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Laser printer missing colour issue

Discussion in 'Troubleshooting and Repair' started by QuantumCheese, Aug 6, 2013.

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  1. QuantumCheese


    Apr 27, 2012
    Hi all,

    I have this colour laser printer i'm trying to save from land-fill here.
    Its a Canon LBP-5100 (the internet would suggest it has the same engine as an HP laserjet 6000 i think)

    OK, the problem - It prints great using Black, Cyan, & Yellow, but Magenta is almost completely non existent. Having got it to print various test pages (including taking off rear cover & pressing hidden test-page button). this is confirmed. what you should get is thin lines width ways across the page of all colours, what you actually get is lines of black, cyan & yellow and a feint magenta line just off centre about 2" wide fading towards the edges.

    Now, i know what your thinking - it's run out of toner. Nope, no suck luck.
    I swapped the magenta & cyan cartridges round to see if the fault followed. The printer informed me i had put the cartridges in wrong :mad: So i carefully removed the chips from the cartridges, swapped them round and tried again, Bingo. the fault stayed in the 'bottom' toner position ie now the test print had no cyan but magenta printed OK. So that rules out a dodgy toner cartridge.

    So, i'm thinking its either something lacking in the HT Bias on the bottom position or the laser is boned? Well, the service manual assures me that the feedback on the laser would pick up a faulty laser, especially as the output is dynamically adjusted constantly throughout a print cycle, so that leaves one of the bias signals right?

    Naturally i've cleaned all the usual suspects (and a few others too for good measure)
    I have the service manual, but it's completely lacking on the circuit front and it also doesn't have info on defeating the front door lock-out so i can try and measure the bias levels. which isn't much help.
    so the next step would to take all the panels off & start prodding probably the ht generation board? Before i embark on several hours of fuming & swearing trying to get into the thing, I though i'd put a little post here & see if anyone has any bright ideas

    Here's hoping :)
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