tone controls on Technics SU-V5

Good day all, plse give some advice. I got a Technics su-v5 integrated amp. The tone controls does not work on one channel, not bass or treble. Can it be only bass or treble faulty pot that makes both not work ? The tone control is in the feedback path. I have not taken the pots out to test them yet out of circuit. I cannot think of much other that can cause it ? Right channel works fine, bass boost work fine on both channels.
Regards
Jan Screenshot 2024-07-17 at 21-33-31 technics_su-v5.pdf.png
 
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My next suspect would be the soldering points of the ribbon cable (?) connecting the board with tone regulators with the main board.
Since neither of the regulator pots in the channel work then it seems likely that the problem is in the common signal path.

Easy way to check would be to feed e.g. 1 kHz into the amplifier and walk the signal and feedback paths either with a scope or DMM (in AC mode). The disappearance spot of the signal should be findable.
 
Madis64, it was indeed an extremely fine crack in a track where the ribbon cable enters. Depending on the orientation of the amp, lying on a side or back, made the fault intermittent, buggering me around,but it is fixed, thanks !
Regards
Jan
 
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