The LED portion of the Vactrol (U3) on my multimeter on diode test (power off on MP-1) positive lead on pin 1 (anode) and negative lead on pin 2 (cathode) shows 1.6V. From the VTL5C4/2 data sheet: LED Forward Voltage Drop @ 20ma: 2V (1.65V Typ.).
With the positive and negative leads reversed shows 2.7V. From the VTL5C4/2 data sheet: LED Reverse Breakdown Voltage: 3V
R50 measures 995 ohms. (1K from schematic, so 0.5% low and in tolerance)
This is the part where I am definitly confused. When I measure directly across R50 leg to leg I get no voltage drop. When I measure from pin 1 of the Vactrol (anode) to the ground side of R50 I get the 14.7V that is being applied to pin 1 of the Vactrol from the output of U2. When I measure from pin 1 to pin2 on the Vactrol I get the 14.7V. Why would I not get a voltage drop from pin 2 that goes to R50 and the other side of R50 that goes to ground?
So if U2 (TL072CP) is not the problem could it be that the sensor side of the Vactrol is inop? How would I test that?