Printer and Photocopier Troubleshooting and Repair Collection : LaserJet II: Where is the fuse?         
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LaserJet II: Where is the fuse?

(From: Tony Duell (ard@p850ug1.demon.co.uk).)

There are 3 fuses/breakers to my knowledge:

The mains circuit breaker is in the AC block (right hand side of the fuser). It's hidden by the fan duct, but that's quite easy to remove if you have the casing off. The breaker is a little rectangular object with a button on top, on the lower PCB. Press down the button and try again.

The Low Voltage PSU contains the other 2 fuses. One is in the mains side, and is a normal 20mm cartridge fuse. If you remove the casing and the bracket round the front of the printer (6 screws), you can see this fuse on the PSU PCB. This fuse is a listed spare part at 'the Printerworks', which is strange, since the only time it should fail is if there's something majorly wrong with the PSU, and no other PSU spares are available.

The other fuse is a thermal one, clamped to the heatsink in the top right corner of the PSU PCB (looking at it from the component side). This one protects the 24 V pass transistor that on the same heatsink. I've never had to replace this one, so I don't know what the rating is.

If all the fuses are good, then you need to do some more troubleshooting. Firstly, check for mains at the 2-wire connector to the LV PSU. If that's missing, then you need to remove the mains input block (easy, once you've removed the fuser), and sort it out. It's _very_ simple, and should pose no problems.

If mains is present at the input to the LV PSU, check its outputs. Remove the base cover, and sit the printer on the right hand side. The LV PSU output is on the 14 pin connector in the middle of the engine control (DC controller) PCB. The front 2 pins are ground, the next 2 are +5 V, and the rearmost one is +24 V. If they're missing, you have a PSU problem.

Good luck in that case - Canon flatly refused to supply me with any spares for the PSU. They claimed that it would be dangerous to repair it. Quite why it's more dangerous than any other SMPS that I've worked on remains to be seen.

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