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[Elantric]

Here's an old doc in my collection I did not compose this!
=========================



Wow. It looks as though I was all wet about balancing the impedance between
the two units and the GK pickup. The schematic shows that 6 signals coming
from the GK pickup are simply wired directly to both unit A and B. The only
impedance balancing and buffering that is occuring is for the guitar's
pickup and the GK volume control signal.

So, sports fans, if you wanted to build your own stripped down "US20", and
you use external volume controls for the two units, you can simiply omit the
volume control connection. The MIDI values over-ride the setting on the GK.
Also, you can the guitar signal to only ONE of the units, or, don't connect
it at all, run it straight to the board. Use the power from only ONE of the
units and you're set.

Of course you lose all of the functionality of the GK. I don't use any of
it anyway. Here's my wiring list in GK pin number order:

GK          UNITA          UNITB
-----        --------        ----------
SIG1       SIG1            SIG1
SIG2       SIG2            SIG2
SIG3       SIG3            SIG3
SIG4       SIG4            SIG4
SIG5       SIG5            SIG5
SIG6       SIG6            SIG6
GTSIG     N/C             N/C       ->Direct to board
SYVOL   N/C             N/C
NC
S1           N/C              N/C
S2           N/C              N/C
+7V        +7V              N/C
-7V         -7V              N/C

[Elantric]

http://www.muc.de/%7Ehm/music/us-20/

Schematics
The schematics is derived from the original Roland part whose schematics if available in the GR-30 file area on yahoogroups. I made two modifications:

    * My clone has the guitar out, and the guitar signal is not routed to the attached GK accessories.
    * Roland did not adhere to its own standards when designing the US-20, see here. The volume buffer has now the 68K resistor.

The Eagle files linked below are Copyright Harald Milz 2001, and put under the GNU General Public License. Commercial use requires a license from me which has to be negotiated, and a license from Cadsoft which is available from their web site. If you make any modifications please send them to me to allow me keeping track of what you did.

The PCB was designed and mostly autorouted using Eagle. The Eagle files are here. A free version of the Eagle layouter for educational use is available from Cadsoft. (Remark: these are the files for board revision 2. The files for rev. 1 are here.) If you have no means to make a double-sided PCB with through contacts I can have some PCBs made for you. They are about 50 DEM each.
Parts
The Eagle partlist is here. The toughest part is getting the 13-pin plugs. In the U.S. they are available from Digikey. In Germany, Reichelt has them. I bought some in Munich (Holzinger, Schillerstrasse).
PCB Assembly

My prototype worked out of the box so it should be quite DIY safe. Rev 1 of the board contains some errors which have been corrected in Rev. 2. Rev 1 is not longer supported except for people who bought the PCB. New projects should always get Rev. 2 from the links above.

Footswitches: I reused a DPDT switch from an older stompbox, and bought a new SPDT switch. Simply solder some 3-wire flat ribbon strands to the solder terminals on the PCB and the respective terminals of the switched (see pic above). The sequence doesn't matter just make sure the middle terminal on the PCB is connected to the middle terminal of each respective switch.

Alternatively, you can connect 2 relays (DPDT and SPDT, respectively) and have them switched by a MIDI device. This way you can change the settings using MIDI. I don't know how much current you can draw from a GK device (the US-20 draws +/- 7V from port A) so be sure not to spoil your GK toys. This is not for the beginner! You have been warned.
The GK Cable
Warning:If you don't have a very fine soldering iron with a sharp tip (10-15W is appropriate), excellent soldering skills, and very calm hands don't even dream of making this yourself.

If you have 2 GK compliant devices it is likely you have one spare GK cable. The easiest way to get 2 short (1 - 1.5m) cables is to cut the spare cable in halves, and solder 13-pin male plugs to the bare ends.

Plank has got a file showing the colorcode to pin assignment in the a.m. file area. It is sort of useful to give you a rough idea but my cable had completely different colors and pin assignments. Use an ohmmeter to find the assignments and document them for your use. There is no magic to this - just make sure all colored wires are soldered to the same pins on both sides, respectively.
Test setup

Soldering the tiny wires to the male plug isn't trivial. My plugs just had 13 pins on the backside, no soldering pockets! Be sure not to use too much solder otherwise you run the risk of getting shorts which can put your precious Roland parts on risk! Double check the wiring! Use a bench vice or something to properly fix the plug before soldering. Otherwise your mental state is on risk.

In order not to get any shorts within the plug you may want to wrap some isolating tape around all pins, or use a thin isolating plastic tube.
Testing 1 - 2 - 3

Lotsa warnings. Don't worry if you have good EE skills. I built the interface including the cables in slightly more than 3.5 hours (the case is still missing, though), and double and triple checked everything to be on the safe side.
Final assembly
Built in a Teko 362 case which is shielded with alu foil, it finally works fine :-)

The PCB mounted to the frontpanel. Since the panel is only 1mm alu I reinforced it with a 1.5mm alu strip.    
View from the side. You can see the sandwich-like construction.    
US-20 clone and faithful companions...

Another US-20 clone project, albeit without a PCB.

To my other electronic projects
Harald Milz
Last modified: Sun Mar 9 20:01:31 CET 2003

[chamav]

Hi,

I was thinking of building a simple 1-in/2-outs parallel box myself similar to the GKP-4 unit. My simple requirements are to fully drive the first output (VG-99) with all the GK output signals of the guitar and drive the second output (GR-33) only with the S1-S6 signals (the synth volume will be controlled on the GR-33 itself).

Here are my questions:

1. I understand from the US-20 schematics that the S1-S6 signals could drive both units in parallel, i.e. no op amps are required, correct?

2. I am wondering about the other output signals 7-13 of the GR-33 unit. Should I leave them open? Especially what should be the case for the VOL (in order to have the max value), SW1 and SW2 (in order to be inactive)?

I attach a very simple schematic of my idea.

Thank you