Online assistance to repair Nakamichi Ta-4a: in protection mode, 18v rail is at 11v

My Nakamachi TA-4A: the problem was that the left channel had less sound that the righ one, so i check it and what i found is that the difference in audio was originating in two parts:

one: in the out of the volume pcb, with the osciloscope i check it and there was a little difference in the ouput of the volume righ vs letf channel at the potentiometer exit, so i dissmantled the potentiometer, the balance part, to clean it and see if that helped, after the cleaning the difference between right and left channel was reduced and the waves on both channels was almost identical on the osciloscope.

Two: after the volume pcb, the signal goes to the Tone control pcb before the amp pcb, when i check it the entrance vs the exit of the signal, the difference in volume between the right and left channel was bigger at the output of the pcb, when i checked it with the osciloscope the signal was different after passing the capacitors that were at the opamp output, so i took the pcb appart and i check the capacitance of that capacitors and one was very low, and was not the same model that the others, and it was on the left channel, so i put new capacitors with the same specifications, i reesambled the device, now the balance between righ and left channel was better, improved a lot, but now the signal out was with less power than the input signal, in consecuence the volume was low even at the max volume, so i check it and the signal droped in intensity after the last two resistance in the circuit before going to the amp, and after passing the contact point of the mute transistors in both channels, :s

so until here i keep searching for the reason of the low volume, and i found now that the voltage from the main board that feeds the tone control pcb was low, from the original 18v + -, to 11v, and i traced the path to the main board, i found the voltage fluctuating in 11v at the exit to the tone control board, and through all the circuit until the conection from the power transformer the voltage was fluctuacting, i checked that section and found one transistor open and two diodes open too, the ones before the open transistor, and the ones that came after the capacitors that are at the exit of the bridge rectifier in the main board, so i changed them hoping that solved the issue, but no, the voltage keep fluctuating, i already check all the transistors, diodes in that area, all seem ok, the last thing i found and what is very weird is the the bridge rectifier in the main board the ac coming from the transformer is 21v but the output dc is 25v, not as the 21v as the schematics says it must be, and i guees that the output voltage in dc must be a little lower that the ac voltage at the input :s, so i don’t know if that overvoltage is making all the circuit fluctuacting so bad and not delivering the 16v at the exit of the powerline, or one of the transistors are bad and that's the one that makes all readings and voltage fluctuating, :s, i tested the diodes in the rectifier bridge and all seems ok, so i don't know, i'm very confuse on what had caused all this and what was the reason, and the final nail in the coffin, now is in protection mode :( , i'm really lost and as i read the post i guess i found the right place to meet the people that can help me :),

so as you can see this is beyond my experience and knowledge, so i please ask for the help and guidance os a seasoned and experience electronic technician or engineer that has worked with nakamichi amps, to find the solution. I hope you can help me please. Thanks.

Jerry