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Pure guess but has somebody already replaced the transformer once, and has JUST ONE of the primary windings reversed ? This would mean they would fight one another, and with larger fuses happily overheat and melt the insulation of the windings.</div>
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<div id="divRplyFwdMsg" dir="ltr"><font face="Calibri, sans-serif" style="font-size:11pt" color="#000000"><b>From:</b> Synth-diy <synth-diy-bounces@synth-diy.org> on behalf of Jean-Pierre Desrochers via Synth-diy <synth-diy@synth-diy.org><br>
<b>Sent:</b> 20 February 2026 01:20<br>
<b>To:</b> synth-diy@synth-diy.org <synth-diy@synth-diy.org><br>
<b>Subject:</b> [sdiy] Questioning on why both of this power transformer primary windings went almost short..</font>
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<p class="x_MsoNormal"><span lang="EN-CA">A customer brought me a guitar preamp that he said ‘blows its internal fuses..’</span></p>
<p class="x_MsoNormal"><span lang="EN-CA">I checked the internals and found that both of the internal 315mA primary windings fuses were blown (F2 & F3).</span></p>
<p class="x_MsoNormal"><span lang="EN-CA">The main 500mA fuse (F1) was ok.</span></p>
<p class="x_MsoNormal"><span lang="EN-CA">So I connected my variac to <u>only one</u> of the primary windings (no fuses) to find that<br>
reaching around 90VAC it started to drain around 1 AMP !</span></p>
<p class="x_MsoNormal"><span lang="EN-CA">I checked the other primary winding alone and it behaved exactly the same,</span></p>
<p class="x_MsoNormal"><span lang="EN-CA">1AMP at around 90VAC.</span></p>
<p class="x_MsoNormal"><span lang="EN-CA">These measures were done with none of the secondaries connected (free wires).</span></p>
<p class="x_MsoNormal"><span lang="EN-CA"> </span></p>
<p class="x_MsoNormal"><span lang="EN-CA">So my questioning is what the heck almost shorted each of these primary windings<br>
exactly the same way..?</span></p>
<p class="x_MsoNormal"><span lang="EN-CA">I connected external AC voltages to each of the secondaries bridge (one at the time)<br>
and there were no shorts on each of them. The resulting DC voltages were as expected.</span></p>
<p class="x_MsoNormal"><span lang="EN-CA">To create those shorts at each primaries it would have needed<br>
that the 315mA fuses would have been replaced by much higher values<br>
with some kind of shorts somewhere in the secondaries side.. ?</span></p>
<p class="x_MsoNormal"><span lang="EN-CA">OR this is a manufacturing default that waited to awake ?</span></p>
<p class="x_MsoNormal"><span lang="EN-CA">Anyway I ordered a new transformer replacement, but I don’t want<br>
to fry it again because of something else that blew the first one..</span></p>
<p class="x_MsoNormal"><span lang="EN-CA"> </span></p>
<p class="x_MsoNormal"><span lang="EN-CA">What do you think ?</span></p>
<p class="x_MsoNormal"><span lang="EN-CA"> </span></p>
<p class="x_MsoNormal"><span lang="EN-CA"> </span></p>
<p class="x_MsoNormal"><img width="802" height="301" id="x_Image_x0020_1" style="width:8.3541in; height:3.1354in" data-outlook-trace="F:1|T:1" src="cid:image003.jpg@01DCA1DD.28EBF930"></p>
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