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The left channel of a phono input on a mixer is outputting only noise. It is measuring -8V DC across input terminals, while the other channel is measuring 0V.

The dual op-amp, a 4580, is getting ~27V in Vcc+.

Channel 1's IN+, IN- and OUT are all ~13.5V in relation to Vcc-. This is the working right channel.

Channel 2 measures IN+ ~5V, IN- ~10V and OUT ~1V.

Should I suspect the op-amp of having failed? Or is this a symptom of something downstream being wrong?

schematic

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  • \$\begingroup\$ Not-off-topic repair question! Shows schematic and shows basic troubleshooting steps (DC measurements of relevant nodes, and comparison with working channel). These significantly narrow down the likely root cause. \$\endgroup\$
    – Kaz
    Commented May 10, 2014 at 7:24

2 Answers 2

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If +Vin is measuring differently to the working channel then I'd suspect the op-amp because that input should be close to GND i.e. +13.5 above the main negative rail which I assume will be -13.5V wrt ground - there is nothing other than the opamp and R38 to cause this problem - check R38 and if that is OK pull the op-amp.

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  • \$\begingroup\$ It was indeed the opamp. I replaced it and the mixer channel works just fine. You made the dj a very happy man. \$\endgroup\$
    – Thomas
    Commented May 29, 2014 at 19:05
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Channel 2 measures IN+ ~5V, IN- ~10V and OUT ~1V.

This is impossible for a properly functioning op-amp for which the following are true:

(1) With negative feedback present, the non-inverting and inverting input voltages are effectively equal

(2) If negative feedback is not present and the non-inverting and inverting input voltages are not equal, the output voltage will be either the positive or negative saturation voltage

Since neither case is true (assuming bipolar power supplies), you should suspect the op-amp has failed.

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