HP IIISi: "50 SERVICE"

I don;'t know the IIISi, but on all the HP (and Canon) laser printers I've worked on, error 50 is a fuser problem. Either the fuser got too hot, or more likely it didn't get hot enough. There is a protection circuit in these machines (certainly the ones I've worked on). If there's a fuser error, the machine will not work unless it's been turned off for soemthing like 15 minutes (this is so if the fuser has rteally got too hot, it's got time to cool down). There's a capacitor on the DC controller board you can discharge to clear this (very useful if you know the fuser is not too hot...), but I have no idea if it's the same on the IIISi, or where that capacitor is. Anyway, the most common cause of this is a problem with the fuser heater itself. On the printers Iv'e worked on, this is a tungsten-halogen bulb down the middle of one of the fuser rollers. If you extract the fuser assmebly, it's generally fairly obvious where the lamp connections are, and it's easy to test the lamp filament for continuity. Most of the time when you get this error, the lamp is open-circuit, you need to replace it. There's also a thermal swtich in series with the lamp inside the fuser. This isthe final protection method, if things go badly wrong this will cut the lamp power and prevent the thing seriosuly overheating. You can test this too, it should show a dead short. If all that's OK, then the proeblem is with the controller circuitry. I could easily talk you through the CX and SX stuff, I've worked on them, I've done component-level repair on them. But I suspect yours will be different. The servie manual will certainly be a boardswapper guide, but it might show interconnections, etc. Have you tried the Australian HP museum? There are some printer service manuals there. -tony