M 49 schematic explanation

[mudevilo]

Hello everyone.
I'm making myself an M49, I have a question.
I more or less understand this diagram, but I do not understand the usefulness of R12 (2Meg) and C8 which seems to me to be a filter capacitor, but why through a 200 Ohm resistor (R4)?
Thanking you in advance.

 

[Khron]

I had totally forgotten about it, but as it turns out, last year i had similar questions Promptly answered by the wiser-than-i members of this forum

https://groupdiy.com/threads/m49c-circuit-details-questions.80237/

 

[mudevilo]

I had totally forgotten about it, but as it turns out, last year i had similar questions Promptly answered by the wiser-than-i members of this forum

Thank you for your answer Khron.
This gives me a start, but it does not explain their usefulness to me... But apparently, R12 and C8 would be useless if we do not use calibration... anyway I would like to understand how they work!