2016-09-07 17:33 GMT+02:00 Doug Ingraham <d...@dustyoldcomputers.com>:
> The most likely cause of what you are seeing is a broken wire when the > plane was originally assembled. The wire was pulled back a few cores and > the end stripped. New wire was soldered to old, insulated and then they > continued threading in that wire. Over the years the solder joint has > degraded or the wire broke at the stress riser found at one end of the > solder joint and now you have an open circuit. I've not heard of this kind > of problem on the Straight 8 but that may be due to the rarity of the > processors. It is apparently a fairly common failure on the 8I core > planes. > > As was stated you have nothing to lose in attempting a repair as the core > is useless as is. A steady hand, good desoldering tools, lots of photos > and you should be able to take it apart, effect the repair and > re-assemble. Keep in mind that the core beads themselves are extremely > fragile so take precautions that nothing gets dropped on it. Broken core > beads are pretty much a death sentence to the memory. Replacements are > unobtanium and if you decided to make the beads you would have trouble > matching the originals well enough to tune the core to work with both new > and old. You would end up making a whole new core assembly consisting of > 49152 beads. You would need to be really determined to attempt that. > > I did come up with an idea that is simply too dangerous to try. Connect a > power supply to the ends of the wire and ramp up the voltage until it just > starts to conduct. This could be several hundred to several thousand > volts. As soon as it starts to conduct the broken ends of the wire will > start to heat and the moment the current starts to shoot up (the resistance > drops) you need to cut power. You will have welded the broken ends of the > wire together. The problem is that if anything goes wrong you are in worse > shape than now and you really only get once shot at it. And the assumption > is that the broken ends are in close proximity. > > Here is wishing you a steady hand and lots of luck! > > Thanks Doug and Anders for the encouragement. I have decided to put the core in a safe place while I am gathering tools (stereoscopic microscope and better tweezers). The core repair project will be on hold until some vacation day when I have plenty of time and there are no disturbances around. Possibly in the mean time I will look into the instruction decoding problem that affects the front panel operation sometimes. I prepared a small video on the non-operating machine: https://youtu.be/8WYiUz4SNi8 /Mattis Lind PDP-8 s/n 351 (non-operating) > -- > Doug Ingraham > PDP-8 SN 1175 >