I have never written anywhere that I forbid anyone to do anything.
I wrote that modifying Chinese electronics is a bad idea, which is true for several reasons.
Cheap PCBs do not tolerate re-soldering very well, the copper peels off from the substrate, even if something works well immediately after such an operation, there is no guarantee that it will work after a month or a year.
Is there any prohibition against writing about it?
PC817 is a strange chip that works in a strange way.
First of all, it is an analog circuit, not a digital one.
Secondly, the input is controlled by current, not voltage, and the output forces current, not voltage.
In addition, the frequency response strongly depends on the input current, a small current means a small propagation time, but also a small output current.
The chip can work really fast (several dozen kilohertz), but then it may not drive the next stage with a small current.
However, when driven by high current, it may have problems already at a few kHz.
There must always be some compromise, assuming that the designer knew what he was doing and did not assume that if the PC817 withstands 50 mA, 20 will be fine...
