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Old 7th April 2014, 06:04 PM   #1
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Default NAD 7155 Preamp help needed

I have been trying to repair a NAD 7155 for a while now and just can't seem to find the gremlin in the preamp circuit. The amp section works fine (bipassed jumpers) but the preamp won't give me any volume. Using the turner gives me a little hotter signal than using the Aux in but not much. The volume seems to adjust a little (maybe 5%) but even at full off the tuner signal still gives almost as much sound this is equally true on both channels.
I have check voltages through out and everything reads as it should. I've checked a ton of resistors, caps and transistors. As it is equal on both channels I tend to think that it would be a power supply issue but for the life of me I can't find it. I can post the schematic if that would help. This thing has been driving me crazy so I would really appreciate any ideas and/or help. Thanks!!!
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Old 7th April 2014, 06:28 PM   #2
Mooly is offline Mooly  United Kingdom
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Have you traced the signal through with a scope to see what is going on ? Could it be physical such as cracked or damaged print ?
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Old 8th April 2014, 02:13 AM   #3
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With the volume being low in both channels you'd be right in thinking the fault is common to both channels and so not necessarily in the preamp itself. The preamp. proper is just 2 transistors and an opamp with the usual tone and volume controls plus loudness switch. The rest is just an active filter on each opamp power rail. There is also a common muting control circuit there - have you checked its operation? (a voltage shift on FET Q603/653 gates when switched) It wouldn't surprise me that that something is amiss there, even the FETs, exposed to external connections (pre out/in) as that point is. There is the more obvious -20 dB level switch too, but I imagine that's been flicked many times already.
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