Wednesday, 22 October 2014

Roland M-240 mixer - power supply replacement (Part 1)

I picked up a beautiful old Roland M-240 24 channel line mixer the other day. Sadly the mixer didn't
have a power supply so it's kinda useless. Roland don't appear to know where I can find a PSU and eBay was equally unhelpful.

The 3 pronged input reads DC -/+ 21.5V and 500mA. It's not your standard Wall Wart.

  Rather than throw it away I've decided to attempt building a new power supply.

Some pics of the circuit board:

This is the underside of the power PCB of the mixer.  It contains 4 voltage regulators: AN7812F / AN7815F / AN7912F / AN7915F. They provide voltages of +/- 12V & +/-15V.
These regulators should be able to handle voltages ranging between 35V to 18V. ..... though I'm not totally sure about how it will effect the rest of the mixer...... So do be careful if you are making these sorts of mods.

Anyway, I decided to test the mixer with a variable DC power supply.
At +/- 18V the mixer works with the exception of the UV meter which doesn't light up.

Nice! +/- 21.5 V and the meter is now also working.:-)

Ok, so we do need at least 21.5 V.

Possible soltions: 

I'm thinking of building one of Ken Stone's PSUs.

Varying the 78XX and 79XX regulators on the CGS14 board will hopefully give me the required voltage
A 7822 & 7922 regulator would be really nice but these are nearly impossible to find.
Maybe the LM317 & LM337 could be used instead.???

The LM317 has three pins: Input, output, and adjustment. The device behaves like an op amp
Vout = Vref (1 + RL/RH)
Vout = 1,25v (1 + RL/RH) .... there might be a small error due to some slight current flows from the
                                               adjustment pin.

It will be fun to play around with and experiment with different resistor values. Also these regulators
look like they could be very useful in future projects where unusual voltages are called for.

Finally, a good tech friend of mine has also suggested I build just the rectifier section of the CGS14 board, leaving out the regulators. (The Roland mixer already has the required regulators on board).

I stumbled upon a very cool site the other day.
There is a useful entry on their blog relating to bipolar power supplies:

They use the LM317 & LM337 regulators.

I'll update this page as I go along.:-)
These ideas are so far untested so proceed with caution if you are doing a similar thing.
I take no responsibility if you connect a power supply to the mixer and it burns out!


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  2. Hi, thanks for posting this. How is the power supply hooked up? There are three connections on the mixer- +21.5, -21.5 and ground- are you using the ground? My electronics knowledge isn't so great- i thought +21.5 and -21.5 would suggest a 43v power supply, obviously that's not right, just connect plus to plus and minus to minus? Maybe they are just trying to make it complicated for us simple folk.