NAD 3020 repairs

The famous George rang.

My NAD's developed a nasty hum, can you have a look?

Yeah, why not...

It's the very excellent NAD 3020.

A particular favourite of mine. These must have been one of the best selling amps throughout it's life...

(There's even a wikipedia article on it here.)

Now, many of these are getting a bit long in the tooth now.... and George's is no exception.

I switch it on, and there's some awful humming which just won't go away. Some of these amps suffer with a hum on switch on, which disappears after a few seconds, after frightening the user. We'll come back to that later.

Remove the screws on either side, slide the lid back slightly and lift it off...

One feature of this amp is the shelf at the back, where all the connections are located. Makes access easy if you're fishing around around the back. It's not without it's issues, however...

Turn the amplifier upside down, and remove the base plate (8 screws, short one's in the middle!) 

This is the underside of the connection "shelf" ... dry and broken joints are normal here, so solder the lot up...

There's some caps showing signs of their age, and have failed. I'm going to change the usual suspects..

... as well as the four main smoothing caps.

You'll need to pull the knobs off, and remove the four countersunk screws securing the front panel, and remove it ... be careful to support it, as we don't want to go to the hassle of removing the power meter's ribbon cable. The power LED will pull out.

Remove the screw securing the brace bar behind the front panel..

... and the other screw securing it on the back 

Two screws attaching the PCB to the brace bar, one through the heatsink.....

and the other towards the rear ...

You can now remove the brace bar to access the whole of the PCB. 

So after a quick cap change (there's 4 2,200 uF 63V caps, two 1,000 uF 6.3V caps (I fit 16V types here , I don't trust those 6.3V elctrolytics!) and two 330uF 63V caps...) 

(The other 330uF 63V is hiding!)

..things are much improved, but there's another fault... I mentioned it earlier. When you initially switch the amp on, there's a very unpleasant hum which decays, leaving the amplifier performing ok.. it's like a "whomp" as it turns on. It's a very common fault...

It's caused by this capacitor (and sometime's its neighbour too). I change them both for 47uF 63v types... whomp gone!

Bias is checked , and is spot on. Just got to button it up, and give it a good testing with "The Lasters" by Fred Deakin.

The guilty parties...