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First of all measure the voltages on the output op-amp power rails. </div>
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If they are reasonable, look at the op-amp itself. The highly attenuated signal could be it passing from a previous stage through the feedback resistor to the output. Op-amps reject PSU hum really well, but not when they are dead.</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 John Ames via Synth-diy <synth-diy@synth-diy.org><br>
<b>Sent:</b> 13 May 2024 01:01<br>
<b>To:</b> synth-diy@synth-diy.org <synth-diy@synth-diy.org><br>
<b>Subject:</b> [sdiy] Non-DIY, but looking for repair advice...</font>
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<div class="PlainText">Slightly not-strictly-OT here, I s'pose, but I'm looking for hints/<br>
advice - I've got a Yamaha CS-5 that's mostly in good working order, but<br>
has two (possibly related) issues. One is that there's a slight 60 Hz<br>
buzz from the PSU - it's present even with the mixer sources turned<br>
off, but the filter can remove it. The other is that the output is very<br>
weak; running direct into my recording workstation, the maximum<br>
amplitude is lower than -30dB! Of course I could pre-amplify it...but<br>
then that boosts the AC hum from "not really audible" to "far too<br>
noticeable" :/<br>
<br>
Now, I haven't had it open yet; before I go poking around willy-nilly I<br>
thought I'd ask for any advice, and raise what may well be an important<br>
point. This is, or was, a Japanese import model, and the print on the<br>
back near the power cord lists it as a 100V device - but somewhere<br>
along the line someone covered that over with masking tape and Sharpied<br>
"115V" on it. I've never had problems running 100V equipment on US line<br>
voltages before, but admittedly it's almost all been digital gear; is<br>
that potentially an issue? And someone might have done a mod to adapt<br>
this thing for that already; any ideas what to look for if I open it up?<br>
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