Looking for some HP LJ3 repair hints and a paper tray

It might do. It might be a lot of other things too. I'll give a basic overview of what goes on in the various modules, I can dig out the schematics (which are not in the HP service manual, this is a somewhat useless document, therefore!), if you want to go further. Firstly, be warned that the 115V and 230V versions of the AC block have different circuitry round the triac, not just different component values. Instructions for the former do not apply to the latter. OK, the fuser temperature is monitored by the thermistor pressed against the roller. This is connected in a simple R's and C's circuit (on the DC controller board), the voltage out from that depends on the temperature. It goes to an analogue input on one of the microcontrollers on that board (I think it's the 'slave' one, but I will have to check). This microcontroller generates a pulse train to turn on the fuser lamp, nothing to turn it off (if you see what I mean). AFAIK it is not a proportional control. Just on/off. This signal goes to the AC block. The upper PCB contains 3 subcircuits 1) A circuit to detect the pulse train ant turn on the optotriac on the lower PCB when it's present (thus turning on the fuser). 2) A circuit to detect problems (pulse input stuck high, current in the fuser when it shouldn't be there, etc), and to turn off the protection relay if this happens 3) A circuit to cotnrol the cooling fan that you can ignore for the moment. The lower PCB contaisn, along with the mains input filter, the fuser control ocmponents. There's a relay between one side of the mains and one side of the fuser, and a triac between the other side of the mains and the other side of the fuser. The relay coil goes to the upper PCB, the triac gate is controlled by an optotriac, the LED side of which also goes to the upper PCB. One thing that's worth doing very early on. Connect some kind of 24V detector (LED + resistor, even a small light bulb) across the relay coil. If it goes out at any point, it means the protection circuit is operating, and this will shut down the fuser. I once had a particularly nasty fault there. The relay would turn off a few seconds after power-up. It even did it when I powered the module from a 24V bench supply (and with no pulse input at all). Every associated component tested fine. It turned out that one of the electrolytics had leaked slightly and put a conductive gunge on the PCB. This was providing enough current to trip the protection circuit (!). Stop guessing ! Yes, there's only one paper path sensor in the SX engine, that's the one on the side of the fuser. A paper jam means that sensor didn't detect / not detect paper as expected. The most common cause of this is that the printer didn't pick up paper from the paper tray, due to a warn pickup roller. That's the D-shaped roller on the spindle above the paper tray, under the scanner block. Replace the roller (the roller alone is cheaper than the complete spindle, and it trivial to change, at least if you had a mis-spent childhood repairing ASR33s!), and the cork (?) pad that holds the paper against it. Of coruse the problem might be in the sesor. It's just a plain slotted optoswitch, I think. Mothing odd about it. -tony