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  1. #1
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    Master Print unit issues on Riso MZ790u

    Hey all,
    I just picked up a second hand Mz790 riso printer and have a little background with taking apart risos (i battled with an old gr machine for years). There are a few issues that are occurring that lead me to think that the master making unit is askew and was wondering if anyone knew how to fix the issues i am having here is the list:

    1. Master making unit makes creaking sound when pressing the master making button on the right side. Ive noticed that if i push the door out a little the sound stops and the door becomes available to open. I also noticed that the track doesnt want to go all the way out because the right side of the master making unit hits the edge of the main machine, with a small effort you can force it out.

    2. Master disposal box issue. when i try to create a master, initially the machine goes through the process of getting rid of the one that is on the drum. the first error that comes up is that i need to remove the disposal box on the left side and remove contents. Once i do that and tape resume, the next error that occurs is to remove the master directly from the drum. Often times after that itll make a blank image and wrap it around the drum and will have to go through that process again.

    3.Information printed comes out crooked on the page. Once i get an image on the drum the image is crooked. There are a few reasons why i think this problem leads back to the master making unit. first being that the master looks askew on the drum. Second thing is that even when doing 2 color, the information is in registration, just crooked.

    2 common error code that occur are t27-248 and t67-250, i know that the first 3 digits of both of those codes are related to "master making motor lock". is there a way to see if the motor is triggering incorrectly? Or maybe just getting it aligned would help? and advise would be helpful and if you can understand my terrible description, please let me know.


  2. #2
    Riso dude 250+ Posts jermyth's Avatar
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    Re: Master Print unit issues on Riso MZ790u

    Yes it sounds very likely the master making unit is out of alignment. If you got a manual then I suggest reading the master making section. In the disassembly part, it goes into all the details on how to remove the master making unit and cautious on reinstalling it correctly. If you don't have a manual, then call a tech if you got one in the area. Not a fun job if you're not familiar with the mz design.
    Good luck.


  3. #3
    Senior Tech 250+ Posts
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    Re: Master Print unit issues on Riso MZ790u

    FYI We have a MZ790 demo machine with 670 prints an made 60 masters we are selling. Call 716-832-4950 ask for ward. Please dont PM me I'm not Ward who is our sales geek.


  4. #4
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    Re: Master Print unit issues on Riso MZ790u

    Hi there, I had a similar problem with my MZ970. It appeared that the MMU (master making unit) was not straight on the MMU guide.
    Remove MMU, switch the machine off and then remove the covers both right and left.
    Check if the MMU is flush with the machine, adjust the engagement of the gears on both the front and rear to ensure that the master making
    unit is not skewed. Check and make sure that the metal beam on the machine and the front frame of the master making unit are parallel with each other.
    You can simply lift the MMU and move it one tooth so that the MMU is back in alignment with the metal beam of the machine again. (if I remember correctly).
    Do be carefull. Never remove or loosen the two screws on the gears! Try and get yr hands on the technical guidance pdf.
    Good luck!


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