HP printer Error 50 == fuser problems

[Tony Duell]

> 
> I am trying to resurect some ancient HP laserjets.  Two Laserjet II's 
> and one Laserjet 4v.  Prior to failure they were all three running fine 
> and giving nice output. Then each just stoped and began to give the same 
> error message, "50 Service".   Just what does this mean?  Is it a 

Fusr problems. The fuser did not get to the right temperature (either 
too hot, or more likely too cold (it didn't heat up at all!).

I can tell you how to sort out the LJ II (SX engine).

Pull the fuser : Take off the cover at the left side (paper tray == front 
side of the pinder) -- one screw on top. Undo the 4 fuser hold-down 
screws and pull the fuser up and out.

At the right side of the fuser is a 2 pin plug that connected to the 'AC 
power supply' module. Measuere the DC resistance between the pins on the 
fuser. It should be a few ohms. If not, then the fuser lamp is open (or 
the protection switch has failed). This is almost trivial to diagnose.

If the lamp is OK, then take off the printer casing, take the fan off the 
top of the AC power supply (a few screws and a plug-in cable), then take 
out the screws holding down the AC power supply and pull that out too. 
Take it apart from the top. There are 2 PCBs in there. On the lower one, 
check the triac (Q801 IIRC), it controls the fuser lamp. The upper one 
contains protection circuity to turn off the fuser if the DC controller 
goes crazy. Check the electrolytics and diodes on this board. It could be 
a transistor failure, of course, but the caps and diodes seem to tbe the 
common failures here.

If you can't find the fault, then I'll dig out my notes and diagrams and 
talk you through the various signals.

-tony