Lightly used Samsung SCX-4100 multi.
When powering on, LCD display says "Warming Up Please Wait" while the fuser heats up. Meanwhile the scanner carriage does a quick "zero" just out a few mm's and back, but continues to run the carriage motor endlessly, banging against the zero stop until it gives up and puts up "Scanner Error" on the LCD.
I got one good copy from it when it didn't do the "infinite zeroing" thing, so I know it can work.
The scanner carriage is just a piece of tin with the plastic bulb/camera holder, riding on a single rod. A toothed belt runs around the motor drive gear and a spring-loaded tension gear at the end of the run. The belt is simply "pinched" in a groove in the plastic bulb holder. When the carriage is run up against the zero stop, the motor keeps going and the belt slips through this groove.
I presume that this is the way it was designed to zero the carriage, just running the motor long enough to run the full length of the bed and stop. But it doesn't stop until it gives up.
There is no "home" sensor. No magnet, no opto, nada. Just a metal tab on the carriage that bangs against another at the top of the run.
I tried holding the belt still, thinking that maybe the motor current is monitored and when it goes high, that's zero. But the motor (a stepper) just skips sync, and tries to keep going.
The under side of the glass has no marks that might be used for zero indication by the camera.
I checked ground (it's tight) and for routing of the data cable from the scanner carriage (it goes through its huge ferrite bead).
How do these el-cheapo models determine zero position?
Where might I look for a problem?
Yes, I know it's cheap. But if I can keep it out of the landfill and have a simple document scanner, that would be great.
Thanks.