Have you tested isolating the amplifier and preamplifier to find out where the problem is located? (remove the bridge on the back)
@Jakobsson, Good idea. I just did and it turns out that the startup/shutoff noise is coming from the preamp.
Illustrating that conclusion, let's start with the right speaker output at startup when the bridge between the preamp and the power amp has been removed:
The startup noise seems to be absent. (although that right spike looks like the intermittent noise I'm also experiencing but I'm hoping that fixing the startup/shutoff noise will fix that as well or perhaps adjusting the DC offset. In any case, one issue at a time)
Conversely, here's the right preamp channel at startup:
We can see by zooming in that it's a 50 Hz sine:
Interestingly, measuring the same right preamp channel at shutoff shows the same sine but it lasts about 2 minutes before tapering off like this:
which feels a bit like a capacitor discharging.
Here's the waveform zoomed in in the middle of the taper:
Not sure what more to make of this but I'm looking forward to hearing your thoughts on this @Jakobsson @kapitiaudio
Illustrating that conclusion, let's start with the right speaker output at startup when the bridge between the preamp and the power amp has been removed:
The startup noise seems to be absent. (although that right spike looks like the intermittent noise I'm also experiencing but I'm hoping that fixing the startup/shutoff noise will fix that as well or perhaps adjusting the DC offset. In any case, one issue at a time)
Conversely, here's the right preamp channel at startup:
We can see by zooming in that it's a 50 Hz sine:
Interestingly, measuring the same right preamp channel at shutoff shows the same sine but it lasts about 2 minutes before tapering off like this:
which feels a bit like a capacitor discharging.
Here's the waveform zoomed in in the middle of the taper:
Not sure what more to make of this but I'm looking forward to hearing your thoughts on this @Jakobsson @kapitiaudio
So you get a burst of 50Hz out of the right channel preamp at startup. Was the original problem only on the right channel? It could be a muting issue since a power supply issue would usually affect both channels. But I'd go back to connecting the preamp back up and looking at both speaker outputs.
The first set of scope shot's on the speaker outputs weren't that useful - too much zooming in. What you need to do is set the amplitude to say 200mV/div and the timebase pretty slow (seconds) (since this noise lasts for 10s at startup). Then hook up both scope channels, ch1 to the left speaker output and ch2 to the right speaker output. Then you should be able to see how both channels behave at turn on at the same time and get a long enough history to further fine tune the scope settings. I would have the scope set to DC coupling so we can see output offset behavior at startup too.
You could connect the speakers to make sure it's actually faulting at the time you're doing the measurement as that's what we are interested in. This may also reduce the measured noise on the output which would be helpful.
The first set of scope shot's on the speaker outputs weren't that useful - too much zooming in. What you need to do is set the amplitude to say 200mV/div and the timebase pretty slow (seconds) (since this noise lasts for 10s at startup). Then hook up both scope channels, ch1 to the left speaker output and ch2 to the right speaker output. Then you should be able to see how both channels behave at turn on at the same time and get a long enough history to further fine tune the scope settings. I would have the scope set to DC coupling so we can see output offset behavior at startup too.
You could connect the speakers to make sure it's actually faulting at the time you're doing the measurement as that's what we are interested in. This may also reduce the measured noise on the output which would be helpful.
The opamps have a very basic powersupply construction limiting voltage from +/-20v to +/- 14.3v only by a resistor and capacitors. My idea is that if the capacitors go bad (high esr)(C311/C312) the opamp may act up a bit before it stabilize. Check that the current limiting resistors is not out of spec.(R311/R312). The muting circuit have some startup delay but maybe its not long enough to coverup the noise.
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I just remembered that there is a switch built in to main powerswitch that is very important for the muting/startup function.
@Jakobsson, I changed out C311 and C312 after you first suggested they might be the cause and I checked R311 and R312 as well and they show 1k in-circuit.
@kapitiaudio, A strange thing happened. I went out to the electronics supply store to get another probe for the scope so that I could measure both the left and right speaker outputs at the same time as you suggested. I rebridged the preamp and power amp and hooked up the speakers as well and measured. For some reason unknown to me, the startup/shutoff noise has now all but disappeared and is only faintly audible now if put my ear right up to the speakers. The only thing I can think of that could explain the improvement is the extra cleaning I did on the brown capacitor goop - I still can't get it all off without removing a lot of components but I did try to break possible shorts it was making by carefully scoring and chipping at it with small screwdriver.
