This has largely been a painless process, largely aided by the set of LSI-11 maintenance manuals I recently got hold of. Compared with their large UNIBUS brothers, the QBUS family of machines are relatively easy to rebuild as the backplanes are much simpler with no wirewrap and much more generalistic (there are no slots dedicated to a particular peripheral). Often its just a case of planning the board layout using a simple set of rules to determine their placement and all fires up well. When it doesn’t it is often just a case of reducing the setup to a simple working configuration and building it back up from there.

For anyone attempting to rebuild a QBUS machines, here are my tips:

1. If you machine is already populated with boards, carefully take note of the locations of each board in your system and then strip all the boards out.
2. Carefully clean/vacuum the chassis to remove any crud/hair/dust that has built up within it
3. Apply power to the empty chassis, for 5 seconds, 10 seconds, 30 seconds, 1 minute to confirm the power supply is working. Check the fans are all spinning whilst doing this.
4. With a multimeter, check all the DC voltages coming from the PSU
5. If all is well, start with a simple setup of a CPU, memory, SLU and bootstrap. If you are lucky you may have a CPU board with onboard bootstrap and SLU (KDF-11B or KDJ-11B), in which case you will just have to insert 2 cards.
6. Hook up a console and setup it to the appropriate baud rate for your SLU (9600 baud is the most common if you don’t know this), set your HALT switch to the down (halt) position.
7. Power on the machine. If all goes well you should see:173000
   @
8. This is an ODT prompt, an interactive utility that allows you to toggle code into memory and start the CPU running.
9. Confirm you can write to memory by depositing some values and reading them back. For example:@1000/ 0000000 123123
   @1000/ 123123

   In the above, the bolder characters are what you need to type. This deposited the value 123123 into location 1000 and then read it back (confirming that the value we had deposited had correctly been stored into memory). Try it for a few locations

10. If all is well, move the HALT switch to the up position (non-HALTED) and hit restart. If all is well, the machine should now start its bootstrap. If you have a bootstrap board or a KDF-11B you should see a "START ?" prompt. If you have a KDJ-11B, you should see a countdown as it steps through its diagnostics before giving you a menu of options.
11. If this fails, then you need to look at the status LEDs on your various boards to get clues as to the nature of the failure. These vary from CPU to CPU:
    • KDJ-11B: This has the best diagnostic output in the form of a seven segment display. This should give you a hex number detailing the nature of the problem. If you refer to uPDP-11 documention you should find a table of codes detailing the nature of the fault.
    • KDJ-11A: 4 LEDs on the CPU card should give you a rough indication of the fault
    • KDF-11B: 4 LEDS on CPU