Based on the observation of the PMC error signal, I started measuring the IMC OLTF. Immediately, it was found that the overall IMC loop gain was too low.
The UGF was ~40kHz, which was really marginal. It had been >100kHz when I have adjusted it about a year ago. (Next entry for the detail)
The first obvious thing was that the SMA cables around the IMC servo have visible degradation (Attatched photos).
I jiggled the signal cable from the demodulator Q_out to the MC servo. The openloop gain seemed fluctuating (increased) based on the cabling.
I decided to repair these cables by adding solder on the shield.
Even after the repair, the open loop TF didn't show any improvement. I checked the LO level and found that it was -16.7dBm.
I traced the problem down to the frequency generator unit (T1000461). The front panel of the unit indicates the output power for the 29.5MHz output is 13dBm,
while measurement showed it was 6~8dBm (fluctuating). The T1000461 document describes that there is only a wenzel oscillator inside. Does this mean the oscillatorwas degraded??? We need to open the box.
I was not sure what was the LO level. I naively assumed the input is 0dBm. Reducing the attenuation of the dial on the AM Stabilizer unit from 12dB attn to 0dB.
This made the LO level -3.3dBm.
Later at home, I thought this nominal LO level of 0dBm could have been wrong.
The demodulator circuit (D990511) has the amplifier ERA-5 (G=~20dB) at the input. Between the input and the ERA-5 there is a pattern for an attenuator.
Assuming we have no attenuator, the ERA-5 has to spit out 20dBm. That is too much for this chip. I need to pull out the box to see how much is the nominal LO for this box using an active probe.
This decrease/increase of the LO level affects the WFS demod too. According to D980233-B, the input stage has the comparator chip AD96687, which can handle differential voltage of 5.5V.
Therefore the effect is minimal.