After closer look found oxidation on protection relay contacts. Cleaning doesn’t help for long run. Relay replacement solved the problem permanently.
Year: 2023
Case 567532. 03/01/2023 . Pioneer QX-9900 receiver . Right channel distorted like thunderstorm noise .
After troubleshooting using ” freeze spray ” for transistors method found 2 transistors 2SA726 in right channel driver board acting bad after cold application . All unit transistors 2SA726 changed to 2SA992 . They have same look but different pinout. Pay attention ! 2SA992 / KSA992 is better in parameters and performance.
Case 456723. 02/26/2023 Marantz 1060 amplifier. No sound on left channel.
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.
Technical review.
Nice small amplifier. Good performance. Nothing special.Power supply section extremely simplified. Classical 
old style engineering design.