QA101B low frequency issue

Hi all,
I use the QA101B Oscilloscope which has a low frequency observing in the displayed waveform. If someone is also using the Scope I have investigated this issue and provide a solution to it.

In the screenshot below you could see the issue, the same behavior you could also see at a low frequency sine. This is annoying because it is already there at 50/60 Hz.

The Problem is the offset setting on the scope input circuitry the AD8370 INHI input. This offset is generated with an OPAMP with a BJT driver circuit (Q183,Q184) to boost the output current capacity. The offset is then filtered by an 220uF Electrolytic. The problem is that the driver circuit is simply the parallel operation of a PNP and NPN BJT with emitter and base connected and the collectors to + and – voltages. This circuit has a dis-connectivity of about 1.4V. If the regulation loop has no offset set, it conducts the NPN due to small leakage currents in the AD8370 INHI input. If then at the normal operation AD8370 INLO input a voltage is changing its level, then the INHI input is affected slightly. This causes a current demand from the offset regulation loop and the OPAMP is shooting to the rail. It takes a certain amount of input voltage to drive the OPAMP into normal operation and this small regulation behavior is measurable at the AD8370 INHI input and hence is visible in the scope trace.

To solve this problem a simple 510 Ohm resistor drain to GND (soldered on the 100nF at the AD8370 INHI input) is forcing the offset loop into a quasi-class-A and the regulation loop is working flawless.

Shown only for channel2, the capacitor where the 510 Ohm resistor could be soldered: