Today, I tried to replace the Reflection Photodiode on South PMC with another 14.75 MHz Resonant (which from test port analysis has TI of 57623 Ohm at 14.75 MHz). Since this another photodiode had been sitting in box for months, I expect it to behave better if the power supply tripping incident caused any harm to the existing photodiode. To my surprise, the error signal turned out to be nearly same. With 6.765 mW incident power, PDH error signal is 900 mVp-p. So I am not sure what is causing this limitation on the output signal strength. From the comparison of sideband strength to carrier strength, we are definitely near 0.88 rad modulation depth. I'll reduce this to 0.3 to avoid power in higher harmonics. But the main concern is that there is some limiting factor because of which the error signal is not going high enough.
The reason I'm worried about this is that PMC lock looks robust enough that it remain locked all night, but whenever I try to scan laser pzt for FSS analysis, the PMCs get unlocked by just connecting a switched off function generator to the laser pzt.