Make sure the inputs for the PD amps are open. This is the current amplifier and we want to leave the input pins open for the test of this circuit.
TP6 is the first stage of the amps (TIA). So this stage has the issue. Usual check if the power is properly supplied / if the pins are properly connected/isolated / If the opamp is alive or not.
For TP8, if TP8 get railed. TP5 and TP7 are going to be railed too. Is that the case, if so, check this whitening stage in the same way as above.
If the problem is only in the TP5 and/or TP7 it is the differential driver issue. Check the final stage as above. Replacing the opamp could help.