I also suspect that the old probe I was using is quite noisy due to this startup measurement where the old probe is on ch1 below and the new one on ch2 above.
I confirmed that this difference is actually in the probes and not the amp channels by switching them and confirming that the ch1 (old probe) was still noisier than ch2 (new probe).
Anyways, with the startup/shutoff noise miraculously out of the way (hopefully for good), I thought I'd try aligning the DC offset and idling current using a multimeter to see if that helps with the intermittent noise. The left channel DC offset was <10 mV already so I moved on to the right channel where I hit a bit of a roadblock. When trying to adjust VR402 to get the right channel DC offset down from 150 mV I noticed that the variable resistors have all been fixed in place with some kind of white glue:
Despite this, before realizing I somehow made it jump to -230 mV while trying to turn it without really managing to turn it at all.
Measuring the speaker output at startup again I now see and hear a lot of the same kind of intermittent noise right away on the right channel (lower side cut off a little bit in the measurement):
Interestingly, the DC offset looks more like -70-80 mV on the scope but in any case, the sudden increase in noise coinciding with the unfortunate DC offset adjustment (more of which I'm afraid to attempt with the VR's glued up) leads me to believe they're connected.
So, that leaves me wondering: Do I try to manually remove the glue on the VR's enough to be able to turn them and perform the alignment procedure or should I look for replacements instead? Am I wrong in thinking the DC offset and noise are connected and Is there some other probable cause for the noise that I should be looking into instead?
@kapitiaudio, A strange thing happened. I went out to the electronics supply store to get another probe for the scope so that I could measure both the left and right speaker outputs at the same time as you suggested. I rebridged the preamp and power amp and hooked up the speakers as well and measured. For some reason unknown to me, the startup/shutoff noise has now all but disappeared and is only faintly audible now if put my ear right up to the speakers. The only thing I can think of that could explain the improvement is the extra cleaning I did on the brown capacitor goop - I still can't get it all off without removing a lot of components but I did try to break possible shorts it was making by carefully scoring and chipping at it with small screwdriver.
I also suspect that the old probe I was using is quite noisy due to this startup measurement where the old probe is on ch1 below and the new one on ch2 above.
I confirmed that this difference is actually in the probes and not the amp channels by switching them and confirming that the ch1 (old probe) was still noisier than ch2 (new probe).
Anyways, with the startup/shutoff noise miraculously out of the way (hopefully for good), I thought I'd try aligning the DC offset and idling current using a multimeter to see if that helps with the intermittent noise. The left channel DC offset was <10 mV already so I moved on to the right channel where I hit a bit of a roadblock. When trying to adjust VR402 to get the right channel DC offset down from 150 mV I noticed that the variable resistors have all been fixed in place with some kind of white glue:
Despite this, before realizing I somehow made it jump to -230 mV while trying to turn it without really managing to turn it at all.
Measuring the speaker output at startup again I now see and hear a lot of the same kind of intermittent noise right away on the right channel (lower side cut off a little bit in the measurement):
Interestingly, the DC offset looks more like -70-80 mV on the scope but in any case, the sudden increase in noise coinciding with the unfortunate DC offset adjustment (more of which I'm afraid to attempt with the VR's glued up) leads me to believe they're connected.
So, that leaves me wondering: Do I try to manually remove the glue on the VR's enough to be able to turn them and perform the alignment procedure or should I look for replacements instead? Am I wrong in thinking the DC offset and noise are connected and Is there some other probable cause for the noise that I should be looking into instead?
OK, so it's established the noise is on the right channel, good.
It would be worthwhile replacing those old adjustment pots on both channels and then doing the DC setup again. A new pot won't create bias hazards during adjustment.
It would be worthwhile replacing those old adjustment pots on both channels and then doing the DC setup again. A new pot won't create bias hazards during adjustment.
If the power supply caps spilledHi everyone - So I have this NAD 7240PE. It's an absolute beast of a receiver that I bought sight unseen from the other side of the country and it arrived pretty beat up. It was super dirty and dusty inside and noisy as hell and the seller proceeded to never return any of my messages 😅. Several of the largest caps had burst at some point and spilled their goop on the PCB, which itself has turned brown from heat in the amp section, it must have been run HOT a lot. The radio tuner side meanwhile is still green as a normal PCB and works pretty well even just using the built in FM antenna.
