Tuesday, 4 November 2014

Roland mixer - power supply replacement (Part2)

This is part two of the search for a replacement power supply for my old  Roland M-240 mixer.
It's morphed into a discussion about the nature of power supplies with one of my good friends who is also a excellent synthesizer technician.

By chance  I have another Roland mixer (Boss BX-16) that also uses a DC +/- 21.5V psu. It's however 300ma .... not the 500ma that the M-240 needs.

I opened it up yesterday:

So simple. The PSU is made up of a stepdown transformer, 4 diodes & two capacitors.

The caps are 2200uf, 35V

How simple !!!!!

The incoming AC power is first stepped down by the transformer (120 - 21.5V). 
Then it's converted from AC to DC by the 4 bridge rectifying diodes & finally filtered with the capacitors. (There don't appear to be any voltage regulators in this PSU).

This circuit uses a centre-tapped transformer.
Most of the circuits like this seem to use 4004 diodes. (I only have 4001s handy).
It's highly likely that the PSU for the 240 mixer uses a similar circuit.
I'll breadboard this when the correct diodes arrive.

If you wish to be really safe add some regulators.


  1. hello Jon, would you happen to have the vero layout?, i just picked up a BX 8 and the PSU is dead.
    many thanks

  2. Hi, was there any resolution?
    Did you get this running with a PSU?

  3. Hello Sam & Mr Bronson,
    So sorry for the slow reply. I have almost forgotten about this thread.
    Anyway, I did build the powersupply using one of Ken Stone's PCBs.
    I didn't bother breadboarding as once I opened up the old PSU it seemed
    to only confirm my suspicions that all we needed was a unregulated PSU.

    All you need are 2 caps, 4 diodes.
    Just build the part o the circuit to the left of the schematic of CGS14
    I'll put up a diagram later on this page to illustrate.
    Let me know if you have any more questions.
    Cheers Jono

  4. Hello Jon, i just want to thank you for this post. Last week i got a boss x8 without the psu, and because of this knowledge you had spread here i managed to build one, using an old transformer from a cordless telephone (!) and that part of the schematic you share. working perfectly with + - 14 volts. thank you very much. greetings from argentina, south america.

    1. Hello, Thanks so much for writing back. I'm glad you fixed your mixer. Well done. It makes me very happy to know that I have helped in a small way.
      Greetings & Peace from Australia.

  5. So the PSU doesn't need to be +/- 21.5V, am I right? > +/- 15V should be fine?


  6. I mean, a 2x15V transformator + 4diodes + 2caps should be fine?

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