Debugging My Circuit Board

Gesi, perhaps you misunderstood the manual. There is nothing to measure at the transformer coils with a normal multimeter. Only their housings which are connected to ground may be used to connect the negative (black) wire of your multimeter as a reference.

When doing so, you may in order to check the power supply systematically, connect the red lead to:
a) the anode (side without white stripe) of D2 and find the voltage of the external power supply, should be greater then +9V
b) the cathode of D2 (side with white stripe) and find the same voltage lowered by ca. 0.6V
c) the anode of D3 an find ca. +0.6V
d) the output (leftmost) pin of REG1 and find ca. +5.7V
e) pin 8 of IC2 and find the same voltage as in d)
f) pin 6 of IC3 and find the same voltage as in d)

If you have all this, the external and internal power supply should be OK and the post-mixer circuitry is alimented.

To check the working point of the audio output stage make sure that you have about the half of the voltage found in d) at pins 5, 6 and 7 of IC2.

Then check the voltage at pin 1 of IC2 and adjust VR2 until you obtain 4.3V.

If you get different voltages, tell me that here, so that we may go on with diagnostics.
If you get correct results until now, we may proceed with tuning.