Hi, I have a NAD 3020 and usable volume has gone. You can still hear it really quietly even with the volume up full. Do you think this is a power transistor or a transformer problem. Cheers. Doug

It's a muting problem, not a power amp problem. The muting circuit on this amp has a very high impedance node, which makes it very sensitive. Right next to the area of the board that contains the muting circuit is a large-ish capacitor that is held in place by a blob of glue. The glue has usually spread out enough to reach some of the muting components. The glue decays over time and becomes slightly conductive. This is enough to keep the amplifier in the muted state, permanently, instead of releasing it about two seconds after power-on.
The picture shows the amplifier with the top removed. The box is where the old glue used to be. The glue is to hold the purple tubular capacitor steady. It is not really necessary unless you have the amp sat on top of your speakers (not recommended).
To remove the glue, unplug the amp from the mains, remove the lid (six screws), locate the area in the picture and scratch the glue away with a large needle or tiny screwdriver. The most important bit is to remove the glue from the bare wires at the ends of the resistors (the brown stripy things).
It may also be necessary to scrub dust etc from around that area, and also from the small, thin wires coming from the on-off switch onto the board.
If this doesn't solve the problem, or it returns, the capacitor that determines the length of the muting period may have failed. This is a slightly less diy repair as you will need a soldering iron and a new capacitor.

It's a power transistor problem, what you are hearing is the pre-amp stage amplifaction, not the power amp! However it's uncommon to lose both channels, so you could be looking at something common to both. Unless the power amp is itself is an IC!