I went at this again in the morning — me being anxious to see whether or not I would succeed — and we have a positive result. The machine is behaving normally.
Since this is not found anywhere on the web, I will spell it out here:
The "ROS motor error" that is characteristic of the LaserWriter 8500 can be caused by a "Raster Optical Scanner" motor that is in need of lubrication. Without it, the motor does not get up to speed in the first seconds after a print job begins, printing is aborted, and an error is reported. Or, to put it another way, the symptoms are that you send a print job, you can hear the scanner motor's high pitched whine as it tries to spin up, and then it stops/ spins down without doing anything, followed by the top two lights on the panel flashing amber. A restart of the printer is then required. Sometimes the printer will print, but it is erratic.
EPOST testing in my case, as I explained last night, showed that the scanner was faulty. The service manual's fix for this is to replace the whole scanner assembly, which these days is pointless as there are almost no parts available, and any parts available would likely have the same problems anyway after all those years. The lifespan of lubricating oil is finite.
The fix in my case was first to check the cartridge sensor assembly, which is a known solution in some cases, and not that hard to do. Mine was fine. (I found, by the way, that Apple supplied the LW8500 with two versions of this sensor; the revised version came with shimming ridges underneath the assembly, intended to ensure contact and solve the problem — and mine had these ridges.) So I decided to dig into the machine. This involved partial dis-assembly of the printer to extract the scanner assembly (a service manual is needed), removal of the scanner assembly cover (four screws and some tape), removal of the motor (three screws), removal of the (white in my case) plastic cap on the end of the motor bearing, inspection of the bearing, and a couple of small drops of light synthetic machine oil applied to the bearing (not too much or there is a danger of it betting flicked onto the lens etc.). These steps are not in the service manual, so that is why I am spelling it out. Reassemble in reverse order. The fix is probably 3-4 hrs work, depending on ability.
I should also add that after lubrication, the scanner motor is now scarcely audible, and that I was able to wash the machine's plastics (as the panels had to be more or less all removed from the machine for dis-assembly per the service manual). I also cleaned the scanner lens and mirrors with compressed air (don't touch!) before final reassembly, since it had to be opened up, and since it is not meant to be open to a world full of dust.
So my LW8500 is spic and span as well as working again.
Since this is not found anywhere on the web, I will spell it out here:
The "ROS motor error" that is characteristic of the LaserWriter 8500 can be caused by a "Raster Optical Scanner" motor that is in need of lubrication. Without it, the motor does not get up to speed in the first seconds after a print job begins, printing is aborted, and an error is reported. Or, to put it another way, the symptoms are that you send a print job, you can hear the scanner motor's high pitched whine as it tries to spin up, and then it stops/ spins down without doing anything, followed by the top two lights on the panel flashing amber. A restart of the printer is then required. Sometimes the printer will print, but it is erratic.
EPOST testing in my case, as I explained last night, showed that the scanner was faulty. The service manual's fix for this is to replace the whole scanner assembly, which these days is pointless as there are almost no parts available, and any parts available would likely have the same problems anyway after all those years. The lifespan of lubricating oil is finite.
The fix in my case was first to check the cartridge sensor assembly, which is a known solution in some cases, and not that hard to do. Mine was fine. (I found, by the way, that Apple supplied the LW8500 with two versions of this sensor; the revised version came with shimming ridges underneath the assembly, intended to ensure contact and solve the problem — and mine had these ridges.) So I decided to dig into the machine. This involved partial dis-assembly of the printer to extract the scanner assembly (a service manual is needed), removal of the scanner assembly cover (four screws and some tape), removal of the motor (three screws), removal of the (white in my case) plastic cap on the end of the motor bearing, inspection of the bearing, and a couple of small drops of light synthetic machine oil applied to the bearing (not too much or there is a danger of it betting flicked onto the lens etc.). These steps are not in the service manual, so that is why I am spelling it out. Reassemble in reverse order. The fix is probably 3-4 hrs work, depending on ability.
I should also add that after lubrication, the scanner motor is now scarcely audible, and that I was able to wash the machine's plastics (as the panels had to be more or less all removed from the machine for dis-assembly per the service manual). I also cleaned the scanner lens and mirrors with compressed air (don't touch!) before final reassembly, since it had to be opened up, and since it is not meant to be open to a world full of dust.
So my LW8500 is spic and span as well as working again.