PT2399 GENERAL QUESTION(S)

1) I checked the pt2399 introducing article on electrosmash which is complete as a reference, I also read different types of its datasheet. What is the maximum input voltage? (Not power supply) Cause I want to make a circuit dedicated to eurorack system. Input voltages (Not CV) also may increase a bit more than usual.

We talk about the input/output levels here: www.electrosmash.com/pt2399-analysis#link51
"The recommended input signal levels are not specified in the datasheet. Testing it, good input levels are around 0.5 to 1Vrms (1.4 to 2.8 Vpp), with these levels the THD is <0.3% (following the datasheet) which is good to minimize the noise in the circuit."

2) WHY PIN 5 IS ALWAYS NO-CONNECTION? It called CLK. So isn't it something useful for microcontroller-based projects?

Pin 5 is an output, thats why is always unconnected, you read there the internal clock that generates the delay, values between 2 and 20MHz, thats pretty fast.
This is a copy paste from the main article

" Pin 5:
It is the System Clock Output pin (this clock frequency is generated by the VCO explained in the Pin 2 section).

This pin gives feedback of what is really happening inside the PT2399. Using an external resistor on pin 6 to set the delay time could be (sometimes) inaccurate, so the pin 5 outputs the exact amount of delay that the signal will suffer.

There are 2 potential applications for this pin:

• Use the pin 5 clock frequency information in a feedback loop to get the accurate reading of the time delay. To do this, a microcontroller needs to be used and also because of the high-speed of the clock frequency (up to 22MHz) a pre-scaler (that would reduce the accuracy of the readings) needs also to be used. It is not clear that the benefits of reading the internal clock would overcome the extra circuitry and complexity of this mod.
• Override the pin 5 signal using an external clock source. In this way, the user can control the amount of delay in a precise way, but again it is difficult to say that the benefits will pay off extra circuitry and complexity.

"

3) There is a not qualified some say "Chorus" effect by importing a LFO signal to pin 2 (Vref). Does it really work?

I have never tried myself but I am pretty sure that it works, there are plenty of people reporting in the online forums that this hack works.

4) But there's another schematic (which works, so many people are using it) by adding that LFO signal to pin 6 (VCO) which causes sort of modulation. The question is would I need this protection circuit to damp all negative values and more-than-fives to not damaging internal circuitry? Cause as I mentioned before, it is for a eurorack use.

That's not an easy answer, the clamping diodes will keep the voltage under control. I am sure that it could work without them but maybe, in the long run, it can affect the internal circuit. I will keep them.