<html><head><meta http-equiv="Content-Type" content="text/html; charset=utf-8"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;" class=""><div class="">Hi All,</div><div class=""><br class=""></div><div class="">I've been studying old drum machines again. This time, I've been looking at the Korg KR-55.</div><div class=""><br class=""></div><div class="">I've come up with one quirk or schematic error, and I'd like you to take a look and see if you agree with me.</div><div class=""><br class=""></div><div class="">The machine uses eight simple squarewave oscillators to generate two "metallic noise" signals for cymbal synthesis. These are called Ring1 and Ring2, because they're generated using pairs of oscillators "ring modulated" together by 4070 XOR gates. The schematic looks like this:</div><div class=""><br class=""></div><a href="https://electricdruid.net/wp-content/uploads/2023/11/RingGenerator.jpg" class="">https://electricdruid.net/wp-content/uploads/2023/11/RingGenerator.jpg</a><div class=""><br class=""></div><div class="">The whole thing is here:</div><div class=""><br class=""></div><div class=""><a href="https://www.synthxl.com/wp-content/uploads/2021/10/korg-kr-55-Service-Schematic.pdf" class="">https://www.synthxl.com/wp-content/uploads/2021/10/korg-kr-55-Service-Schematic.pdf</a></div><div class=""><br class=""></div><div class="">The schematic gives us frequencies for the eight squarewaves. We also know that the general pattern for such oscillators is Freq = k/RC, so it's just a question of working out what k is in this case (using the LM339 on a 5V supply).</div><div class="">It occurred to me that we can assume the designers wanted the tuning pots roughly in the centre, so if we feed in the required Frequency, with the midpoint R value and the given capacitor, we can work out k. That gives us this:</div><div class=""><br class=""></div><div class=""><table style="margin: auto; width: 1081.1875px; border: 1px solid rgb(102, 102, 102); color: rgb(0, 0, 0); font-family: -webkit-standard; background-color: rgb(255, 255, 255);" class=""><tbody class=""><tr class=""><th class="">Required Freq</th><th class="">R</th><th class="">C</th><th class="">K</th></tr><tr class=""><td style="text-align: center; border: 1px solid rgb(153, 153, 153);" class="">272 Hz</td><td style="text-align: center; border: 1px solid rgb(153, 153, 153);" class="">125 KΩ</td><td style="text-align: center; border: 1px solid rgb(153, 153, 153);" class="">22 nF</td><td style="text-align: center; border: 1px solid rgb(153, 153, 153);" class="">748000</td></tr><tr class=""><td style="text-align: center; border: 1px solid rgb(153, 153, 153);" class="">8529 Hz</td><td style="text-align: center; border: 1px solid rgb(153, 153, 153);" class="">49 KΩ</td><td style="text-align: center; border: 1px solid rgb(153, 153, 153);" class="">8.1 nF</td><td style="text-align: center; border: 1px solid rgb(153, 153, 153);" class="">3385160.1</td></tr><tr class=""><td style="text-align: center; border: 1px solid rgb(153, 153, 153);" class="">633 Hz</td><td style="text-align: center; border: 1px solid rgb(153, 153, 153);" class="">125 KΩ</td><td style="text-align: center; border: 1px solid rgb(153, 153, 153);" class="">9.1 nF</td><td style="text-align: center; border: 1px solid rgb(153, 153, 153);" class="">720037.5</td></tr><tr class=""><td style="text-align: center; border: 1px solid rgb(153, 153, 153);" class="">9073 Hz</td><td style="text-align: center; border: 1px solid rgb(153, 153, 153);" class="">49 KΩ</td><td style="text-align: center; border: 1px solid rgb(153, 153, 153);" class="">1.5 nF</td><td style="text-align: center; border: 1px solid rgb(153, 153, 153);" class="">666865.5</td></tr><tr class=""><td style="text-align: center; border: 1px solid rgb(153, 153, 153);" class="">1458 Hz</td><td style="text-align: center; border: 1px solid rgb(153, 153, 153);" class="">125 KΩ</td><td style="text-align: center; border: 1px solid rgb(153, 153, 153);" class="">3.3 nF</td><td style="text-align: center; border: 1px solid rgb(153, 153, 153);" class="">601425</td></tr><tr class=""><td style="text-align: center; border: 1px solid rgb(153, 153, 153);" class="">13500 Hz</td><td style="text-align: center; border: 1px solid rgb(153, 153, 153);" class="">49 KΩ</td><td style="text-align: center; border: 1px solid rgb(153, 153, 153);" class="">1 nF</td><td style="text-align: center; border: 1px solid rgb(153, 153, 153);" class="">661500</td></tr><tr class=""><td style="text-align: center; border: 1px solid rgb(153, 153, 153);" class="">731 Hz</td><td