After opening the housing it was obvious that someone has already played with transistors in amplifier stage.
For test purpose I used two power supplies with current limitation to supply the power stage. I connected them to points marked with yellow boxes on the circuit diagram below. After turning power supplies on, I observed that without input signal both power transistor (Q13, Q16 - blue ovals) are turned on completely. My power supplies entered constant current mode. I removed both power transistors (Q13, Q16 - blue ovals) and turned supply again, this time I observed that R87 (green box) overheated. I measured voltages across R84 and R79 (blue boxes) and realized, that Q12 and Q15 (green ovals) are fully turned on, while they shouldn't. While inspecting pairs of transistors Q5-Q6 and Q7-Q8 I noticed that in each pair there are different types of transistor, while they should be actually the same. I swapped Q6 and Q8 as they should be according to the circuit diagram and after that the amplifier (still without power transistors) draw approx. 20mA. I soldered power transistors back and tested the amplifier. It works well.
The problem was, someone was trying to fix this amplifier or to test transistors but mixed them up while putting them back in the circuit. This caused a shoot through current, the power transistors survived that but the fuse tripped.
On the very beginning, when I observed that there is high current flowing from both positive and negative supply rail, I thought it is something with phase inverter, and not with upper or lower power stage. I did once mixed up transistor while fixing a power stage and after that I can say, I learned my lesson and this time was able to find the problem quite fast.
