It's a circuit using the minimum number of transistors.
During playback, the power amplifier is driving the loudspeaker, and during record it is an oscillator? driving the erase head.
I would suspect a dirty rec-play switch. If it makes a bad connection, C22 will float and it can oscillate. Try clean the switch, move it back and forth a hundred times (with no tape in).
I would also suspect the electrolytic capacitors given the age, e.g. C23, C24, C26 if there is strange low freq. oscillations.
What is "ZU"?
C23 looks like it provides tiny +ve feedback which bootstraps the input to increase the input impedance.
R42 seems to increase ripple rejection on the PSU rails, so check the rails have no excess ripple from a bad capacitor or PSU. R47 seems to be for the usual DC feedback for the power stage.
T10-C you open a jumper and connect an milliammeter there and adjust R48 for 5mA bias current, and R40 might be to set DC offset 3.7V at T10-E. I think you would do both adjustments back and forth because the trimpots interact with each other.