style="text-align: center; border: 1px solid rgb(153, 153, 153);" class="">125 KΩ</td><td style="text-align: center; border: 1px solid rgb(153, 153, 153);" class="">7.5 nF</td><td style="text-align: center; border: 1px solid rgb(153, 153, 153);" class="">685312.5</td></tr><tr class=""><td style="text-align: center; border: 1px solid rgb(153, 153, 153);" class="">3946 Hz</td><td style="text-align: center; border: 1px solid rgb(153, 153, 153);" class="">125 KΩ</td><td style="text-align: center; border: 1px solid rgb(153, 153, 153);" class="">1.5 nF</td><td style="text-align: center; border: 1px solid rgb(153, 153, 153);" class="">739875</td></tr></tbody></table><div class=""><br class=""></div></div><div class="">This is all pretty reasonable, with values between 600,000 and 750,000, which is not much of a spread. However, that second row is an exception - it's way out. Working out the average value, excluding 8529Hz, gives us k = 689,002. We can then put that k value back in and see where the tuning pots need to be in theory:</div><div class=""><br class=""></div><div class=""><table style="margin: auto; width: 1081.1875px; border: 1px solid rgb(102, 102, 102); color: rgb(0, 0, 0); font-family: -webkit-standard; background-color: rgb(255, 255, 255);" class=""><tbody class=""><tr class=""><th class="">Required Freq</th><th class="">Required R</th><th class="">Midpoint R</th></tr><tr class=""><td style="text-align: center; border: 1px solid rgb(153, 153, 153);" class="">272 Hz</td><td style="text-align: center; border: 1px solid rgb(153, 153, 153);" class="">115.1K</td><td style="text-align: center; border: 1px solid rgb(153, 153, 153);" class="">125 K</td></tr><tr class=""><td style="text-align: center; border: 1px solid rgb(153, 153, 153);" class="">8529 Hz</td><td style="text-align: center; border: 1px solid rgb(153, 153, 153);" class="">10.0K</td><td style="text-align: center; border: 1px solid rgb(153, 153, 153);" class="">49 K</td></tr><tr class=""><td style="text-align: center; border: 1px solid rgb(153, 153, 153);" class="">633 Hz</td><td style="text-align: center; border: 1px solid rgb(153, 153, 153);" class="">119.6K</td><td style="text-align: center; border: 1px solid rgb(153, 153, 153);" class="">125 K</td></tr><tr class=""><td style="text-align: center; border: 1px solid rgb(153, 153, 153);" class="">9073 Hz</td><td style="text-align: center; border: 1px solid rgb(153, 153, 153);" class="">50.6K</td><td style="text-align: center; border: 1px solid rgb(153, 153, 153);" class="">49 K</td></tr><tr class=""><td style="text-align: center; border: 1px solid rgb(153, 153, 153);" class="">1458 Hz</td><td style="text-align: center; border: 1px solid rgb(153, 153, 153);" class="">143.2K</td><td style="text-align: center; border: 1px solid rgb(153, 153, 153);" class="">125 K</td></tr><tr class=""><td style="text-align: center; border: 1px solid rgb(153, 153, 153);" class="">13500 Hz</td><td style="text-align: center; border: 1px solid rgb(153, 153, 153);" class="">51.0K</td><td style="text-align: center; border: 1px solid rgb(153, 153, 153);" class="">49 K</td></tr><tr class=""><td style="text-align: center; border: 1px solid rgb(153, 153, 153);" class="">731 Hz</td><td style="text-align: center; border: 1px solid rgb(153, 153, 153);" class="">125.7K</td><td style="text-align: center; border: 1px solid rgb(153, 153, 153);" class="">125 K</td></tr><tr class=""><td style="text-align: center; border: 1px solid rgb(153, 153, 153);" class="">3946 Hz</td><td style="text-align: center; border: 1px solid rgb(153, 153, 153);" class="">116.4K</td><td style="text-align: center; border: 1px solid rgb(153, 153, 153);" class="">125 K</td></tr></tbody></table><div class=""><br class=""></div></div><div class="">As you can see, all the "Required R" values are pretty close to the midpoint R values. The fifth line, 1458Hz, is pushing it a bit - we have 150K as the maximum value from the resistor+trimmer, so that's a bit close to the end, but it's still within, just about.</div><div class=""><br class=""></div><div class="">The BIG EXCEPTION is that second line, which needs an R value five times smaller than the available 39K+20K trimmer. What's going on? Is this just a mistake on the schematic? Is the frequency wrong, or is it the cap value that is wrong?</div><div class=""><br class=""></div><div class="">Any thoughts much appreciated.</div><div class=""><br class=""></div><div class="">Thanks,</div><div class="">Tom</div><div class=""><br class=""></div><div class=""><div class="">
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