Probe circuit for measuring higher voltage on oscilloscope?
OK, so to answer my own question.... Despite whatever is shown in the schematics, despite R4 actually being a 2M resistor that's soldered on the board, the input impedance of the DSO138 scope is 1 MegOhms (at all ranges, 1V, 0.1V and 10mV). Weird? Yes, and I don't have an answer to that.

I started out by getting ~18 Megohms worth of resistors and four 47p capacitors in series and made a probe (complete with pen holder and all) and realized it had become a close to 20x probe (compensated with a 150p capacitor at the BNC connector to give a square output with the 1KHz test freq). Attenuation was greater at DC (about 20x) than at 50Hz (about 18x) or 1KHz (about 16x).

Next, I got 9 Meg worth of resistors, accurately measured with the multimeter. Connected seven 100p caps in series, and connected those parallel to the resistors. The reading is almost exactly 10x at DC and the 1KHz test pulse. Hardly any compensation required.

The caps are required at anything other than DC. Even at 50Hz, output became distorted without caps, almost a triangle wave. So, not possible to measure voltage or do anything useful with only a resistor divider, even at 50Hz.

Tips: Ceramic SMD capacitors crack easily. Do not overheat or apply pressure due to board flex (I used a thin 5mmx40mm PCB). Better to connect the string of caps with flexible wires on both ends. A bunch of caps and a few series resistors are required to increase the voltage rating. 1/4 W through hole resistors are rated at 250V, caps are rated at 50V. I used 4 resistors and 4 caps for the 20x, so max 200Vpp. 4 resistors and 7 caps in the 10x, gives me max 350Vpp. Better to use a safety margin and not go to the limit, especially at lower frequencies.

Spice simulation does not match actual results exactly due to capacitance changes in the capacitors as well as due to construction (and the coaxial cable). I estimate DSO138 impedance to be 1Meg and input capacitance &lt;100pF.