This is a list of spare modules I have in my collection:

G7273 X 10 – U – Bus Grant & Non-processor grant
M105 X 8 – Q -Device selector for PDP-11
M4002 – U – KWV11-C Programmable Real-Time Clock
M7390 – Unknown
M7513 – Q – RQDX extender for RQDX1
M7555 X 20 – Q – RQDX3 MFM Winchester and floppy disk controller
M7559 – Q – TQK70 TMSCP controller for TK70
M7680 – ? – RK05
M780 – U – KL11 Teletype transmitter & receiver for KL11, 110 baud
M7800 X 3 – U – DL11 Async transmitter & receiver
M7816 – U – DQ11-AB Bus selectors and block check
M7820 – U – KW11 Interrupt control, 7-bits, 1 per PDP11 peripheral
M7822 – U – DU11 Synchronous serial line interface.
M7823 – U – KW11-W Watchdog timer module
M7846 X 3 – U – RX11 RX01 floppy disk controller
M7860 X 2 – U – DR11-C M786+M105+M7821; general device interface to PDP11
M7864 – U – DR11-L DR11-L, 2 word unibus input interface
M7946 – Q – RXV11 RX01 8″ floppy disk controller
M8020 – Q – DPV11-M Single-line serial EIA sync interface
M8300 – O – KK8E Major registers
M8310 – O – KK8E Major register control
M848 X 2 – O – KP8E Power fail and auto-restart
M8554 – U – DTE20 (Unibus Interrupt Control) [ KL-10D ]
M8946 – Unknown
M8951 – Unknown
M920 – U – UNIBUS connector
M930 X 5 – U – Bus terminator
M9312 – U – Bootstrap terminator
M9396 – Unknown
M935 X 2 – OMNIBUS bridge
M981 – Unknown
M9970 – U – M997, H854 Facing Edge

Over the last couple of weeks I have spent most of my available ‘PDP’ time bringing rebuilding as many of my QBUS machines as I can.

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

For some light relief from rebuilding the more complicated UNIBUS machines, I decided to take a break and tackle something a bit simpler, a QBUS based uPDP-11. I have a few uPDP (BA-23) chassis lying around so I picked one of these as the basis for the machine.

As the machine had only left service a mere 5 years ago, I thought I would take a chance and test the power supply the evil way. With all the boards removed, I powered on. All started up well and I confirmed the correct voltages with a multimeter. So far so good

Before I started installing cards I took a break to download the required manuals from Bitsavers (http://www.bitsavers.org) and was deeply engrossed when my nose picked up the first warning signs that all was not as it should be. Sure enough, I looked up to find smoke streaming out of the back of the PSU.

I quickly removed the power and confirmed there wasn’t an actual fire inside the machine. After airing the machine for 10 minutes to rid it of the worsed of the smell, I removed the power supply and opened it up. I was met with an impressive scene of fluff, grease and a variety of insect remains but surprisingly no signs of damage or exploded capacitors.

After a thorough clean, I returned the power supply to the chassis and tentatively put the power cable back into the chassis. With fire extinguisher on hand I waited to see if the smoke returned. After 10 minutes and no smoke, I decided to risk a power on and eureka, all came up well and voltages were still as they should be.

So I was really rather lucky here, but I guess the moral of the story is never power on an old machine without first thoroughly inspecting it.