I already find the distorted sound in one of the op-amp when I  feed the signal in the amp input - and + IN and also the OUTPUT of the op-amp the other side of the op-amp is working I attach a photo so you can see where it is does.
does it has to do with the resistor or the diode?

thank you very much for the guide it is very helpful on my part.

thanks for the help I have no signal tracer I will make one as you suggested.
I will make an update when I already have signal tracer.
can you teach me how to use the signal tracer beco'z I have no idea how to use it.
does I have to hook up the first wire to the speaker amp board and the other wire for tracing the board correct me if I'm wrong?

I already test the voltage 15v and I also replace all the 3 tl072CP and also the LM13700 chip.

Work done:
1.) I connected to INPUT to LINE OUT result - working without speaker
2.) I connected to LOOP IN to LINE OUT result - working without speaker
3.) I connected to LINE IN to LINE OUT result - working without speaker
4.) I connected to LOOP IN with speaker result - working
5.) I connected to INPUT with speaker result - not working (when you max the volume, comp and the bass knob volume the sound is small and distorted and loud sound occur randomly.

OP AMP
LM3886TF - 1 PCS (connected to the big heat sink)
LM13700M - 1 PCS (16 PIN located at the back of the motherboard)
TL072CP - 3 PCS

I connect different speaker but It's still the same.

I cleaned it but no success what should I do next?

I plug it to the Main In and Pre Out and test it to another amp both are clean.

Roly thank you for the reply
You are right maybe it is a Soundking in China they are very similar.
By the way as you said I measure the two big cap C29 and C32 both are 40 volts, and also C33 and C34 are also 15 volts.

I test to plug my bass into the loop IN input and it is ok but if I try to plug it to the Master Input the problem arises, it is a LM3886TF.

Hello to everyone can somebody help me my bass amp was not working properly when I plug my input jack and start to play the sound is low and distorted and randomly pop a loud sound to the speaker and I can't find a schematic diagram on the internet can you please help me thanks in advance.
here are the pics..