The ghost in South Cav Refl RFPD

• "Water doesn't boil if you watch it". I routed +5V through unused pin 6 on the sub-D 9 connector and used a breakout box to keep monitoring the +5V rail when in operation. The tripping just stopped happening!
• Until this point, I was fairly sure that closing the RF cage lid or the case lid was causing the problem, but not it is no more happening.
• This is an irreproducible problem and I can't delve more time on it. If this happens again, I'll look at it then.
• I did replace a capacitor at the output of the 5V regulator today with a higher rating (25V) tantalum capacitor, but i don't think that was the reason for this issue. I also saw -0.7V in first power cycle but not in the next ones.

PDH Error Signal

We have been experiencing an extraordinarily low PDH error signal on the South FSS. I think I have found the source of the problem here but not the cause.

• I used an external mixer, ZFM-2-S+ and directly fed the downconverted signal into test port Test1. I wanted to check if the onboard JMS-1H is faulty.
• But the error signal directly from the external mixer was the same as the mixer output channel on FSS box.
• But the mixer output channel is supposed to have a gain of 2.61, which is not happening.
• The problem is, I tried taking transfer functions of parts from the test port to the mixer output channel, and everything looks like design spec (Elliptical filter and inverting amplifier). But when I compare the error signal at the start of gain stage and at the input to the FSS box, Test1, I see that somehow after going through elliptical filter, the signal is suffering a loss of 1/3 and hence the later gain stage is just making it same again instead of actually amplifying the signal.
• I'll look into this more tomorrow. I don't understand how the transfer functions are from SR785 look ok but the actual ratio of error signals seen on the oscilloscope is wrong. Do we really need the elliptical filter?