Power Line Monitor

Our green polarity testing bulbs had grown so dim it was really hard to see them. But there is a solution. It involves replacing the bulbs with new ones and increasing the resistors on the board that drive the bulbs.

The original bulbs wore out in less than two years due to being over-driven, increasing the resistance decreases the current significantly allowing for a much longer bulb life. Thanks to some great work by Dick Mason, we can fix this issue.

The steps are:

  1. Remove front panel from electrical box - 4 screws
  2. Unplug 2 cables from front panel to power supplies
  3. Remove 4 hex nuts and remove board from front panel

The 4 NE-2G green bulbs are 6 x 12 mm in size. There are also 2 plain NE-2 (amber) 6 x 12 mm bulbs for the open ground indication. I didn’t touch the circuit of these 2 bulbs as they are almost never (hopefully!) lit, and should last forever.

I replaced the original series resistors with 220K, but you can use up to 470k, for long life and not too dim a bulb for the 4 green bulbs. Before you change these resistors, you should follow the traces and verify that your board layout is the same.

The attached photo shows these as well as the adjusting pots to calibrate your volt meters. Just be careful of the high voltage, both as for shorting anything while it is hot, and coming into personal contact with any high voltages!


General layout of the board with part locations.
Taking the old line monitor apart is very easy.
The main circuit board before I got my soldering iron out and removed the (4) ‘dead’ lamps and (4) 47k resistors
All the new components have been installed. Yes I used .5 watt resistors where they should have been the smaller .25 watt ones but they fit just fine. I used 220K resistors in place of the original 47K ones, Dick used 470K, ones to get this level of light, choose what you like.
Quick check to make sure there were no mistakes, also a chance to see what the monitor said vs my DVM. DVM said 120, Monitor 122, so I tweaked the pots until the monitor also read 120
All put back together.

So there you have it, hope it works well, I have high hopes!


I did this update 6 years ago and the lights still look like new today (10/2015)