My new project

My PDS-1D, serial number 277

As you may be able to tell from the above, my Imlac is in pretty sad physical condition.  There's quite a bit of rust on the chassis and it's been banged around a bit.  But I'm remaining optimistic, I think under the rust-covered exoskeleton is a heart of gold.  Or silicon.  Or not.  Anyway, we'll see.  Let's take a closer look.

With the front cover removed, we can see the card cage, with a few dozen cards installed.  These contain the majority of the logic that comprises the computer -- both data and display processors, I/O and display controllers. These are installed in wire-wrapped slots; you can see a few empty slots indicating potentially missing hardware -- not really a great way to start things off.  Fortunately, it does not appear anything vital is missing (more on this later).

Of interest is the blue wire-wrapped prototype board in the lower-right corner.  I'm not entirely sure what purpose it serves yet, but it incorporates 3 74181 4-bit ALU chips, likely forming a 12-bit ALU.  Based on this and its location, I suspect that it is a hand-made equivalent of the LVH-1 "Long Vector" hardware option which normally would sit in the slots directly adjacent to it (and is currently missing.)  (The LVH option allows the Imlac to draw "long" vectors -- without this hardware, any vector longer than 9 units or so has to be manually split into many smaller segments, adding a great deal of computational and memory cost.)

To the right is a close-up shot of one of the cards installed in this machine.  This particular example is a D/A converter board used to drive the X output to the display.

 Now let's traipse on over to the rear of the machine.  The rust's looking a bit more fierce here.  Other PDS-1D's I've seen just have a rear cover here, but mine has a set of core memory (two 8K core planes) and an extra power supply to run it.  The cover for the core memory is missing and as a result, the core logic board has been exposed to the elements for awhile, resulting in significant corrosion to many of the components on it.

A close-up shot on the exposed core memory logic board.  That's not the kind of thing I like to see.  Cleaning that up and ensuring good connections is going to be a pain.

Two captive screws unscrewed and that back cover (plus memory assembly) swings out to reveal the wire-wrapped backplane.  Fortunately, things here look great.  No signs of corrosion, tampering, or damage from small rodents.  And it's wonderful that the backplane is so easily accessible -- this will greatly aid in debugging.

A closer shot of some of the wire wrapping.  Kind of a maze of wires.  Hopefully I don't have to spend too much time tracing this out.

On the right side of the machine is one half of the main power supply.  This is a fairly standard unregulated linear supply, which is fortunately pretty simple.  We have two gigantic transformers to take the 120V AC and drop it down to something closer to the lower voltages we'll need to run the computer, a set of large diodes to rectify that AC, and some gigantic electrolytic capacitors (those big silver cans) to smooth the supply out into happy clean DC.

On the left side of the machine is the regulator for the supply -- this takes the unregulated DC from the right side and regulates it to a nice even 5V, 12.5V, and 15V for the computer to use.  Both sides of the power supply look very good -- there's a bit of corrosion but not on anything that should impede operation.

I was fortunate to get the "Control Console" along with this machine.  This is a separate unit from the main processor, and plugs into the backplane via two very long ribbon cables.  Using this console, one can examine/modify memory, single-step or halt the CPU, or examine processor registers.  This will be absolutely essential in debugging this machine.

The console's in great shape and hopefully will not need too much work to get going.

And of course no machine is complete without a keyboard.  This is effectively an ASCII keyboard (8-bits parallel output) with a few interesting keys on it (see below).  It's in great condition and I don't foresee any issues with it.

 

The interesting keys on the Imlac PDS-1D keyboard, apparently geared toward some manner of text processing.

So that's a brief tour of the machine.  Here's a rough assessment of what I've got ahead of me:

The Good:

• Based on the schematics, the boardset I have is fairly minimal, but complete.  I have the main CPU logic, the X/Y D/A converters, two Asynchronous serial ports, one synchronous serial port, the IOT controller and a DMA controller.  The memory system I have is different than the one in the schematic set, but it looks to be complete.  I do not appear to have the Long Vector hardware, but it may actually be provided by the wire-wrapped prototype board.  The Long Vector hardware isn't required to run the machine, but a lot of fun software (like Maze War) requires it, so it would be really nice to have.
• The wire-wrapped backplane is solid and should not be a source of issues
• The Control Console and Keyboard are in great shape, as are the power supplies.
• I actually have documentation for 90% of the stuff in the machine, both hardware and software.  While it's not extensive, it's far better than nothing.

The Bad:

•  Astute readers may have noticed there was no display covered in the tour.  That's not an oversight -- the display is missing.  As this was a custom vector-based display, this is unfortunate: given the tiny number of Imlacs left in existence, the chances are approximately zero that I'll be able to find another.  On a positive note, it should not be incredibly difficult to hook up an oscilloscope as a makeshift replacement -- the Imlac appears to output simple analog X/Y signals.  I hope that long-term I can build a suitable replacement, perhaps using a vector display from an old Asteroids arcade game or something similar. 
• The corrosion on the core memory control boards is going to be a pain to clean up.  I expect that many chips will need to be replaced.
• The rust on the chassis doesn't look incredibly pleasant.
• The 10% of the hardware I lack documentation for (namely the different core memory system and the custom wire-wrapped board) may end up being a source of problems.  Keeping my fingers crossed.

Basic Plan of Action:

Roughly, I plan to attack the restoration of the machine as follows:

1. Document what I have; take lots of pictures and scan the individual boards for future reference.  Check the boards for obvious damage or other faults while I'm at it.  This is already done.  I've put up these pictures and scans here and here, feel free to peruse them.
2. Rebuild the power supplies.  This entails (at minimum) replacing the old electrolytic can capacitors with new ones.  Some people believe in reforming old capacitors rather than replacing, I happen to think it's voodoo, especially with capacitors pushing 40 years old.  I'll be holding onto the originals so I can restore the system to its original state if I need to.  Once I have the power supply running within tolerances I can move onto step 3.
3. Power up the CPU, sans memory subsystem.  I should be able to drive the CPU from the Control Console to do very minimal functional testing.  Let the debugging commence!
4. Clean/repair/test/debug the memory subsystem.
5. Rig up an oscilloscope as a display, test/debug!
6. Test/debug I/O so I can load programs from a host PC.
7. ...
8. Profit!

I see a lot of debugging in my future.

Until next time, keep watching the skies!