synthgeek's blog

This is a toy drum machine with 8 playable pads (not velocity sensitive), 4 percussion-sound buttons, 30 preset rhythm loops, and a built-in speaker. There are two choices of drum kit for the pads, “classic” and “electronic”. I added a 1/4″ line-out jack (switching, so the internal speaker is cut off when something is plugged in) and a pitch knob.

Here’s a shot of the board, pre-bend, with the clock resistors marked:

There are two resistors there, on a daughterboard attached to the main board at a 90-degree angle- this daughterboard also contains the main IC/ROM, which is of the “black blob” variety. Adding parallel resistance to just the top resistor (thus lowering the resistance there) with a pot allowed only upward pitch changes, while putting the pot between the upper side of the top resistor (top in the photo, that is- it’s actually the bottom side of the device if you were looking at it head-on, like you were playing it) and the bottom side of the resistor underneath it (closer to the main board, not down from the circle in the photo- both resistors are inside the circle) allowed the pitch to be shifted slightly higher and much, much lower, with less resistance here giving a lower pitch.

The yellow wires which lead up & out of the photo are the output connections. I believe the 8-pin IC next to them is the amplifier.

I was unable to find any other bends that were very interesting. However, if one were to find a non-working one (or just not like it enough to keep it around), there are a few things that would make fine salvage. The drum pad triggers are piezo discs, there are a few slide switches, and the smaller board which is screwed in at the corner there, facing the opposite direction of the main board, has the two small slide pots which are used for the pad & rhythm track volumes. Also, the built-in speaker is around 5″, and not too shabby for a toy like this- might be useful for a small lo-fi amp of some sort.

Here’s a video:

The TR-66 is an old Roland rhythm machine, containing only preset rhythms, and having no sound editing capability. However, it contains the same or similar circuits as a more well-known Roland machine, the CR-78. This is the first in a series of posts which will document my modifications of this old beatbox. The first bend I’m documenting in these posts is something I found awhile ago, but haven’t made permanent yet. I’ve decided to go the breakout box route, to avoid having to drill multiple holes in the case. It’s sort of a strange design, where the outer wooden casing is a box open in the front and back. Four screws come out of the bottom, and then the innards (which are built on a metal frame with the front & back panels attached) slide out.

I should note that conventional wisdom says it’s a bad idea to circuit bend 120 VAC devices such as this, and that I make no guarantee that attempting to do this with your TR-66 won’t cause damage.

Taking a look inside, you will see trim pots for setting many sound parameters- and handily, most are even labeled. There are a total of 8 trim pots on this board- the ones labeled “103″ are 10k pots, the ones labeled “501″ are 500 ohm. I’m thinking of trying 1k pots for those, since 500 ohm seems to be harder to find.

You may also notice that it seems the outputs for each drum sound are labeled on the board- this should make it easily possible to include level adjustments for each sound, as well as other mods such as individual outputs if one were so inclined.

The first order of business will be making a hole to thread the wires for the breakout box through. I’ve decided to make the hole in the rear panel- partly because that seems “right”, but mostly because the case design makes that the easiest way to go. As the picture shows, there’s plenty of open space back there.

I would like to note that it was a lot harder to drill through the relatively thin plate than I thought it would be. I used a fairly small drill to make a pilot hole, then followed up with a step drill to get it to the size I wanted. I went with a pretty big hole, since there are several pots I plan to replace, as well as a couple of other mods.

The first bend/mod is what I call the “distortion mod” – connecting a pot between the two points shown in the picture below gives you a sort of distortion effect that increases as you turn the knob. I used a 10k pot, because that’s what I had on hand.

the solder points for the distortion bend

That’s it for the first installment (because I’m out of unspoken-for parts). More to come, though it could be awhile as this is pretty low on my priority list.

if I were the sort to listen to this kind of crap, I probably wouldn't have a job right now