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Rebuilding / upgrading a Roland MX-5 stereo 4-channel desktop mixer

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  • Rebuilding / upgrading a Roland MX-5 stereo 4-channel desktop mixer

    I bought one of these years ago at a university surplus sale. It was always really hissy and at the time i was intimidated by the weirdness of SIP opamps and the sheer number of components in it.

    Decided i wanted to use it if i could upgrade it. After replacing near a hundred components, it's really not bad at all.

    Opamps: They built this out of C4570HA and M5218ML opamps, which are SIP-9 and SIP-8 respectively.

    The C4570HA isn't terrible for the era but we can do better. The M5218ML is claimed by the datasheet to be as good as a 741 and absolutely needs to go.

    The NJM2068L is superior to these chips in every regard and only about 60 cents each from Mouser.

    With regard to the SIP-9 formfactor -- near as anyone can tell they did this so that you didn't have to pay attention to the orientation when installing. When replacing a SIP-9 dual opamp with a SIP-8 part, pin 9 is not connected, and you install the SIP-8 part backward -- with pin 1 to pin 8, pin 8 to pin 1.

    If you want to re-cap, you will need somewhere around 70 10uf 16v caps, around 54 of which shouldn't be taller/longer than 8mm or else it will get pretty awkward to install them. There are also half a dozen 1uf 16v caps. Since they use 5mm lead spacing for caps throughout the board, you can fit small films in these positions. The main power reservoir cap is 470uf 16v and there are two other 100uf 16v power caps (on mine, one of them is tacked to the back of the board and glued down). Physical size is a concern for these, too. Not a lot of room between the control panel and the component side of the board.

    The headphone section is substantially improved by a few component changes. They used one of the pitiful 5218 chips here with a 100uf cap and 1K resistor in series on the outputs. Replace the 5218 with an NJM4556AL, the 100uf caps with 470uf 16v and replace the 1k resistors with 4.7r or maybe just jumper wires. The 4556 can sink up to 70mA so it will have no problem driving any dynamic headphones.

    When disassembling be aware that although all of the screws look the same, the screws securing the headphone jack have a different thread pitch. Keep them separate. Also, the one with a star washer goes on the corner of the board where the shielding foil folds over.

    So, your shopping list:

    NJM2068L x8
    NJM4556AL x1
    10uf 16v 8mm x70(ish)
    470uf 16v x3
    100uf 16v x2
    1uf film with 5mm lead spacing: x6 (Wima, AVX, certan Panasonics)

    My MX-5 mixer is now hiss-free and sounds pretty good as an instrument preamp on channels 1-4. Probably does a fine job with dynamic mics too but i haven't tried that yet.

    It might be reasonable to use this with up to a 15v regulated supply rather than the original Boss effect pedal supply but i haven't tried. Potentially, with higher voltage ratings on the caps, you could go up to 36v. Squeezing 50v caps between the main board and the front panel could get challenging - you might have to install them on the back side of the board, where there is plenty of room.

    I haven't been able to find a schematic for this mixer, but the google searches are a mess due to the fact that certain japanese convertibles are way more common.
    Last edited by Timpanogos Slim; 05-11-2018, 10:30 PM.
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