Help me decipher analog OP-AMP circuit from the mid '90

[Houseman]

Hi. It's me again. In previous post I was asking to decode some diode, now it comes with the full circuit I have discovered so far and I need your help.
It comes from a PHASE CUT controller for small solenoid valves that got magic smoke and rather than change the whole assembly that costs 2000 USD I want to find out the fault and repair it by myself.. the valve is powered via a 24VAC 50Hz transformer and controlled via a 0-10V DC circuit that needs to be is isolated from the mains. As You can see in the schematic the control voltage is fed to a first opamp. A differential amplifier. It should output a reference voltage that is related to the difference in the input but I cannot understand why so many biasing resistors. D9 is a zener Diode. Vcc is so at 8V.
The output is then fed (through a RC filter?) to U2 and here comes the troubles..
Any help to find out the top right part of the circuits? The 24AC voltage is full bridge rectified by D11 and D12 and C5. Ok so far.
What are those antiparallel diode? why a small capacitor C1? Even a simulation in LT spice does not help me..
Any help?
Best regards and thanks

 

[moffy]

Classical? it's no Mona Lisa, but it does look confusing. Are you sure the diagram is correct? D5/D6 would make sense if they went from gate to source i.e. back to back zeners protecting the gate but as shown, no idea.

[Houseman]

With term classical it was meant not digital part like nowadays. No mention to arts