Just a thought and I can't see this on diagrams but quite a lot of amps have a 'ground lift resistor' around the inputs to the power amp. Usually around 1 to 10 ohm and it is there to help improve background hum. It artificially breaks any ground loop. I can't see that on the NAD but you never know if something like that was added in production.
It certainly is, ive had to do it many times for mechanical things in the past
I thought i had it earlier as the PNP transistor in the pre amp was giving a very spurious reading of only 245mv with the meter, so i replaced it, but it didnt make alot of difference.
i also disconnected the ground from that board in case it was having an influence, but alas not.
I have made a list of all components to ground that i need to check and ill plod through it over time untill i get it.
I know i have made progress with this as the original issue was with it wandering regardless, so i havent let my head drop too much.
the grounding must be the issue somewhere and i wont give up untill ive found it.
both channels operate completely normal at lower volumes, so i know i am close, my head just cant take any more for now, and i need some dinner!
I thought i had it earlier as the PNP transistor in the pre amp was giving a very spurious reading of only 245mv with the meter, so i replaced it, but it didnt make alot of difference.
i also disconnected the ground from that board in case it was having an influence, but alas not.
I have made a list of all components to ground that i need to check and ill plod through it over time untill i get it.
I know i have made progress with this as the original issue was with it wandering regardless, so i havent let my head drop too much.
the grounding must be the issue somewhere and i wont give up untill ive found it.
both channels operate completely normal at lower volumes, so i know i am close, my head just cant take any more for now, and i need some dinner!
I would do some very basic tests to begin with. Working from the diagram check that C613 reads zero ohms to other known ground parts in that channel, for example to the cathode of D601 and to R601. If it does then check that those points read zero ohms to the negative speaker terminal and that that in turn reads zero ohms back to the reservoir caps.
Compare those results with the other channel and the other amp you have.
Also if you are 'acquiring' these amps from the usual suspects 😀 then thy may have a very unknown past and have been worked on before.
Compare those results with the other channel and the other amp you have.
Also if you are 'acquiring' these amps from the usual suspects 😀 then thy may have a very unknown past and have been worked on before.
oh some of these have definatly had stuff done previously and i always bare that in mind,and i can tell what is and isnt original.
but im quite picky what i buy.
like i said to ian, i do it because i enjoy it but it gives me pleasure getting something back out there that is in good condition and will give someone pleasure for a few years.
meant to say i had alreay removed the ground side legs of C613/4 (this was -234mv ground) to make sure it wasnt that point causing the issue, i have then been using that as a point in reference
all of these other points have the same reading exactly as C613 and 4, some i have not checked yet, but this is using the case as a ground, and i will use the casing as reference for all.I have replaced all the parts to the board that make up the grounding to casing.
i also did some continuity checks between opposite channels D101-D102 egand all points have good connections.
if you use the ground where the main caps are they are all 0v, but because i have now looked at 2 seperate 3020 units i have here, they are all the same, the cap ground is not in continuity with the casing, so i am disregarding that as an issue, and this is why i know there is a simple issue somewhere
on the other amps the ground is no more than 1mv on all the below using either case ground or cap ground as a reference.
D601/2-234mv ground
C615/6-234mv ground-these i replaced last week as they were very shabby looking and unbranded
R601/2-234mv ground
R657/8-234mv ground
R701/2-234mv ground
R625/6-checked and ground problem exists with items removed
C613/4-234mv ground
i wont get the chance to check the rest untill next weekend now, it may even be elsewhere, but i have already eliminated the pre amp
i had to give the lounge back to the wife 😉
all of these other points have the same reading exactly as C613 and 4, some i have not checked yet, but this is using the case as a ground, and i will use the casing as reference for all.I have replaced all the parts to the board that make up the grounding to casing.
i also did some continuity checks between opposite channels D101-D102 egand all points have good connections.
if you use the ground where the main caps are they are all 0v, but because i have now looked at 2 seperate 3020 units i have here, they are all the same, the cap ground is not in continuity with the casing, so i am disregarding that as an issue, and this is why i know there is a simple issue somewhere
on the other amps the ground is no more than 1mv on all the below using either case ground or cap ground as a reference.
D601/2-234mv ground
C615/6-234mv ground-these i replaced last week as they were very shabby looking and unbranded
R601/2-234mv ground
R657/8-234mv ground
R701/2-234mv ground
R625/6-checked and ground problem exists with items removed
C613/4-234mv ground
i wont get the chance to check the rest untill next weekend now, it may even be elsewhere, but i have already eliminated the pre amp
i had to give the lounge back to the wife 😉
LOL.
I really don't know what to advise without actually seeing one for real. It would be a bit odd to have a floating chassis but there are clues to that in some other NAD diagrams if you look at the different earth symbols. There are small caps tying the zero volt (ground) line to chassis. That is not uncommon and works at AC not DC.
You have to be logical with this. Use the centre point of the reservoir caps as a ground and confirm all the circuit grounds (components that go to ground) read dead short to that point. If they do then you look elsewhere for the fault. Stick to that principle, that the reservoir cap centre point has got to be a ture ground. Everything ground must read electrically back to that point.
