a 270 with V5R4 minimum. Newer is always better. Even a low-end model 270 is ridiculously slow. 150 and 170s are even worse, especially when running a current OS.
The model 170 dates from around 2000 and is very reliable hardware wise. a lot of spare parts are available. I would avoid the double wide models, which I think is feature 2290. Those models are too heavy for one person to lift on their own. On the legacy models like the 170 I would definitely choose a twinax system console. For that to work you need a twinax controller card in the system. If the 170 does not have one, it is easy enough to buy on ebay. Install is simple task. To boot and logon to the system you need the QSECOFR password. To reinstall the OS from tape you need the DST password that was in effect when the tape was created. To program the system PDM, SEU and the RPG compiler is needed. PDM and SEU are for editing. RPG is for compiling. Many models on EBAY will come with the OS installed, but no editor and no compiler.