Andrew Rossa [Cakewalk]
It's an easter egg that was put in to the module as already mentioned. Does nothing to the sound :)

Odd that the default is off.  Was that to keep in in line with the fifty shades of grey interface ?
 
I find the lit state so much better, seems a shame that most people would never discover it.
 
 

and what page of the 2000 page manual is this in?  How the heck to you guys stumble across this?.. and yes the buss compressor does look better with the blue light....and ...I don't need to ctrl + alt either... just move the mouse after clicking on the screw....watch it move, then click the meter for the light to come on.

I should have listened to John...  I tried to catch it when it fell and ended up smacking it down a furnace vent
 

Can confirm what Scook & dlesaux write.
Some time ago, I had colorized the screw to help make it easier for me to see as shown here:
"Due to recent PhotoBucket TOS changes regarding 3rd party hosting, I have removed this link." 8-7-2017

Is is not based on the hardware  device it was modeled on. That screw could on the hardware device
 calibrate to -20 -18 -12 RMS.
 
?????

What a screwey idea!
 
:-)
 

fitzj
Is is not based on the hardware  device it was modeled on. That screw could on the hardware device
 calibrate to -20 -18 -12 RMS.
 
?????

Digital doesn't need that kind of calibration. Real VU meters are mechanical with both moving parts and electronic components. Each will degrade over time. To compensate a screw is used to adjust the needle's deflection. Another reason I hate them. 

Beepster

ampfixer
 
If you are referring to the R37 screw, it adjusts the high frequency response on an LA2A and most emulations. I find the effect is easier to hear when you sweep that control. Even on the real deal it is a subtle difference.

Yup. Subtle but really worthwhile. It has so few controls anyway it's pretty easy to give the screw a twist just to dial it in for the specific source. It really does depend on the material feeding it.
 
Can't even describe exactly what it does. It's like... the "slightly better sounding" control. Like some weird variable comb filter control that hones what the compressor works harder on.

From the Universal Audio manual:-
 
"The LA-2A was designed for use in broadcast applications. The audio signal in FM broadcasting undergoes pre-emphasis and results in a 17 dB boost at 15 KHz. Due to this increase in signal level, transmitters are subject to over-modulation. The LA-2A provides a control (R37) which controls the amount of high-frequency compression. This potentiometer is factory set for a “flat” side-chain response (clockwise). Increasing the resistance of this potentiometer by turning it counter clockwise will result in compression which is increasingly more sensitive to the higher frequencies."
 
HTH

I thought the CA2A documentation was adequate

R37 (High Frequency Pre-emphasis trim). This control adjusts the gain reduction frequency response. The original hardware unit was designed for leveling in radio and TV broadcast applications. Due to an increase in high frequency content in FM broadcasting, the R37 control adds gain reduction at frequencies above 1kHz, based on the program material. Use this control to reduce or prevent over-modulation caused by the pre-emphasis (high-frequency sensitivity). When set to the FLAT position, gain reduction is applied equally on all frequencies. When the control is moved toward the HF position, gain reduction is increased on the high frequencies. Experiment to achieve a good balance between low and high frequency limiting. This control is often useful for de-essing vocals (cut down sibilance by compressing high frequencies), but the FLAT setting is recommended for most music applications. To adjust the knob in fine increments, hold down the SHIFT key while moving the knob.

 
Still does not explain "What Turning the PC-76 Meter Screw Does"

scook
 
Still does not explain "What Turning the PC-76 Meter Screw Does"