I'm repairing an old and badly damaged nad 7020, and while I managed to get it work, I still got a silent but noticable noise on one channel, and I couldn't find out why.
What I did:
-Replaced some bad transistors and inserted some missing ones (I also replaced the 2n6553/6556 drivers with bd139/140, becouse one of the originals was missing)
-Recalibrated the idle current & dc offset according to the service manual
-Replaced all the electrolytic caps in the amplifier and power supply (it had the noise before I did this)
What I know about the problem:
-The right channel is perfect
-The left channel is usable, but it has the noise, and I think it sounds worse than the right - it's hard to decide
-When I turn on the amp the noise is very loud then it gets almost muted
-If I unplug the preamp-poweramp jumpers, the noise remains loud - So I guess the grounding from the preamp signal silence it in "normal operation"
-If I turn on soft clipping it helps a lot, but still not solves the problem
-If I unplug the amplifier card (Q601-Q606 and the regulated power supply) the noise is gone, but I'm not 100% sure that the final stage of the power amp still works this way.
-Q601-603-605-607 seems ok (measured in circuit)
-The noise sounds like white noise (like FM when it's not tuned to station)
The FM part of the radio doesn't work, but I don't think it's related (and it might works, it's just recalibrated to the old hungarian 66-73MHz FM band)
If I had an oscilloscope I would try to trace the noise, but unfortunately I don't have one.
Service manual: http://p10hifi.net/planet10/manuals/NAD7020early_servMan.pdf
But it's not accurate - it doesn't mention the amplifier card, and some modules, like the power regulator is completly different, looks more like the one in the 3020b (3020b schematics: http://p10hifi.net/planet10/manuals/3020b_schematic.gif). The part numbering is completly wrong (the numbers I used above is based on the service manual - it's probably not the same in the device)
What I did:
-Replaced some bad transistors and inserted some missing ones (I also replaced the 2n6553/6556 drivers with bd139/140, becouse one of the originals was missing)
-Recalibrated the idle current & dc offset according to the service manual
-Replaced all the electrolytic caps in the amplifier and power supply (it had the noise before I did this)
What I know about the problem:
-The right channel is perfect
-The left channel is usable, but it has the noise, and I think it sounds worse than the right - it's hard to decide
-When I turn on the amp the noise is very loud then it gets almost muted
-If I unplug the preamp-poweramp jumpers, the noise remains loud - So I guess the grounding from the preamp signal silence it in "normal operation"
-If I turn on soft clipping it helps a lot, but still not solves the problem
-If I unplug the amplifier card (Q601-Q606 and the regulated power supply) the noise is gone, but I'm not 100% sure that the final stage of the power amp still works this way.
-Q601-603-605-607 seems ok (measured in circuit)
-The noise sounds like white noise (like FM when it's not tuned to station)
The FM part of the radio doesn't work, but I don't think it's related (and it might works, it's just recalibrated to the old hungarian 66-73MHz FM band)
If I had an oscilloscope I would try to trace the noise, but unfortunately I don't have one.
Service manual: http://p10hifi.net/planet10/manuals/NAD7020early_servMan.pdf
But it's not accurate - it doesn't mention the amplifier card, and some modules, like the power regulator is completly different, looks more like the one in the 3020b (3020b schematics: http://p10hifi.net/planet10/manuals/3020b_schematic.gif). The part numbering is completly wrong (the numbers I used above is based on the service manual - it's probably not the same in the device)
First remove the jumper plugs between PRE OUT - NORMAL IN - LAB IN
RCA-s, (this way you will hear only the power amp stage) if the noise is still there, then the source of it is in the power stage, if not, the it comes from the preamp section. If the noise source is the preamp, then turn your volume down completely, if you still hear it, then it is the volume-tone stage (check Q501-Q510, also the coupling capacitors).
RCA-s, (this way you will hear only the power amp stage) if the noise is still there, then the source of it is in the power stage, if not, the it comes from the preamp section. If the noise source is the preamp, then turn your volume down completely, if you still hear it, then it is the volume-tone stage (check Q501-Q510, also the coupling capacitors).
Done that already: "If I unplug the preamp-poweramp jumpers, the noise remains loud - So I guess the grounding from the preamp signal silence it in "normal operation"
So the noise not just remains, it gets louder.
So the noise not just remains, it gets louder.
It has nothing to do with grounding, since the plugs couple only the signal path, not interrupting grounding. In this case anyway the problem is your power stage, so continue checking transistors, diodes, and electrolytics there. It can even be a noisy resistor (not uncommon with carbon types). I suspect Q601 (or 602) after your description.
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