Printer and Photocopier Troubleshooting and Repair Collection : Apple Laserwriter IINT 'clicks' with int..  
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Apple Laserwriter IINT 'clicks' with intermittent fuser error

"I have a LaserWriter IINT with an intermittent fuser error. When the printer is switched on, sometimes it cycles normally and works fine; but sometimes, the startup cycle sound is followed by a soft click, and a few moments later the green light gives way to double blinking red. I have attached a voltmeter to the fuser lamp contacts; when the soft click is heard, no voltage reaches the fuser."

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

The click is almost certainly the fuser protection relay dropping out. This relay is located in the AC input block assembly at one end of the fuser, under the main cooling fan.

This relay is on the lower board in that module, and is driven by the protection circuit (a few transistors, not hard to trace out if needed) on the upper board. It operates if the fuser drive signal from the DC controller is active for too long at a time - it prevents the fuser from overheating if one of the CPUs on the DC controller goes crazy

OK, things to check:

  1. Look for dry joints on both boards in the AC input block. They are VERY common here.

  2. Check the thermistor in the fuser. Clean it, check for dry joints, check that it is pressing against the fuser roller.

  3. Reseat all the connectors on the DC controller board associated with the fuser and AC input block.

  4. If that doesn't cure it, you'll need to hang a scope off the fuser drive signal (one of the 4 wires from the DC controller to the AC block - the other 3 are ground, +24 V and fan speed control) and see if the fault is in the DC controller (Signal does stick high) or the AC block.

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