I bought a 1991 SD-1 for very cheap, $100. I knew it had issues but wanted to give it a try. I have discovered there is limited information on these synthesizers. When powered on it fails the keyboard calibration. It all has a note playing intermittently, I think it is the low B flat, as that key is not working. Also, 4 other keys seem to also activate this B flat but distorted, when pressed. All the other keys function but of course the other note is kind of tapping in the background I have found many articles on this and have tried everything. This is a later model so it has the ribbon cable between the 2 coil boards,
- I have verified continuity of the coil board connection ribbon cables with a multimeter and no issues.
- I have cleaned all the coils with rubbing alcohol, it had some dirt and grime, nothing real bad though, no oxidation. I did find quite a bit of oil on the coil boards, no scent. I thought maybe blown capacitors, but they are not bulging or oily. I also found a guitar pick and dime inside so who knows.
- I have removed all of the keys and it still fails the keyboard calibration so it is not a defective key
- I traced the coil to a transistor and the one for the bad key and it reads a little over 20ohms resistance, base to emitter, compared to all the others which read 10.31 ohms resistance. So I thought, it’s a bad transistor, I ordered a replacement 2n3904 transistor. I checked the resistance before I installed and it was high like the failed one, I had to order many and they all tested high. This leads me to believe that this is not the way to test the resistor, although this 1 connected to the bad coil reads much higher than any other.
- I tested a couple of resistors connected to that coil and they seem fine as in the same resistance for each color code.
- I have checked the power supply line voltage according to the repair manual, although I am not sure what voltage I am supposed be getting there, 120v on the main connection, should everything read 120v on the AC Line measurement? I also tested all the power supply voltages as per the repair manual, Figure 5 page 8, everything is in spec.
- I have done a Reinitialization
I am sure I have forgotten something, Any help would be appreciated