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<font face="Arial">In my experience, the main difference between the
rev C and rev G chips is that the rev G need a larger offset
voltage range than is supplied by the existing synth circuits. <br>
<br>
I've successfully used rev G chips in several P600s with no
circuit changes required -- the only time there was a problem was
if the unit had the Gligli firmware upgrade, which uses a
different tuning procedure.<br>
In most cases, I found that a Gligli modified P600 with rev G
oscillators would tune correctly if the oscillator trimmers were
turned hard left. In the few cases where this didn't work, the
resistor change mentioned below solved the problem.<br>
<br>
Using a rev G in an OBXa, I had to reduce the value of the 1Meg
resistor from the Initial Frequency trimmer to get enough range to
scale the oscillator as per the service manual procedure.<br>
<br>
If anyone has had different experience with either of these
synths, please let me know.<br>
~GMM<br>
</font><br>
<div class="moz-cite-prefix">On 2/25/2020 7:35 AM, Tom Wiltshire
wrote:<br>
</div>
<blockquote type="cite"
cite="mid:AB499511-EF4A-4DD6-A09F-DC2C8368797C@electricdruid.net">
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Hi All,
<div class=""><br class="">
</div>
<div class="">Does anyone know what the differences are between
early and late revisions of the CEM3340?</div>
<div class=""><br class="">
</div>
<div class="">I have a customer who is trying to fix a dead
oscillator in an OBXa. He’s replaced the faulty CEM3340 rev.C
chip with a modern Rev.G which fixes the dead voice, but the
voice won’t tune. Swapping in a Rev.C from another voice cures
the problem, so we’re pretty sure it’s down to differences
between the chips.</div>
<div class=""><br class="">
</div>
<div class="">The only thing I’ve been able to find is a Analogue
Heaven thread where someone mentions a note in the Sequential
Circuits documentation about reducing the value of the resistor
on pin 1, but they don’t reference where this is found:</div>
<div class=""><br class="">
</div>
<div class="">
<blockquote type="cite" class="">"Modification To VCO Circuits<br
class="">
For correct scaling, all Sequential synthesizers with Rev G
CEM3340 VCO chips need the 26.7K 1% resistors (part #R-146) in
the VCO circuits to be replaced with 25.5K 1% resistors (part
#R-516). This change MAY affect tuning of Prophet-5, -10,
-600, and -T8s.”</blockquote>
</div>
<div class=""><br class="">
</div>
<div class="">(From <a
href="http://analogue-heaven.1065350.n5.nabble.com/CEM3340-revG-reissue-td135312.html"
class="" moz-do-not-send="true">http://analogue-heaven.1065350.n5.nabble.com/CEM3340-revG-reissue-td135312.html</a>)</div>
<div class=""><br class="">
</div>
<div class="">This isn’t a massive change (-5% or so) but it was
obviously important enough to warrant the effort. Oberheim had
wads of Engineering Change Orders for a lot of their stuff, and
I’ve checked the ones I have but haven’t found anything relevant
to VCO chip revisions.</div>
<div class=""><br class="">
</div>
<div class="">Does anyone have any clues?</div>
<div class=""><br class="">
</div>
<div class="">Thanks,</div>
<div class="">Tom</div>
<div class=""><br class="">
<div class="">
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word-spacing: 0px; -webkit-text-stroke-width: 0px;">==================<br
class="">
Electric Druid<br class="">
Synth & Stompbox DIY<br class="">
==================</div>
</div>
</div>
<br class="">
</div>
<br>
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