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<p>Hi all,<br>
</p>
<p>Well 8.1n is not a E12/24 value at all, 8.2nF would be. </p>
<p>Not consistent to use 8.1nF for 8529Hz, and in the next closest
frequency 1.5nF (9073Hz)</p>
<p>Its plausible that this is a typo, and it's really 1.8nF. <br>
</p>
<p><br>
</p>
<p>Best,</p>
<p> René<br>
</p>
<p><br>
</p>
<div class="moz-cite-prefix">Am 13.11.2023 um 14:44 schrieb Tom
Wiltshire:<br>
</div>
<blockquote type="cite"
cite="mid:87289F44-90B3-4F0B-A232-83D4016F07A4@electricdruid.net">
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<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="moz-txt-link-freetext" moz-do-not-send="true">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="moz-txt-link-freetext" moz-do-not-send="true">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="">
<div>==================<br class="">
Electric Druid<br class="">
Synth & Stompbox DIY<br class="">
==================</div>
<div class=""><br class="">
</div>
<br class="Apple-interchange-newline">
</div>
<br class="">
</div>
<br>
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</blockquote>
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