Reel to reel equalization

Hello everybody!
I just bought a Technics Rs1500 reel to reel recorder. It comes with a Nab equalization. I would like to modify it to have a switchable IEC equalization that is used on some master tapes.
I have been looking at the schematics(I have attached them). It appears to me that the equalization is obtained with the feedback circuit circled in red plus the one circled in green which changes based on tape speed (the transistor Tr127 is connected to the speed switch).
The circuit circled in red has (if I have done my calculations correctly-I got the formulas here:Andy's Active RIAA Phono Equalization Design Page) two time constants:the first around 3180 μs which is the bass lift and the second around 25μs which is not a time constant of the NAB equalization but I suspect it is then corrected by the second circuit (circled in green).
Now my questions are:
1)Is my reasoning correct? (I'm new in the electronic world)
2) If I wanted to implement the Iec equalization for 15IPS (Two time constants:∞ and 35 μs) what should I do? I think I could increment the value of the of the capacitor 162 or resistor 182 until I get a bass roll off under 20 Hz (∞ is not possible, I've read that people place it somewhere under 20 Hz) and then decrease the value of the capacitor 169 or trim pot vr105 or resistor 181 till I get the correct frequency response ( high frequency roll off at 4500Hz) taking into account the circuit circled in green (I would probably need a calibration tape to do this correctly).
Am I at least partially right? Or completely wrong?
Thank you very much


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    pb rs1500.jpg
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I will just playback not record. Anyway I wanted to do a switchable eq. I think it shouldn't be to hard to place somewhere there a relay that can change from one equalization to the other. With this solution I could still record with the Nab equalization and then listen to it.
The last link is very useful. I have actually seen it before. It has two issues in my opinion. First it wasn't tested and the author doesn't seem to be 100% sure. I was not able to found a RS1520 service manual (the Rs1520 was a version of the same deck that came with a switchable equalization)to double check his findings. Second it doesn't change the lower frequency lift off which stays at 50Hz. On other Iec playbacks systems I read that it is usually placed under 20Hz.
I don't know how big this difference could sound but since I will anyway have to open the deck and do some soldering I thought that I should try to go with the most correct solution.
Thank you all for all the help so far! I really appreciate it!:)
Hello guys,
I just acquired one RS1500 and one RS1506 from Japan in good condition.
Realizing that it would be great to modify directly the EQ from NAB to IEC on the board, I would be very interested to know if Benji90 you had found a way to do so!
I also searched for the RS1520 service manual but for now could not put my hand on it.