YAMAHA MCS2 BREATH CONTROLLER BUGFIX Robin Whittle 25 August 1994 ------------------------------------ The Yamaha MCS2 is a MIDI merging device with inputs for a variety of controllers - including a breath controller. There is a bug in the hardware design which causes many of them to fail to put out the full 0 to 127 range from the breath controller. It seems that this was hardly a problem with the original BC1 breath controllers (little box with curved metal panel) but is more likely to be a problem with the BC2s - the headset style units which have been sold more recently. I have not investigated the differences between these two units, but they are clearly different. The BC2 must make different demands on its power supply than the BC1. We don't really need to know about this however, because the bug is easy to fix. The voltage of the external power supply would also affect this problem, so a workaround may be to try a different supply. Remember that Yamaha use the opposite polarity assignments to everyone else. The problem is in the circuit which gives the BC2 its negative supply. Its positive supply is the +5V rail inside the MCS2. There is a 4.7K from the rough negative supply to a zener and cap hanging down from the +5V rail. This reference goes via a 56 ohm resistor to the base of a transistor - identified on the PCB as "3". This is meant to be an emitter follower to drive the breath controller negative supply at about 0.6 volts above the reference. This brings us to -> . . . . . . . . The BUG <<<<< The problem is that the collector and emitter of the transistor have accidentally been reversed in the printed circuit layout. The transistor - type A777 - is a PNP device and like other bipolar transistors, it still works if you use its collector as the emitter and its emitter as the collector. The gain of the transistor in this configuration will however be much lower. The gains of transistors may vary a lot anyway, but the reversed gain could be all over the place - it is not something which would be specified or checked in manufacture. The gain depends entirely on the particular transistor. The SOLUTION Locate the dysphoric transistor. It is near the CN2 connector that goes to the slider pot board. Place the MCS2 with its right end facing towards you and the bender end away from you. The transistor is a handsome, rather formal, entity with three pins in a straight row. There's nothing wrong with this dude - he's just doing its job. The screwy circuit board is the problem. It is easier to get this elegant black silicon butler to cross his legs and adapt to this crazy world than to mess with the printed circuit board. Note the current orientation of the transistor - its flat is towards you. Pull up the main board (lots of screws) and desolder the transistor. Holding its pins downward and its flat towards you, cross over the left and centre leads. The right pin - the base - goes back in the holes it came from. Cross the leads with appropriate bends so they will never touch. Solder the poor confused guy back in - life will be easier now. As they say in the classics - Assembly is the reverse of the disassembly procedure. Squirglinax! -------------------------------------------------------------------------- Robin Whittle 9 Miller St. Heidelberg Heights 3081 Melbourne Australia zcirrw@minyos.xx.rmit.oz.au Real World Interfaces - Hardware and Software, Design and Consulting - Especially for Music First Principles - Research and Expression Human factors in technology adoption Ph +61 3 459 2889 Fax +61 3 458 1736 After June 1995 Ph +61 3 9459 2889 Fax +61 3 9458 1736 ---------------------------------------------------------------------------