$20 to the first person to give me the tip that gets my NAD 3020 working

[thefunkybeat]

Here's the deal...

I'm a relatively novice repair tech who knows how to avoid getting electrocuted and can fix most basic problems with audio gear. I've got a NAD 3020 on my repair bench that I'm fixing for a friend that I could use some help figuring out. I'll give $20 to the first person who gives me the tip that leads to this thing getting fixed. (for realz!) I know it's not much, but I'm not making much on it either, so that's the deal.

Here's the service manual and schematic.

Here's the clues:

• I checked it over for loose components and cracked solder joints. There were a few loose connections, but otherwise (I think) everything's connected properly now.
• I just put 4 brand new power transistors in it (Q615,Q616,Q617,Q618)
• FUSE 3 and FUSE 4 were blown and I replaced them with new 3A fuses
• It turns on
• It passes distorted garbled audio
• Turning the volume control up results in more distortion, but not more volume
• after a few seconds, R643 and R644 start smoking and charring

If you need more clues, just ask and I'll do further testing or take pictures or measurements if you need me to. I just want this thing fixed and OFF my bench! I really will send the money.

Thanks guys!

[Nrik]

Replace Q607 and Q608, and ofcourse R643 + R644

[Geek]

Hi,

DC voltages usually tell a lot.

In order of probable cause...

1) Check Q609, Q610, D605 and D606 for shorts.
2) Check Q613 and Q614 for shorts.
3) Check Q607 and Q608 for shorts.
4) Check for bad solder joints.

Bah Nrik, you beat my post by SECONDS!

Cheers!

[h_a]

Well, in these bjt-output stages usually also the drivers get fried when the outputs go. This fits also well with R643 getting hot as there is excessive current coming from the rail.

I would check everything south of Q605, means Q607, Q611, Q615.

What worries me is where all this current goes to without blowing up more resistors. Does Q613 get hot? Likely the bias generator Q609 is also not properly setup, so check that as well (R641).

Have fun and good luck, Hannes

[chris661]

I suppose you could make a 1/4" jack input and have it as a guitar fx box...

Anyway.

It sounds like there's not enough power to the main chip thing - the usual reason for distortion is the PSU is being pulled down, meaning there must be somewhere else all that current is going to, but it's not meant to. I'd look for shorts from the PSU capacitors onwards - clearly, there is a leak somewhere.

[stratus46]

Originally Posted by thefunkybeat

Here's the deal...

I'm a relatively novice repair tech who knows how to avoid getting electrocuted and can fix most basic problems with audio gear. I've got a NAD 3020 on my repair bench that I'm fixing for a friend that I could use some help figuring out. I'll give $20 to the first person who gives me the tip that leads to this thing getting fixed. (for realz!) I know it's not much, but I'm not making much on it either, so that's the deal.

Here's the service manual and schematic.

Here's the clues:

• I checked it over for loose components and cracked solder joints. There were a few loose connections, but otherwise (I think) everything's connected properly now.
• I just put 4 brand new power transistors in it (Q615,Q616,Q617,Q618)
• FUSE 3 and FUSE 4 were blown and I replaced them with new 3A fuses
• It turns on
• It passes distorted garbled audio
• Turning the volume control up results in more distortion, but not more volume
• after a few seconds, R643 and R644 start smoking and charring

If you need more clues, just ask and I'll do further testing or take pictures or measurements if you need me to. I just want this thing fixed and OFF my bench! I really will send the money.

Thanks guys!

What are the power supply voltages. I know it says +28/-28 . Is that what they are? Without a load on it, what is the DC output voltage? Were the power transistors you took out actually bad? What is the +29.9 voltage really? Did you check Q611, Q612, Q613 and Q614?

G²

[rem280]

Check the dc offset there is an adjustment procedure in the manual you posted. If it's way out it could explain the distortion if the output is always being clipped in one direction and the resistors getting hot because of constant current flow

[Mooly]

Without having the unit to actually work on it's quicker in the long run to do several tests first.

1. First fit a 60 or 100 watt bulb in series with the mains to limit damage while testing.

2. Concentrate on one channel, so remove the outputs from the other together with the drivers... in other words, get that channel so it doesn't smoke and you can concentrate on just one.

3. On the channel you are working on... make sure everything is connected... no load... and switch on. I'll assume you are working on the "top" channel.

4. If R643 smokes and the bulb lights brightly remove Q605 and test again. The bulb should be dim and no smoke. The voltage on the outputs (emmiters) will be -28 (rail)

That's a first step...
Gut feeling with this... if you replace those burnt resistors and check the 6 transistors in the output side of the amp you will find the fault. You can add a wire link from base of Q611 to base Q613 to short out the vbe bias generator to help fault finding too. That will remove bias from the output pair.
Are you certain all the transistors are OK and fitted correctly.

[Duo]

Shorts in Q609/610 and D605/606 will cause a class C output stage bias condition, but won't smoke anything. This just increases distortion and you can measure the voltage between the bases of Q611/612 and Q613/614 to see if it's shorted. If not shorted, you'll get a voltage around 2.3V. You need not worry about this until you have the smoke problem sorted.

If R643/644 are burning, something is pulling too much current on the negative side of the amplifier's output stage. If Q607/608 were shorted, you'd be unlikely to get much audio at all and the output offset would be astronomical (speaker cones trying to escape their baskets). It seems more like Q613/614 are having problems and the error amp is doing all it can to hold the output near zero and nuking the upper part of the circuit in the process. Try replacing Q613/614; at least test them for shorts. (If you're really fast, measure Vbe on them while the power's on).

Also measure the voltage across R651/652 and tell us what it is; it's supposed to be around a volt (1.1V based on schematic numbers).

Other than the above; are you certain you replaced the output transistors with the correct types and installed them in the correct places?

Please tell us what the output offset voltage is too (volts DC on the speaker terminals).


Cheers

[Duo]

Also check that D605/606 aren't blown and gone open. Check the same for Q609/610.

Check R645/646 as well. If they are blown, the bias transistor will turn off and the output stage will try to cook.