UPDATE
Have ripped the top off and with no inputs, volume to zero, line straight selected, no load and VR1 - 4 untouched I measured the following:
L Ch - across pins 21(+L) and 20(-L) = 79.5mVDC
R Ch - across pins 19(+R) and 18(-R) = 87.5mVDC,
using a Fluke 73.
Prescribed values are 50mVDC &...