Have you tried to set bias current for the amplifier? I see there 2 trimmers - RT461L and RT461R, one for either channel and the schematics show to 2 resistors R400L and R400R with arrows coming from the trimmers but they don't mention about the target current we should read there.
Also I wonder what the metal box under the trimmers is. It is marked L461 OSC BLOK.
Oh, I just noticed it's under the tape recorder adjustment, it should be 4mV across either of these resistors by adjusting the trimmers herein mentioned. I am not going to use the tape player, just the radio, so I suppose I don't have to care about these settings?
I've not been involved with any of the components you mention, I'm afraid my technical skills don't go that far.
You don't need technical knowledge. Only a flat screwdriver, multimeter and schematics.
There's an instruction with drawings in the service manual.
Trimmers are variable resistors or potentiometers, they have a slit on top to be turned with a flat screwdriver. There are plenty of trimmers on the board, most for adjusting radio tuners, we are interested in the two I mentioned above. You connect probes of your multimeter to both sides of a resistor I mentioned and you turn the corresponding trimmer with screwdriver until the multimeter shows 4mV. You do it with the resistor and trimmer with symbol L (left channel), then the same with trimmer and resistor with letter R (right channel). I understand it's adjusting recording current for the tape head. They advise it as the last stage of the head maintenance. Before that there's playback adjustment with Azimuth screw wherever and whatever that screw is and measuring at DIN OUT for highest current.
I am new to boomboxes, I suppose that most of them are adjusted in a similar way so someone could clarify. Personally I am interested if there is any adjustment process for setting bias and idle currents for amplifier as at is in separates which are less for casual listening than a boombox though.