After troubleshooting found few cracked soldering joints on preamplifier and tone control board.
Problem solved by re-soldering bad joints.
Nice amplifier. Good sound performance.
Technical review .
Pluses – this amp has own character in technical design . Complete power DC source separation for each channel. Independent power transformer for each channel gives you stability. Speakers Protection realized on soft start, self control and sound time delay idea.
Minuses – high concentration of heat emitting components in some areas.Some areas on PCB boards overheating and damaging soldering joints after time .