Another thought... we know the NAD's have weird FET muting. Is that OK? Nothing odd going on with that. Thinking more of the distortion on that, it wouldn't cause the ground voltages you see.
different symbols in the same diagram or just different models, im only aware of a few models that used a different symbol, that was the 3130/3020e/3225pe(proberbly others)
some certainly use a conventional 'earth' symbol, marantz and rotel have used this on occations
maybe just different draughtsmen
(FET muting)The early series 20 amps didnt have this feature(pretty sure), introduced later in the 3130/3140/3240-then 3202E/i were the first and all the 'pe' amps
ok ill do that a bit later today, i dont need alot of kit out to do that
i was going to do it this way but opted for the long winded approach lol 😉
i let you know what i find👍
some certainly use a conventional 'earth' symbol, marantz and rotel have used this on occations
maybe just different draughtsmen
(FET muting)The early series 20 amps didnt have this feature(pretty sure), introduced later in the 3130/3140/3240-then 3202E/i were the first and all the 'pe' amps
ok ill do that a bit later today, i dont need alot of kit out to do that
i was going to do it this way but opted for the long winded approach lol 😉
i let you know what i find👍
OK, have fun 😉
The different ground symbols do have different meanings... this is not 100% definitive to what you might encounter but it gives an idea of the basic differences and what they mean.
http://lednique.com/ground-earth-ch...which ground the components are connected to.
https://www.rapidtables.com/electric/Ground_Symbols.html
The different ground symbols do have different meanings... this is not 100% definitive to what you might encounter but it gives an idea of the basic differences and what they mean.
http://lednique.com/ground-earth-ch...which ground the components are connected to.
https://www.rapidtables.com/electric/Ground_Symbols.html
so here we go, nothing conclusive with the below.
all referenced back to the resevoir cap grounding and all voltages stable.
i guess im going to have to check the rest of the amp
D601/2-0.6mv ground
C615/6-0.6mv ground
R601/2-0.6mv ground
R657/8-0.6mv ground
R701/2-0.6mv ground
R625/6-0.6mv ground
C613/4-0.6mv ground
The above are ok? ive never had exactly 0v on any of these amps
one thing i found odd though
speaker terminals
L neg to ground 0v
L poss to ground 0.6mv
R poss to ground 6.9mv
R neg to ground 0v
all referenced back to the resevoir cap grounding and all voltages stable.
i guess im going to have to check the rest of the amp
D601/2-0.6mv ground
C615/6-0.6mv ground
R601/2-0.6mv ground
R657/8-0.6mv ground
R701/2-0.6mv ground
R625/6-0.6mv ground
C613/4-0.6mv ground
The above are ok? ive never had exactly 0v on any of these amps
one thing i found odd though
speaker terminals
L neg to ground 0v
L poss to ground 0.6mv
R poss to ground 6.9mv
R neg to ground 0v
Those look fine.
The speaker terminal measurement, POS to ground is just the DC offset of the channel and NEG to ground should be zero because the negative return should be thick copper print.
The other very small voltages (0.6mv) sound consistent with the small volt drop along thinner copper print that would be caused by small milliamp currents to parts of the amp flowing in it.
That at face value all looks good.
The speaker terminal measurement, POS to ground is just the DC offset of the channel and NEG to ground should be zero because the negative return should be thick copper print.
The other very small voltages (0.6mv) sound consistent with the small volt drop along thinner copper print that would be caused by small milliamp currents to parts of the amp flowing in it.
That at face value all looks good.
ok thats cool
im concentrating my efforts elswhere now.Ive has suspision about the volume control, or what it does directly to affect the centre .
0 ohms to about 5k (quarter turn) centre stable at about 2mv, but then the more you increase it, it just goes crazy and both channels are affected, although the left is worse.when at anything more than 1/4 turn its anything from -200mv up to +1.2v
I dont think it is the control itself because i took it out and checked it with the meter and both sides seem pretty consistant when i dial it up and down.
so its onwards from this point realy
so this is the current cenario
centre stable with or without the pre out links in even turning up volume to full-no movement on voltage
the minuite you play music it goes wild when you turn it up, on both channels.
not much time left today, back to work tomorrow so got to get ready for that👎
centre stable with or without the pre out links in even turning up volume to full-no movement on voltage
the minuite you play music it goes wild when you turn it up, on both channels.
not much time left today, back to work tomorrow so got to get ready for that👎
Random thoughts while I think...
The chassis to ground voltages you had and that were different between two different amps. If the chassis does not connect to ground then any slight leakage differences would cause massive differences in apparent readings but would be of no consequence. Its like saying the leakage in one amp is like the resistance of a foot of damp string and in the other a couple of yards of damp string.
The latest issue... I would use the scope and ground the probe to the reservoir cap ground and then look at all the rails as you turn the volume up.
You're not using a bulb are you when testing with speakers and music? That will cause the rails to drop dramatically and the amp will do weird things.
The chassis to ground voltages you had and that were different between two different amps. If the chassis does not connect to ground then any slight leakage differences would cause massive differences in apparent readings but would be of no consequence. Its like saying the leakage in one amp is like the resistance of a foot of damp string and in the other a couple of yards of damp string.
The latest issue... I would use the scope and ground the probe to the reservoir cap ground and then look at all the rails as you turn the volume up.
You're not using a bulb are you when testing with speakers and music? That will cause the rails to drop dramatically and the amp will do weird things.
No I never do when putting a load on it. This amp must have had multiple this wrong with it originally. I'll know it better than my own wife by the time I've finished lol
The base of Q501 needs to be around 600mv negative with respect to the emitter for it to turn on (negative because its PNP). So it looks reasonable if you +54mv on the base and +619mv on the emitter.
If the emitter of Q505 is at +2.6v as you show then it all sounds good DC wise. Although you might expect zero volts at that point the circuit lacks DC precision and so to prevent C517 ever being reverse biased the design will be set to make what should be the midpoint of that stage remain positively biased.
If the emitter of Q505 is at +2.6v as you show then it all sounds good DC wise. Although you might expect zero volts at that point the circuit lacks DC precision and so to prevent C517 ever being reverse biased the design will be set to make what should be the midpoint of that stage remain positively biased.