From arjan@prognosis.nl Mon Aug 04 05:02:35 2008
Subject:Modifying a Korg CX3 for 220V AC operation

Hi,

I started this topic in my previous thread

What is it about the CX-3 sound that I like so much?
<../../message/49899>

but I figured to get some more response I should give it its own thread
with proper subject title so here goes.

I have ordered a Korg CX 3 (the modern digital version) from Sweetwater.
Even though I'm in Europe I had to order it from them because it is no
longer sold anywhere in Europe.

As it turns out the CX3 appears not to have an auto-sensing
power-supply, it will only take 110/117V. I don't really want to use an
external transformer so I'm looking at modifying the internal PSU to
take 220/230V.

Bruce Wahler mentioned that

"The CX-3 is NOT a "universal voltage" model. IIRC, it can be changed
to the
other voltage range, but it requires a soldering iron. Don't hold me to
it ..."
So that made me think that the PSU was an actual old-fashioned
transformer and I could just tap the secondy winding at a different
point, but having obtained the Service Manual it apears that it is a
switching power supply
after all.
By switching power supply I do not mean that it will take different
input voltages, I am merely referring to the design of the PSU.

Now does anyone here have any information on what the best/easiest
approach would be to make this unit take 220 instead of 110? I have
thought of three different ways:

* If the PSU uses a half-wave rectifier, it may be possible that Korg
used the rectifier stage to double the incoming voltage when used with
110V external power and that for 220V external power the rectifier stage
only is modified. Some power supplies actually allow switching between
these two modes of operation with an internal or external switch but if
Bruce is correct this is not the case in the CX3, hence the soldering
iron.
If anyone has any details on the schematics and changes required please
let me know. Unfortunately the Service Manual has only limited
schematics on the power supply board. Only the stages coming after the
actual PSU are detailed, you can clearly see the well known 78L05
regulator being used as well as a BA033T which also appears to be a
regulator probably used to generate 3.5 V.
If I knew what to change exactly this could be the simplest solution by
far.

* Completely replace the PSU with something else.
I won't be able to assess this possibility until I open her up
* Include an internal 220->110 converter and reroute the AC
receptacle wires to this converter and feed back the 110V into the main
circuit.
Again I won't be able to see if there is room for such a converter until
I open the unit
* There is of course a 4th way and that is to use an external
transformer. The main reasons I don't want that is because a) if I
forgot it I won't be able to use the thing and b) if I or someone else
inadvertently plugs in a power cord straight to 220 it may fry my CX3.

If any of you have any information to help me that would be greatly
appreciated. I'm an Electrical Engineer so don't worry about me being
electrocuted, I know what I'm doing. Also I will be testing the unit
with my external 220->110V converter that I already have lying around
before I start to mess with it and I should be able to test my
modifications on the PSU board in isolation.

BTW expected delivery date of my CX3 is August 7, this Thursday [:)]
Unforunately I don' t have my 220->110 converter here, I'll be picking
it up over the weekend. [:(]

Thanks,
Arjan

P.S. My Service Manual, which I ordered from Parts i Parts, turns out
to be Revision 1.00, Provisional Version. If anyone has a more complete
or uptodate service manual please let me know.
<../../../../message/49899>
What is it about the CX-3 sound that I like so much?


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