"Developed frequency-changing whine of bad bearings. Not really sure whether a fan motor or main motor."
(From: Tony Duell (firstname.lastname@example.org).)
A service manual probably wouldn't be a lot of help. The SX printers (HP LJII, Apple Laserwriter 2, etc.) are very modular, and AFAIK, the official way to repair them is to replace the defective module. Of course it's possible to repair them at a much lower level than that, which is what I do to my pair.
There are 3 motors:
By finding out when the noise occurs, it's possible to figure out what is causing it.
Firstly the good news. I have never had a main motor bearing fail. They're ball races anyway (Yes, you can strip the motor down and rebuild it). I've never had noise from the gear train either.
Now the bad news. Fan failure is common. The bearings are bronze bushes, and they fail. Sometimes lubrication helps a lot, sometimes things are just too far gone.
Lower fan. Remove the toner cartridge and paper tray. Turn the printer upside-down and remove the screws that hold the base cover on. Remove the cover. The fan is obvious, and is held down by 4 screws (there's a bracket at one side that comes off as well), and it plugs into the smaller of the PCBs in the machine. Remove it. You can then unscrew the motor from the fan and slide the motor + blades out. Then pull the blades off the motor, remove the circlip, and pull the motor apart. Examine the shafts and bearings. If they're not too badly worn, a drop of oil (3-in-1 or similar) will probably cure the fault. Otherwise you need a new fan - parts are impossible to obtain.
Upper fan. Remove the paper tray and toner cartridge. Undo the screws and remove the outer casing. Remove the 2 screws and lift out the fan duct + ozone filter (on top of the AC block). The fan is under it and is held down by 3 more screws and a 2-pin plug on the upper (fuser protection) PCB in the AC block. There are at least 3 different upper fans in use, so I can't give directions for dismantling them. But in general, the motor can be unscrewed from the side of the fan, and the bearing on the other side pulled out. Inspect them as above, and try a drop of oil.
Editor's note: If you buy a replacement for this, it will likely be a 24 VDC muffin style fan (like in your PC, except that is 12 VDC). HP or Apple will likely charge you $70 or more for this part!
(From: Frank Reid (email@example.com).)
If the upper fan fails, the fuser assembly will overheat and do about $200 damage. Cleaning the bronze bushing often solves the problem.
An obscure and obnoxious squeak sometimes comes from the anti-static brush on the end of the upper fuser roller (end opposite drive gear). A bit of high-temperature grease on the nearby felt pad will fix it.