The "Tape 1 Monitor" led on my C370 lights up spontanously without me
activating it, and when I try to shut off the led by pressing the "Tape
1 Monitor" button it will only dimm a little, and I cannot get signal
via any other inputs (CD, tuner and so on).

If I turn the amp off and on, the other inputs will work again, but the
"Tape 1 Monitor" led is still on. After a while (hours or days) the led
will go out by itself and the amp functions properly for a day or two;
then the problem will occur again. I sent the amp to the local
NAD-dealer here in Denmark (HI-FI Klubben) but they did not find a
solution as the problem occurs now and then only

wrote:
The "Tape 1 Monitor" led on my C370 lights up spontanously without me
activating it, and when I try to shut off the led by pressing the
"Tape 1 Monitor" button it will only dimm a little, and I cannot get
signal via any other inputs (CD, tuner and so on).

If I turn the amp off and on, the other inputs will work again, but
the "Tape 1 Monitor" led is still on. After a while (hours or days)
the led will go out by itself and the amp functions properly for a
day or two; then the problem will occur again. I sent the amp to the
local NAD-dealer here in Denmark (HI-FI Klubben) but they did not
find a solution as the problem occurs now and then only

Sounds like maybe the "Tape 1 Monitor" tact-switch is leaky or damaged. It
happens, and can cause a problem like the one you described. If you
described the problem exactly this way to the repair dealer - they should
have replaced the switch on general principles. We're talking about a 1.00
part, give or take.

Mark Z.