I'm so sorry about the missing information. I was working on a TR 7520 for this particular issue. It was quite frustrating, in that the error code 6004 covers a large variety of potential problems. But taking the time and trouble to put a good shop light in the mix, shining on the interior of the printer, and overriding the switch that is turned off when the cover is lifted, I was able to observe the machinations of the printer. It might be unsafe, but proceeding carefully and doing this while being rested and alert, you maximize your potential to isolate the error. In this case there was a "blob" of ink that had "jelled" on one of the plastic gears that rotated to move the print head. There was also some gummy ink covering a piece of plastic in that area, that did not appear to be something that might have been there when the printer was new. So applying a solvent, which was isopropyl alcohol using a small paint brush, I was able to remove the ink that had built up. IN addition, I had a can of aerosol silicon, which I sprayed onto the white plastic, and then onto a cotton rag made from a T-Shirt, and wiped the metal track that the print head slid across, I also used the alcohol and tiny brush to clean the contacts on the print head, as well as pouring onto a paper shop towel, I placed the Print head onto the wet towel to clean out the outlets on the print head. It was in this way I cleaned everything on the print head that I could see and touch, without removing the case, then I let the parts dry, and cleaned the contacts on the cartridges before reinserting them. As it turned out I was called away from the printer by a client for about twenty minutes before attempting to start it up. VOILA' the printer by-passed the code 6004 error and posted a new one, that indicated the that the rear input tray had an open flap or something, which I corrected, and the thing came to a ready state. Then the printer had some documents in the "cache" and they began printing. The first page was unclear, but it printed on both sides, and the back of that page was better, but the next page on the printing was a good as it could get. Sorry to take so much time in responding, and the procedures I used may seem intuitive, but to me they seemed insignificant, but they were in fact necessary to get it working.
