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I believe C7, C8, and C9 in the schematic are meant to represent capacitance on the modules to be powered - ie. not part of the power supply itself. In my own modules I generally just put capacitors to the rails from ground, so at the moment there isn't really
a "C9", but since it's for a modular synth, it needs to be able to handle modules that do have it.<br>
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<div id="divRplyFwdMsg" dir="ltr"><font style="font-size:11pt" face="Calibri, sans-serif" color="#000000"><b>From:</b> Liam Wall <liam.wall@gmail.com><br>
<b>Sent:</b> Wednesday, August 26, 2020 11:39 PM<br>
<b>To:</b> ackolonges fds <ackolonges@hotmail.com><br>
<b>Subject:</b> Re: [sdiy] Power Supply Start-up Issues</font>
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<div dir="auto">From the description there the diodes are not the problem. C9 is the problem, with the diodes being a (partial) fix.
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<div dir="auto">Do you have an equivalent of C9 in your circuit?</div>
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<div dir="ltr" class="x_gmail_attr">On Wed, 26 Aug 2020, 12:26 ackolonges fds, <<a href="mailto:ackolonges@hotmail.com">ackolonges@hotmail.com</a>> wrote:<br>
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<span style="font-family:Calibri,Helvetica,sans-serif; font-size:12pt; color:rgb(0,0,0)">Hi All,</span></div>
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<div><span style="font-family:Calibri,Helvetica,sans-serif; font-size:12pt; color:rgb(0,0,0)">I've been having issues with a very standard +/- 12V power supply design for a modular synth. Every once in a while when powering up, the positive rail doesn't rise,
and it needs to be reset to work. After some trial and error, and research, I found mention of this issue in Douglas Self's 'Small Signal Audio Design' book, where he shows a circuit almost identical to my power supply. A screen grab of the related page can
be found here: </span><a href="https://pasteboard.co/Jo8PuIu.png" id="x_m_893671547408173809LPlnkOWALinkPreview_0" target="_blank" rel="noreferrer"><span style="font-family:Calibri,Helvetica,sans-serif; font-size:12pt; color:rgb(0,0,0)">https://pasteboard.co/Jo8PuIu.png</span></a><br>
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<span style="font-family:Calibri,Helvetica,sans-serif; font-size:12pt; color:rgb(0,0,0)">This paragraph from that page describes the issue I'm having perfectly :
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<span style="font-family:Calibri,Helvetica,sans-serif; font-size:12pt; color:rgb(0,0,0)"><i><br>
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<span style="font-family:Calibri,Helvetica,sans-serif; font-size:12pt; color:rgb(0,0,0)"><i>"</i></span><i><span style="font-family:Calibri,Helvetica,sans-serif; font-size:12pt; color:rgb(0,0,0)">D2, D4 will prevent damage
</span><span style="font-family:Calibri,Helvetica,sans-serif; font-size:12pt; color:rgb(0,0,0)">in this case, but leave the power supply vulnerable to start-up problems; if its output is being
</span><span style="font-family:Calibri,Helvetica,sans-serif; font-size:12pt; color:rgb(0,0,0)">pulled down by the
</span><span style="font-family:Calibri,Helvetica,sans-serif; font-size:12pt; color:rgb(0,0,0)"></span><span style="font-family:Calibri,Helvetica,sans-serif; font-size:12pt; color:rgb(0,0,0)">15 V regulator, the</span><span style="font-family:Calibri,Helvetica,sans-serif; font-size:12pt; color:rgb(0,0,0)">
</span><span style="font-family:Calibri,Helvetica,sans-serif; font-size:12pt; color:rgb(0,0,0)">15 V regulator may refuse to start. This is actually
</span><span style="font-family:Calibri,Helvetica,sans-serif; font-size:12pt; color:rgb(0,0,0)">a very dangerous situation, because it is quite easy to come up with a circuit where start-up will</span><span style="font-family:Calibri,Helvetica,sans-serif; font-size:12pt; color:rgb(0,0,0)">
only fail one time in 20 or more, the incidence being apparently completely random, but</span><span style="font-family:Calibri,Helvetica,sans-serif; font-size:12pt; color:rgb(0,0,0)"> presumably controlled by the exact point in the AC mains cycle where the
supply is switched </span><span style="font-family:Calibri,Helvetica,sans-serif; font-size:12pt; color:rgb(0,0,0)">on, and other variables such as temperature, the residual charge left on the reservoir capacitors,
</span><span style="font-family:Calibri,Helvetica,sans-serif; font-size:12pt; color:rgb(0,0,0)">and the phase of the moon. If even one start-up failure event is overlooked or dismissed as</span><span style="font-family:Calibri,Helvetica,sans-serif; font-size:12pt; color:rgb(0,0,0)">
unimportant. then there is likely to be serious grief further down the line.</span><span style="font-family:Calibri,Helvetica,sans-serif; font-size:12pt; color:rgb(0,0,0)"> Every power supply</span><span style="font-family:Calibri,Helvetica,sans-serif; font-size:12pt; color:rgb(0,0,0)">
start-up failure must be taken seriously.</span></i><span style="font-family:Calibri,Helvetica,sans-serif; font-size:12pt; color:rgb(0,0,0)"><i>"</i></span></div>
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Unfortunately after stressing how important it is to fix such an issue, no solutions are mentioned... I've tried removing all of the protection diodes, increasing and decreasing the value of the smoothing caps, tried many different makes of voltage regulators,
and also tried an NTC thermistor to slow the in-rush current, all to no avail. <br>
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I'm starting to run out of ideas, so if anyone on the list has come across a fix it would be great to hear about it.
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