After troubleshooting found missing voltage on collector H702 transistor ( 4-5 volt) in driver section.
Problem solved by replacing R708 resistor 18k.
This time we did ” lazy” replacement . New 2 resistors 10k + 8k installed right on top of bad resistor. For saving time.
Nice small amplifier. Good performance. Nothing special.Power supply section extremely simplified. Classical old style engineering design.
Problem solved by OUTPUT PLUG REPLACEMENT.
Technical design review.
Pluses- simplicity, build simple as possible. Minimum wires used in design . Servo loop channel method ( input signal controlled by output signal) realized in schematic. This method has own advantages and minuses. Some called it “over kill” and unnecessary, some – perfect and high performance support method. Elimination input electrolytic capacitor in signal path is questionable. Debates in process already for over 30 years. Still unique .
Low power unit use for functioning. Good detailed attention to phono input section.
Minuses- no ventilation holes, using no soldering contacts on major transistors ( oxidation )
Power supply filter capacitors installed close to heat emitting heat sink ( heat destroy electrolytic ), not shielded balance pot wires.
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 .