nad 3020e - distortion both channels

johnlon said:
ok just starting to look at it.

other symptom is that the sound volume is capped at a low level .

one volume knob.is eg 90 degrees it gets no louder at all. no effect at all.

like there's no juice to squeeze on the power
Click to expand...
Stick a 1khz tone in on the aux and with the scope, take it to clipping in to a dummy load. If the top of the sine wave is flapping, it's a possible sign that you smoothing caps are loosing their mojo. Pull it back to just before clipping and measure the RMS voltage. Use V²/R (where R is the dummy load) to find what max wattage you are actually achieving.
Don't forget there are smoothing caps on the pre as well as the big ones.
 
DatunWalnut said:
Just a thought: Another point of commonality is the mute fets on the output of the preamp (Q309 & 310). Try scoping the Source of both of them to a 1Khz square wave and compare it with the Drain. if the signal diode or the 22uF/100v cap were going faulty, it might cause noise to be injected at that point.
Click to expand...
hi I have a scope. it does have a basic signal out used for setting the probes and I could I guess use that as a signal gen.

q309/310 are default conducting and I measured them at 20ohm which is a little below spec of 30.
So I assume you mean calibrate the probe then probe the JFETS and see if there's a significant difference in the square wave?
what would bad look like?
 
so hopefully new fuses have arrived when I get home.

I have been through almost every transistor and diode in this thing and find nothing sick.

I did find the 10nf cap adjacent to the fuses was not secure and fixed that.

it seems suspicious that the fuses blew and the cap next door wasn't soldered properly but this does just seem a coincidence to me.

please correct me if that isn't a coincidence.

I see no short to ground on the output of the discrete bridge rectifier that the blown fuses lead to. those diodes are healthy, but they are also beefy.


what I did notice was my son had the amp on it's 4ohm setting but was running on 8ohm speakers.
but my understanding is that that isn't a risk, whereas running at 8 ohm and using 4ohm speakers might be a problem?
is that correct?

i've also heard that this amp deals with that kind of thing rather robustly but again please comment?

-++

is it worth taking the chance powering it up?

i don't have the 60w incandescent lightbulb safety rig yet and am away for a few days so would like to try it tonight to put myself out of my misery.

advice appreciated. I should probably wait right??
 
Corrected NAD 3020e service manual.
I've fixed a bunch of pages that are cropped badly including the parts list.
Also corrected, imho, the schematic legend of the transistor pinout.


Elektrotanya for electronics experts

Electronics forum for electronics experts to repair defective appliance.
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ok having gone through every semiconductor and all good.
and checked shorts to ground from power rails.
and soldering down the little 10nf cap by the blown fuses to the power section.

I replaced the fuses and it's back to life.

I checked the output voltage and they are at 32mv which I understand is ever so.slightly high.

but

there is the idle.current thing to check at the two.pin headers, but do I need to remove
the 1ohm resistors that sit across those pins.

anyway ...

reading the voltage across those shunts seems adequate to measure current. The two resistors actually read as 0.3 ohm. With the power on I read 0.02mv across those pins. 0.02mv/0.3ohm=66uA.
But 0.02mv is 1000 times less than the 26mv mentioned in the manual.

but should I change anything in that respect? to.up that voltage/current.

this guy experiences the same very low voltage and no adjustments changed it...


Hmm ok apparently I'm supposed to remove a solder joint from the other side. I guess that's for tomorrow.

what would a low idle current indicate. I'm terms of.health or sound quality? distortion at the cross over??

---

btw q508 hits 60c - is there any point moving it to the heatsink?
 
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So Team Karma....
there are two power supplies in this thing right?

there is a regulated one that seems to drive preamps etc

and there is a "unregulated"/"loosely regulated" supply form the output stage . is that right?

what do.you call the second one?

and it was the two fuses going to that output PSU that had both blown (and the adjacent 10nf had never been soldered properly and had a leg lifted).

given that I've checked every semi and the big power caps look ok and there is no obvious rail shorting and replacing the fuses seems to have gotten it working..... why would both fuses.pop? i can imagine one popping due to age or so but why.both? is it possible that one popping causes a kick that hurts the other? eg a transient in the transformer or something?

not sure what to think

+++++

also what are your opinions on doing any further kind of renewal work. I don't like the idea of changing stuff for the sake of it. but I know some folk differ.

