That was quick! I had an 11/23 on my workbench that had had that backplane mod, so I took it apart to take a picture, to help you, but I guess that's OBE... :-) It's not clear that that's really a hardware problem. DEC makes assumptions about how many memory CSRs there will be, etc, and some third-party board 'fail' when they don't meet those expectations. I have a Clearpoint QED1 (I think that's the one, didn't check) that does that. I have a PDF of the "User Information Manual" for the Clearpoint Q-RAM 88B. (Oh, I see, later message says you found it.) No prints, alas. Anyone have any we can scan and put online? I would tend to use diagnostics as 'help', not my main tool in diagnosing faults. This is particular true as really bad issues can prevent the diagnostics from loading/running, so if you get skilled at fault analysis without depending on them, you're better situated to deal with the failures that prevent using them. As other have said, lacking the prints, pulling chips (if they are socketed) ought to enable you to work out which chips goes where. (I'v worked out the chip layout for a number of un-documented memory cards this way.) I don't know about your 88B, but the one I have photos of, and my other Clearpoint cards, the chips are in sockets. With luck yours will be too... If not, ask if someone has one with sockets (alas, I don't have an 88B), who can do the mapping for you. Important: once you have worked it out, pass it along! I'm trying to upload all the data I've collected about cards for which no documentation is available to the Computer History wiki. Could be. I have an KDJ11-B which has stuck bits, and that's the CPU board, so yours could be to. It's definitely worth having another small QBUS memory card to use in fault isolation when debugging. M8044's are readily available on eBay for about $20. They are only Q18, so not usable in the same machine as Q22 memory, but they are useful for debugging. I would definitely invest in one. (If you luck out and get a bad one, send it to me, and I'll swap it for a known good, tested, one. I've fixed a whole bunch of them, got to the point where the last one I did for someone I didn't even have to pull out my 'scope! I could tell from the symptoms exactly which chip to replace! :-) Well, like I said, I do have a KDJ11-B which will run the on-board startup diagnostic, but which has a bit stuck hard on the QBUS interface! So those machines seem to be pretty resilient. Although as you point out, if he can load and run the diagnostic, it's probably not the CPU. Yup. If some locations work, and others don't, it's almost certainly the memory. And since he's only dealing with a single memory card, it's probably not the bus drivers/receivers on that card, either. One can use ODT on the KDJ11-B to poke around, and find the envelope of the problem. (See comments above about not relying on diagnostics! They are better for saying something's busted, than for accurately telling what _is_ broken.) With that, and a chip->memory map, it should be fairly easy to replace the offending chips. Noel