Imlac PDS-1 success!

After 3 years of part-time debugging, I've gotten my Imlac PDS-1 booting original software! I got an Imlac PDS-1 shortly after VCF East 1.0 and began to study the documentation and tinker with the machine. This was an original 4K word PDS-1 'Alpha' Imlac (meaning it does not have long vector support, and an odd non-linear display mapping designed for text). This PDS-1 had seen some modifications in it past lives. The original 4K core memory was one of the weak points, and had been 'upgraded' to use 8K words of static ram. But for several reasons the machine was unable to address the top 4K. Additionally the main CPU was rather unstalble, often crashing even when running 2 or 3 instruction loops. The quality of the logic power supplies was very poor, with high 120 hz ripple. This made me think the power supply needed to be re-capped, etc. There was also an excessive amount of high frequency noise as the original Imlac Engineers had left out the usual decoupling caps from 99% of the the logic for both processors (display and main CPU). After some power supply work, and several weeks of evenings of soldering small caps to the boards to tame the logic supplies the machine was able to run hand-toggled programs on the main CPU. The Display Processor was another story. Any attempts to draw short vectors with a -y displacement produced +y axis vectors. This was chased down to an original Imlac option board that expands the display processors address space from 4K to 8K. With that option pulled (as the machine could only address 4K anyway) I finally got the machine to reliably display the HELLO test pattern. But the basic stability problem soon returned. One recent evening I got a little lucky and found that the linear regulator for the +5 volt supply was dropping out of regulation due to a high frequency oscillation on one of the pins of the regulator IC. This was repaired, and I once again had a solid machine to work on. I discovered that for the main CPU to address 8K several modifications were needed, I had to change some wire-pairs in the external programmers console box (which already had the 8K address switch and indicators), as well as debug the logic on the SRAM modification that added the extra bits to the memory address and program counter registers. Imlac's hardware engineers also decided that the transistors driving the console lamps did not need base current limiting resistors, so they pull a TTL output to ground when lit. This nailed the new address bit low as far as the rest of the machine was concerned. A set of TTL buffers restored the new address bit with an absolute minimum of modifications. Dallas NV SRAMS replaced the srams, so the 'appearance' of core memory returned. At this point I thought I had a solid 8K machine, except for the display processor anyway. A plug-in replacement for the stock Imlac serial I/O board (110 baud!) was designed and slipped into the correct slot, with the ability to load Imlac software (stand-alone) But the machine would not boot correctly. I discovered that the bootstrap loader was having problems with the 4K address boundry, and indirect address transfers were not correctly setting the high order memory address bit. More debugging of the sram modifications brought me to a missing connection, and now I really had a working 8K word Imlac main CPU with a 4K display processor. Even after this it would not boot correctly, until I tweaked my serial interface hack slightly, and tested several binary images from Tom Uban's liberary. Then early tonight I successfully loaded and ran a terminal emulation program. It took well over 150 hours of on-again, off-again debugging, but seeing a page full of green flickering vector text was worth every moment. And there is nothing like a 16 bit mimi with real incandescent bulbs flickering on the panel! I've still got some debugging and repair to do, and then the cosmetic restoration of the display and keyboard section will begin. I imagine that in about a year more, I'll have the machine in a very cherry state, excepting that the original core has been replaced. But even that upgrade is 'vintage' (having been done in the early 1980's). So maybe the upgrade was not completed until 2005, but thats a minor detail really. I've restored five or six machines from the 1960's (this Imlac is circa 1969), but by far this has been the most envolved job yet. Most other machines need a repair or two and then worked fine, but this Imlac has been a very different experiance. Some may argue that I should have removed the sram mods and re-installed the original core. Alas, the original core (which I have) was never very reliable, and some of the Imlac software of the most interest to me requires 8K. There is also a (good) argument that the modification to sram is very much a part of the machine to preserve and has been in the machine for over 15 years. Perhaps some day soon I'll be using this Imlac as the system console on one of my HP 2116's. That would be just about the 'right' amount of blinking lights, for a single user system anyway.