Thank you all, really many answer, i did not expected to so many answers.
Yes i am beginner. I only build some amplifiers from 0 to functional in my youth but i have no experience in trouble shooting and repairing stuff. It was easier to build as new with schematic in font than to search defective components.
So..for now i have an ( quite good) digital multimetre, and an cheap chinese ESR metre.
I will try to diagnose the problem only with them, but i am afraid that i have to desolder the component to test from pcb. From what i read i see that excepting the capacitors no measurement is trusty on the pcb or ..in circuit. I am afraid to damage something at de soldering at soldering again.I have no good soldering gun ( but i can buy one)
For that reason i readed that using an oscilloscope will be easier to isolate the part of the circuit with problem..
Even so , even a cheap oscilloscope as the mentioned (DANIU ADS5012H) will provide some benefits in diagnosing my damaged amplifiers over a simple multimetre?
If no, i will forget it and try to repair only with classical methods.
Thank you again for all.
PS:the story is:
my main and most expensive amplifier ( a sansui au 9900) i bought about 2 weeks ago with 1000 euro
:
-at the beginning it has almost no bass and had strong hiss on right channel
-I tested with input from other source as pre amp and found that it sounded excellent in separated position with external preamp ( so i supposed that the internal sansui pre amp was quite defective)
-after that i deoxided it very good. After that he recovered and had a very strong bass, but hiss still present strong hiss in right channel.( the hiss it is audible in all the room, and decrease as the amplifier is warming up)
-after 3-4 days of intensive use the bass and sound stage became more and more good. It seems that capacitors suffered a sort of burning as much as i use the amplifier. So excepting the strong hiss all was great with it and he have a beautiful sound.
but suddenly 2 days ago in a morning it started to pop and crack very very loud from ( i suppose) both channels. So loud as my children started to cry..and i thought i damaged my speakers..
I never used from that moment.
(Similar symptoms i have with another sansui au 505 but the pops are not so loud. The pops are much much softer as volume.)
Thank you