I understand there might be minimal safety related upgrade to prevent a future catastrophic smoke emission and then there are other optionals. some folk do a full recap and resistor upgrades.

but what's the opinion here on all that? ie if it sounds ok to me (nothing to compare with) and isn't doing anything suspicious.

++++++

i.will try the idle current thing tonight and also the square wave injection and scoping.
 
Btw it seems clearly we still get an audio signal from the amp even when the main power to the output is dead. this is a little surprising.
and the sound is low level
and distorted
and doesn't respond to the volume control very much

= symptoms of a total power loss on the output side

obviously I should have taken the top off before asking a question here but I've never messed inside an amp at all before (just digital stuff) and didn't know what to expect. so sorry about that beginner error
 
There are a number of supplies in this amp. I count 5, plus some circuitry around the main+/- supply. These PE amps are a bit above my level of understanding, so hopefully one of the experts can chime in here.
 
Ok so I think it's done it again.

Both fuses to the power amp section have popped.

It seems a symptom of the failed power section fuses is that you still get sound somehow but massive distortion and no amplification.
Here is a sample...
New video · Monday, Mar 23

Need help diagnosing.

Last time it popped them I checked every semiconductor in it and they all tested ok as far as "diode checks" are concerned, and the JFETS were 20-30 ohm as expected. Could it be a short somewhere or a cap that intermittently shorts or a resistor that intermittently shorts?

I'm also checked bottom for dodgy traces, though at that point i.was.looking.for breaks.

I have just visually inspected the caps and see nothing obvious.

I assume if.i replace the fuses it will.work.again for a while.

Is it possibly due to in rush current because of me turning it on/off with the volume knob not down??

Nothing looks.cooked.

clueless


Any clues appreciated.
(lost my spare fuses , grrr)
 
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That supply (the PE) is an odd bird . . . Looks like a "base" voltage (with the fuses that fail) and another, higher voltage winding (on two other fuses) that also drive the rail (just not sure the purpose . . . since no external signal to modulate it) which is likely where the inimal drive to the outputs comes from when the fuses die.
 
i have the E not PE

could it be a bias problem?
or the 2 mains caps

the little fuse.board has two unoccupied spaces for the PE components.
?
 
OK . . In post #17 you said:


NAD3020E
NAD3020PE
5553125800 "w-08"
TATUNG TT-294-v0

So was not sure . . . (but that certainly makes it simpler, and a *LOT* harder to see how you get output with open fuses!)

You said the fuse board has the open spaces for PE components . . . what about the main supply board itself?
 
yea sorry.
that's what the PCB says.
sorry

it's weird isn't it.
dis you watch the video?

I have no doubt if I switch out the fuses that it will work fine for a few weeks

might trace the PCB when I get home and see if it makes any sense
 
johnlon said:
yea sorry.
that's what the PCB says.
sorry

it's weird isn't it.
dis you watch the video?

I have no doubt if I switch out the fuses that it will work fine for a few weeks

might trace the PCB when I get home and see if it makes any sense
Click to expand...
The only vid I saw from you was an image of a speaker . . . Nothing interestiing like the amp internals . . . (and the other guy wasn't showing yours, so again, marginal value.)
 
Btw here is a copy of the service manual for the nad 3020e with corrections I've made (fixed damaged pages and corrected the schematic)..

NAD-3020E-3220PE-sm-corrected.pdf

drive.google.com drive.google.com

looking at the schematic there does appear to be a relatively high resistance part through the power amp without passing through the semis.
r404,r432 looks about 5k'ish ..Is that enough to explain the very weak sound? even though no power?
 
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