Checking the rundown sources with a multimeter: One could try and measure the nets denoted as +S64 +S4 -S1 -S16 and -S256. They are R213 pins as well, except -S256. -S256 is accessible at the source of Q206. The mosfets drive the current sources either into the integrator input or into ground. All five voltages should be near zero with -S256 the largest.
With DCV, 10 V, 10 PLC and input short i get: +S64 = 5.6 mV, +S4 = 0.1 mV, -S1 = - 0.1 mV, -S16 = -1.7 mV, -S256 = -24.3 mV.
I also probed integrator output with a scope, see above. I could see runup and rundown with different slopes. Guess it requires some hours to find, record and measure each slope.
Regards, Dieter