500 ppm LCR meter LCD noise problem

[nophead]

When cased my LCR meter fails to trim in the open circuit mode unless I disconnect the LCD cable. I have added an earth screw to the case but I notice the earth connection is via a 2R2 resistor R43. What is the design rationale for this? Wouldn't a low resistance connection give better screening?

Do I need to put screening between the LCD board and the main PCB? If so why isn't this included? Does it work without for anybody?

I am thinking perhaps a rectangle of blank PCB material on spacers. What have other people done?

[nophead]

Screening the LCD PCB seems to have done the trick.



I also had to make a few tweaks to the case, see my blog for more details http://hydraraptor.blogspot.com/2016/03/elektor-500ppm-lcr-meter-case-tips.html

Summary
Extra Parts required

5 off M3 x 16mm countersink screws.
5 off M3 plain nuts.
5 off M3 shake-proof washers.
4 off M3 x 6.4mm spacers.
1 off M3 x 5mm spacer.
Metal screening for the LCD PCB.

Tweaks

Drill and countersink the earth screw hole.
Expand the hole for the USB connector to give clearance all the way round.
Chamfer the inside of both LED holes.
Reposition both LEDs slightly.
Shorten the ribbon cable.
Crop the LCD pins.
Fit an earthed metal or foil screen over the LCD PCB.