I switched out the busted caps and cleaned it up and it's working a lot better but there must still be some component(s) fried or something since the amp has a ca. 10 sec period at startup and turn off where it plays this kind of ground saw wave noise through the speakers similar to touching an input jack your finger. After that stops, there is still this random, intermittent popping sound in both channels underneath the audio signal but it decreases slowly until - when the amp is warmed up perhaps - the noise is gone. At this point, the amp works almost noiselessly and sounds absolutely superb.
I'm telling you, the combination of buttons called “Bass EQ” and “Loudness” really make this amp drive way more nice low-end from my two Artcoustic wall mounted speakers than my other, newer NAD C316BEE ever could. It also sounds much more dynamic and enjoyable, no doubt partly due to the pretty complex BJT-based Class G topology in which separate higher voltage power rails are switched to momentarily when triggered by a powerful enough input signal, ultimately resulting in higher efficiency and dynamic range I believe.
Anyway, in my search for knowledge that could help in diagnosing and fixing the issue, I came upon some 7240PE repair threads on here and some people (e.g. Mooley) seem to have the skillset and experience needed to diagnose this kind of stuff so here I am, asking for some tips on what to look into on this amp 🙂
I can record the noises in question if it helps, as well as measure anything with a multimeter. I also have a power supply and even an old analog Agilent Oscilloscope that I bought cheap from my old student job employer (ICEPower) as they had stopped using it in favor of the new digital ones. It has been sitting in a cabinet unused since because I have yet to buy some probes for it - which I will if it will help fix this amp!
P.S. Here's the service manual which includes the amp schematic on page 13 and here's also some technical info on how the Class G voltage switching works for those interested:
View attachment 1438390
If the power supply capacitors spilled their chemical contents and have since been replaced one would have to consider what gave rise to this occurence and consider any damage to the diodes in the rectifier bridge and the transformer secondary and primary winding and any plastic capacitor bridging the latter. If any diodes in the rectifier bridge allowed passage to the full ac output secondary voltage of the transformer. That would cause a disaster with untold consequences.that could have compromised the transformer in the way of breakdown of insulation and leakage between primaryHi everyone - So I have this NAD 7240PE. It's an absolute beast of a receiver that I bought sight unseen from the other side of the country and it arrived pretty beat up. It was super dirty and dusty inside and noisy as hell and the seller proceeded to never return any of my messages 😅. Several of the largest caps had burst at some point and spilled their goop on the PCB, which itself has turned brown from heat in the amp section, it must have been run HOT a lot. The radio tuner side meanwhile is still green as a normal PCB and works pretty well even just using the built in FM antenna.
I switched out the busted caps and cleaned it up and it's working a lot better but there must still be some component(s) fried or something since the amp has a ca. 10 sec period at startup and turn off where it plays this kind of ground saw wave noise through the speakers similar to touching an input jack your finger. After that stops, there is still this random, intermittent popping sound in both channels underneath the audio signal but it decreases slowly until - when the amp is warmed up perhaps - the noise is gone. At this point, the amp works almost noiselessly and sounds absolutely superb.
I'm telling you, the combination of buttons called “Bass EQ” and “Loudness” really make this amp drive way more nice low-end from my two Artcoustic wall mounted speakers than my other, newer NAD C316BEE ever could. It also sounds much more dynamic and enjoyable, no doubt partly due to the pretty complex BJT-based Class G topology in which separate higher voltage power rails are switched to momentarily when triggered by a powerful enough input signal, ultimately resulting in higher efficiency and dynamic range I believe.
Anyway, in my search for knowledge that could help in diagnosing and fixing the issue, I came upon some 7240PE repair threads on here and some people (e.g. Mooley) seem to have the skillset and experience needed to diagnose this kind of stuff so here I am, asking for some tips on what to look into on this amp 🙂
I can record the noises in question if it helps, as well as measure anything with a multimeter. I also have a power supply and even an old analog Agilent Oscilloscope that I bought cheap from my old student job employer (ICEPower) as they had stopped using it in favor of the new digital ones. It has been sitting in a cabinet unused since because I have yet to buy some probes for it - which I will if it will help fix this amp!
P.S. Here's the service manual which includes the amp schematic on page 13 and here's also some technical info on how the Class G voltage switching works for those interested:
View attachment 1438390
and secondaries.
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