How I fixed a 17-year old HP scanner...

Fixed! I've had this HP LaserJet 3330 multi-function printer (3300mfp) since 2002, but one day it started making a clicking/grinding noise, which is a common issue in this printer. The key to fixing is taking apart the entire unit and cleaning the mirrors and lens on the scanner unit (see YouTube video)

What exactly is the issue? It turns out that the scanner during startup tries to find its home position at the edge. When the scanner lid is closed, the light will reflect down onto mirrors. There are 5 of these mirrors that eventually bounce the light towards a CCD sensor:

I had done this mirror cleaning a few years ago and it worked miraculously. However, this time around the problem started happening again but the mirrors seemed fairly clean. What I didn't realize and what the repair videos don't mention is that I also had to clean the lens.  You can clean the front easily but to get to the back of the lens, the board on the back of the scanner unit has to be removed. This article seems to suggest you can get access to the scanner unit without taking the entire printer apart, but if you need access to the board, there are really no shortcuts.

There were small glue residue on the screws, so I had to use a power screwdriver to remove them. You can see in the pictures how the lens is fogged up:

After cleaning with a Q-Tip and alcohol, this lens starts to become cleaner:

Upon putting everything back, I noticed the printer continued to show "warming up bulb" on startup. I think the problem could be traced to some of the residue from the glue when I removed the board causing issues on the board. I managed to scrape away the residue and use painter's tape to lift it off the board. 

To verify whether the circuit board that I had just removed was at issue, I also started using a voltmeter and testing the ground and power (Vcc) pins of each of the integrated chips on the board while the board was plugged in while leaving the gray scanner lid off. There are a few commodity parts, which includes the CCD sensor (UPD8861), an inverter (74vhc04), a Schmitt trigger, and a voltage regulator that outputs 12 volts.

It suddenly worked! And I didn't have to buy a new unit (51000104-0000 on Ebay).

Note: I did try to reset the printer's memory by holding down "*" and # on power-up.  But I stopped doing it after it didn't seem to help.

Other notes from the HP forum here.