NAD 3020e low volume and distortion... fried 68R resistors

[byron99]

Hi all,

I'm new so please go easy on me. My NAD 3020e developed this fault a couple of years ago, that is, very low volume on both channels with distortion and smoke coming out of the unit.

It sounds like the same issue that appeared in a thread a few years ago (which reached no conclusion) although my model has different reference designators - but when you compare the schematics, I can see they are the same family of amplifier.

Here's the thread for your info:

nad 3020 fried resistors

My "hot" 68R resistors are R437 and R438. I've already used a multimeter to test Q405/6, Q407/8 and Q411/2. They all seem fine (some I tested in situ, others I removed to test them).

Oddly, the fuses F501/2 (F503/4 in the attached schematic) were blown. In this condition, there was no more smoke. But when I replaced them, R437/8 started smoking and glowing again.

The only other info I can offer is that one electrolytic looked melted (C517, a 330uF/63V) which I replaced with a 150uF/63V part.

Any help will be really appreciated - I loved this amp and the Cambridge Audio A1 that replaced it just doesn't cut the mustard for me (I can't afford more than that right now!)

Thanks,
Barry

[colwaiting]

Hi, well I ve had a 1020,(pre, now playing happily) and 3020 as well as the 2100 amp around at times, and i can tell you they are a bear to fix.
Here, i would draw your attention to the +43 v rail (sorry you know that) I would be suspicious of all the SD669's and 649's I see here.
Drivers: take 'em out, leave em out for a while. Check for heat stress, test 'em Diconnect +43V altogether. Mebbe you can repower the mains rails (32v) mebbe not
(that'll be your test for the outputs)
Now.... take a good look at the 669/649 pair that runs the +/- preamp rails, because 1-you may want to pre-amp only this unit and 2- ive found that NAD lets these run HOT, rather overheating the traces, and causing wipe-out of the filter caps (over the years) It is quite possible the 669, responsible for the +43 V rail, is fried, letting 50 V thru...
I do not, myself, feel the amp stage is worth saving-- I'm not quite sure about the 2955/3055 outputs, as I remember All-3055 in the early 3020, but I found the amp I had lacking in clarity, vis a vis the others I had around--A NAD 2100 (Lovely, but rather flakey), and the Rod elliot p3a.. (good)
Note: to use my pre (an Ebay item) I had to reflow the solder anywhere near the regulators, and replace all low-voltage Caps, and fix or abandon a panel switch, (tape/mon) and on. That and the rca's were trouble for hookups. Situation AFU, normal

Good luck-- Doug

[eggontoast]

You have a fried driver or output transistor somewhere you must have missed it/them. I would also change that cap to the correct value or at least put another 150uF in parallel with the existing one. I have attached a schematic for the 3020e for you.

[byron99]

thanks for the schematic eggontoast, I've been working from the 3020i one which appears pretty close.

i've found a couple of other threads that appear to have a similar issue as me, which point towards the output transistors and drivers/biasing as you suggest;

here:
NAD AMP.....sound distorted....fuses blown

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

I think in my case the outputs are definintely gone (Q415 and Q417 are s/c across collector/emitter) but the drivers (Q411/Q413) and biasing transistor (Q409) appear fine. I'll go and replace these devices and see what that does


col waiting, wow, so much info in there, thank you. I got a bit lost in the middle, but it felt like generic advice to the 3020 rather than specific to my issue: I will try to internalise it!

[eggontoast]

I have repaired quite a few NAD 3020's and most of the time its just the O/P transistors, they don't usually take out the drivers etc unless someone has kept replacing the fuses ;-)

Don't forget to replace those 68R resistors as well, 1/2 watts are fine.

[byron99]

Success!

A little more than £5's worth of components (including base and emitter resistors as suggested in the other threads) from Farnell and it's working again

It looks like I damaged one of the drivers (Q413) during debug, so I replaced it (a 2SB669A) with a BD139 which could handle the same Ic and power; it appears fine. On setting up the idling current I found the left channel (the one I'd been working on) would jump randomly. I traced this to dry joints on Q409 (which also sits on the large heatsink), probably caused by me when I was replacing the output transistors. I replaced the 68Rs with 1/4 parts, like the originals.

The only other thing is I noticed that Q508 gets hotter than Q507 (they both have small heatsinks bolted onto them). I wonder if this is related to C517 which I replaced with a 150uF/63V part (it was 330uF but I didn't have any of those). I shall replace that asap.

Many thanks for your help, this has been an enjoyable learning process!

[colwaiting]

The schematic (3020e) is a good bit different around q 505/506, so we had a case of the blind leading the blind. I still recommend making sure the preamp supply is happy, with reflowed solder and replacement electrolytics ( not the whole amp, just the major ones pertaining to the preamp +/-, (possibly also that +/- 40v supply, which is designed in an identical manner)