The control circuit is a mystery to me so I would appreciate any help any one of you could provide, specially a complete circuit of a modern chlorinator.
i've just picked apart my chlorinator and find the identical circuit. Mine has pacific-sel markings on the board; although the controller header is 2x5, not 6 pin (four pins are connected to 0v from the toroidal transformer) and the rectifier on the left of your drawing is a schottky SD241P like the other rectifiers.
If I understand the workings of this chlorinator correctly, the two power resistors are used for current monitoring across the electrolyser (via voltage measurements at pin 3 of the connector on your drawing).
I believe that the controller uses PWM (pin 4 on your drawing) to change the duty cycle on the mosfets, adjusting the voltage bit by bit to get an optimal current across the electrodes. Or rather an optimised V-I ratio as the lower the voltage the less overpotential being created.
this has two purposes I guess - if there is v low current then the device should shut off as there may be a build up of chlorine gas or a disconnect; if there is current but it is too low then the controller will keep hunting for an optimal voltage/current mix; and after a while go into a failure condition because there is insufficient salt, excessive scaling or degraded coating on the electrodes).
the current is then reversed periodically to descale the electrodes. the reversing happens by powering the relay coil attached to pin 5 of the header on your drawing.
I am not sure what pin 6 on the header is for; although measuring the voltage difference between pin 6 and pin 3 would give some useful information about the resistance between the electrodes which could be used, perhaps, for safety and measuring adequate salinity. and I also can't see the point in the multiple schottky rectifiers, mosfets and power resistors. redundancy i suppose - they are all carrying high power and so will generate heat. possibly the chosen components are only just in tolerance so sharing the load between them reduces service calls.
I doubt whether the design (at an industrial, low maintenance level) has changed much in terms of your search for a more modern version. but an idea at a hobbyist level might be to use a motor driver board that is high current and uses full n-mos h-bridges to avoid the heat costs in the p-mos chips. they are not expensive (£10 -
https://www.robotshop.com/uk/cytron-13a-5-30v-single-dc-motor-controller.html). An advantage of these boards is that they also have PWM inputs to allow you to change 'speed' (voltage) and, of course, direction. so easily hooked up to an arduino, pi, esp32. you'd want a way to measure current. easy enough to do with an ACS712 (run one of the outputs from the motor board to the ACS712 and the from the ACS712 to the electrode.
For safety, you'd want to (have to) add a mechanical flow switch or a gas trap in the cell. again, a micro-controller is a good device for testing the state.
you can get ruthenium oxide coated titanium at sensible prices from ali-express. a decent chunk (suitable for a couple of 100m3 pools) seems to go for 250eur shipped. you'd need to pay import duty of course. then the components for the controller can probably be sourced for <£30. 3d print a mount for the electrodes and buy some PN16 fitments to house the electrode, some heavy duty wire for the cables to the electrode and it seems like you're done!
You do need a heavy duty transformer as well of course. but probably your existing system has one of those; as well as loads of heat sink material that you can reuse for other projects.
Just as a disclaimer - I have not tested the board I linked to, nor thought through every aspect of what might not work substituting an electrode for a motor; but it feels logical at first blush. Get proper advice before using!