One of my old topics has information about this.
"Posted 20 June 2014 - 09:39 AM
Next thing that I will try, is to cut the pin no. 7 of the TA7658P chip. That's the preamp for the tape deck with integrated ALC (automatic level control).
http://pdf1.alldatas...BA/TA7658P.html
In the datasheet, pin 7 is named as "ALC control input". And I figured it could possibly disable the ALC so we could get the box working better with "Line in"."
"Posted 29 June 2014 - 01:05 AM
Yess! Cutting the pin no. 7 seemed to disable the ALC altogether. Though cranking the bass knob (at over "6" or three o'clock) muffles the treble, but only if treble knob is turned up.
Also one plus side is that it does not need much level on the line-in now. So a lower powered mp3-player should also be enough to get a good volume. These are just based on a few minutes of playing but will test more and update, but for now, seems reeeeaally promising!"
EDIT: Now that I read those, I noticed the thing about treble and bass knobs acting weird. I have to test soldering the pin 7 back on, and see if it has any effect on the tone controls...