Backlight Modification for Roland TR-707 Drum Machine

Back in 1985, I was born.

But that’s beside the point; that same year, the Roland Corporation also manufactured my TR-707 Drum Machine! I received it as a birthday present in 2004 (or thereabouts), and after playing with it a few months, me being the hacker that I like to think of myself as, I proceeded to modify it in many different ways. Among them, I wired a TON of toggle switches to the panel, forming a matrix of connections between various sound effect data lines.

SWITCH BANK MODIFICATION

These are fairly well-documented circuit bends, and most of the connections create some very unique sounds and effects that I can instantly recognize when I hear in a track (I say that like it happens a lot, but it doesn’t and all I can really think of right now is Last Step (aka Venetian Snares) – “Last Step”). I used it to make some videos recently (before the mod I made this post to discuss was performed):


I have yet to find any articles on the backlighting of a TR-707 or similar device (or perhaps this is just me being too lazy to figure out just what similar machines might exist and searching for “ backlight mod”), so this was something I had to figure out on my own. The LCD in this machine is based on Twisted Nematic technology, so simply putting a light behind the display did nothing for me; in fact, less than nothing. It prevented the LCD from showing anything at all! I later realized this is because this was completely diffused light source. The way the display normally works is there is a reflector with a polarizing film, so that ambient light from outside passes through an outer polarizing film, then the LCD, another polarizing film attached to a reflector, then back out again. Thus, the light you see is polarized in one direction throughout the layers; the segments show up as dark spots in the reflected light because the liquid crystal is twisting the light out of phase with the rest that is reflected behind the LCD, and the outer polarizer thus blocks it from getting through; the segments represent light that is blocked.

LCD CONTROLLER BOARD

ACRYLIC AND LCD

So I knew what I needed finally, which was another polarizing film like that found on the outside of the LCD. Long story short: I had an old broken TI-89 graphing calculator which is conveniently based on the same technology, and the film is just slightly smaller than the TR-707 LCD; in fact, it is the same size as the silk-screened grid on the thing, which is all that needs to be backlit anyway! Excellent!

Now for the important, non-ranting bits of information! My backlight consists of a 1/8″ thick piece of mirrored acrylic cut to fit behind the LCD.

LIKE A GLOVE

I then took some white LEDs, and ground them down to fit flush against the edge of the acrylic; in total, two sets of three, powered by the -10V regulated supply I had to make with an LM317 because I burned out the transformer on board many years ago (that’s another story to tell altogether, but suffice to say that I didn’t know what was messed up for 3 years and only fixed it last year at some point, finally).
ACRYLIC CLOSE-UP

I also went ahead and put another transformer in from a wall-wart so that the whole thing can be powered from a normal computer-style power cord. There’s also a small fan to keep the whole thing cool.

MODIFIED POWER SUPPLY

The other edges of the acrylic are covered with foil tape to keep light inside, the surface was roughed up a bit to improve diffusion of the light, and the TI-89 polarizer is stuck there. The whole thing is then stuck behind the LCD and everything put back together. Fairly simple idea, but I’m the only one I know of who has documented this process. To do it to other machines, it may not require more broken TI-89s, because the polarizing film can be purchased online in a few places, such as this one.

SCREEN IN DARK, UP CLOSE

It is wired with the backlight on all the time because the LCD is readable in both normal conditions and the dark so I didn’t care about setting up a separate switch for it. I find it works nicely, despite some swirls of distortion in various parts (due to the polarizing film not being meant to be removed and placed elsewhere). Finally, I used GIMP to mock up some new labels for the panel (that was a lot of fun, I swear), and printed out on white adhesive label sheets.

BACKLIGHT ON, NORMAL AMBIENT LIGHT FROM LAMP

BACKLIGHT ON, NO AMBIENT LIGHT

BACKLIGHT OFF, NORMAL AMBIENT LIGHT FROM LAMP

3 thoughts on “Backlight Modification for Roland TR-707 Drum Machine”

    1. Fixed it! I changed hosts recently and forgot to re-upload all of my pictures. Thank you for pointing it out to me! :)

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