Bob's SBC6120, PDP-8 Single Board Computer kit, and its companion FP6120 front panel kit, are not continuously available because they are inherently low-volume specialty items of great interest to a few people. Even in production volumes of 50 to 100 units at a time, the kits' custom made components are much more expensive than they would be if they were produced in high volume. So these unique kits are primarily of interest to a rarefied population of true computing enthusiasts and computing history buffs. (I happen to think they are spectacularly cool! :)

To give you some idea of the sort of custom machining that goes into the components of these kits, here's an excerpt from a posting Bob Armstrong made on January 8th, 2009, as he was researching the possibility of producing another run of kits (this was the last run of kits from which I purchased three):

... and a bit later in the same posting, Bob discusses the situation with the front panel kit's special-order white C&K paddle switches:

To get a better idea about the nature of the kits, you can download and read through the single-page brochures, assembly guide & user's manuals, and browse the schematics for both the SBC6120 single board computer and its companion FP6120 front panel:

What's a “partial kit” ??

A partial kit consists of all of the custom-made, special-order, and hard-to-find bits and pieces required to assemble a complete, working PDP-8 single board computer with its matching front panel: This includes the printed circuit boards, a Harris Semiconductor HD6120 single-chip 12-bit PDP-8 microprocessor, all of the custom-programmed gate array logic (GAL/PLD) chips, RAM, and EPROMs containing the SBC's BT6120 BIOS code — essentially all of the real meat of the kits, consisting of everything that you can't otherwise order off-the-shelf from various online electronics suppliers:

You can find a complete bill-of-materials (BOM) at the back of each user manual. This combined parts list, excerpted from the back of both user manuals, shows with  yellow highlight  the parts Bob Armstrong will be supplying. You'll see that it also provides the supplier and stock numbers of everything else you'll need. So obtaining the rest of the bits and pieces should not be a problem.

If you aren't familiar with the terrific online sources for parts, you'll want to check them out too:

This combined partial kit will ONLY provide the hard (or impossible!) to find bits & pieces, the custom-ordered and custom-made parts, circuit boards and programmable parts. You will still need to add IC sockets, resistors, resistor packs, capacitors, and so on, as detailed in the combined parts list. All of these things are readily available from the sources specified, and possibly your corner Radio Shack store. Moreover, the members of the SpareTimeGizmos Yahoo forum are generally very helpful and willing to help get you out of any tight spot.

One of the guys (Robert Ribbeck) who has built Bob's kits in the past, who hangs out in the SpareTimeGizmos Yahoo! forum, and is well known to the people there, has stated that he will make up a kit of all of the rest of the parts required to complete the combined kit for anyone who would prefer to have him do the parts scrounging, leg work, and ordering. Robert has done this in the past for these and other partial kits, and everyone appears to have been satisfied with what they received.

He has stated that the cost for everything NOT provided by Bob Armstrong would be $175 with red LEDs, or $185 for the same white LEDs I used with my own kits. (This includes taxes. US shipping would add $10 and non-US shipping would add $35.) I prefer the white LEDs, since all PDP-8's up until the very last models — the 8/f and 8/m — used white incandescent lamps (the 8/f & 8/m did use red LEDs). The white LEDs, therefore, look a bit more authentic to me and somewhat less “electro-generic” <g>.

If you are interested in taking Robert Ribbeck up on his offer to round up everything else you'll need to complete the kits, you do not need to do anything about that now. Just know that the option will be available to you if you choose.