Bending a PSS-460: Finding bends
In part 1, I opened the Yamaha PSS-460, gained access to the main board and identified some chips and safe areas of the board.
In this entry, I’ll be outlining how to find bends, both of the simple short variety and the resisted short variety.
In this entry, I’ll be outlining how to find bends, both of the simple short variety and the resisted short variety.
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Traditionally, bends have been found by using a pair of jeweler’s screwdrivers connected with a wire. Holding both screwdrivers and playing the instrument often requires more hands than I have. Instead, a pair of insulated alligator clips connected with a wire allows hands-free connections.
To start, I’ll be focusing mainly on the sound chip, a Yamaha YM3812. An integrated circuit (IC, or ‘chip’) is like a miniature computer. It interfaces with the rest of the circuit board with a series of wires or other electrical contacts (called pins). Pins are numbered counter clockwise from the top left of the chip. Pins are generally dedicated to one of three purposes: Power. ICs need power just like the rest of the circuit board. There will be at least two pins that carry power to the chip (one ground and one ‘high’ - 3 or 5 volts.) In datasheets, these are usually denoted as GND and VSS, V5, V3, V+, etc. In general, shorting a power pin to another pin will result in a crash or at least cancel the output of that pin. According to the YM3812 data sheet, the top left pin is VSS, the bottom left pin is GND. Control signals. Often broken up into Clock, Interrupt (IRQ), and Command. These are used to synchronize the chip with another chip, or pass commands between them. Shorting these usually results in highly unpredictable behavior, including crashes and partial lockups. Data. Usually in groups of 8, data pins are used to transmit digital information in and out of the chip. In many cases, the same pins can be used for both input and output, and the YM3812 makes use of this. On this chip, pins 10,11 and 13-18 are data pins. Shorting data pins is the most common way to get relatively stable sound modifications. An easy way to try all combinations of pins Start at the top left pin. Attach one clip to this pin. This is the ‘fixed’ clip. Using the other clip, try all the other pins in sequence. Move the clip from the top left pin to the next counter-clockwise pin. Then try every other pin, leaving out the top-left pin, since you’ve already tried 1 shorted to 2. As you progress, you can skip every pin the ‘fixed’ clip has already tried. |
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It’s a good idea to take notes during this process. A 24-pin chip has 276 possible bends.There are six types of bend effects found on the sound chip:
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Adding a reset switch.
Early on, I discovered a reset bend. Shorting the VSS and GND pins (top left and bottom left) on the sound chip resulted in the entire device resetting. During testing, it is often useful to have a quick way to reset the device, so I attached a pushbutton switch (momentary on) to act as a reset button. If you don’t find a similar reset bend, you can instead install a switch between the batteries and the circuit board, or, if wall-powered, use a surge protector switch. This is basically the same method used to add a control to an effect - a pushbutton switch in particular is useful for controlling a permanent effect. | |||
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Here I’m testing a crash bend by adding a variable resistor (potentiometer) to search for any interesting effects near the crash threshold.
At first, it may seem that pause or crash bends are useless. However, each of these may only be useless with 0 resistance. You can try each one with a potentiometer in series with the clips, to see if anything interesting happens with more resistance. Often there will be a very narrow range of resistances in which random effects happen; above this range, the bend has no effect, and below this range the crash/pause happens. If you find a range like this, it is very useful to have a multimeter handy! |
April 27th, 2008 at 11:24 am
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