Roland Juno 6 / Juno 60
The IC, which had to be replaced, was IC48 on the mainboard (you find it on the service manual on page 9 at the right border on the lower half; coordinates P-38). While searching the faulty IC, I recognized that the Juno uses a so called "Total Gate"-signal, which enables the gates of the individual voices. This enabling is done by the NAND-Gates in IC48 and IC49. My idea now was: if I replace the internal total gate by an external gate signal, then I can trigger the notes, which are held on the keyboard. I did a quick test and it worked basically. I found that I have to use both total gates (the internal and the external), to avoid unwanted notes. To combine the two I use a unused NAND-Gate in IC49; a second Gate is used as inverter. This is the original circuit:
In the following picture we see, that I have cut the total gate line (blue), and feed its signal instead over the switching contact of the new gate-input-socket. The two transistors are used as buffer and inverter. The NAND-gate IC48-A combines the original total gate-signal and the external gate-signal. The the second NAND-gate IC48-B simply is an inverter again
The next picture shows the PCB layout in its original state:
The blue markings in the next picture show, where to cut the PCB traces. Cutting between the legs of IC48 looks more delicate than it really is:
The red markings show the new wiring:
And here a picture from the real pcb. The yellow and the orange cable are the +5V and Ground which I need for the transistor on the buffer pcb. Please keep in mind that the graphics before show the view from the top side. This is the view on the bottom side
And last but not least, the prototype board with the socket and the transistor buffer.
And now something completely different: this Juno plays like a charm - as beaten as it may look like.
