Display bright, but no characters [solved]

Hi,
Working on getting my Mission Shruthi working. The filter board works and sounds great, god, what a great and rich collection of filter sounds!
The display on the control board lights up, and is certainly bright enough, but there are no characters on the display. It doesn’t seem to matter what setting the 5K variable is at, R21. When I push buttons and turn the encoder it does step through the pages (LEDs work), so I think the control part seems to be working. This is the second display I’ve tried with the board, there must be something else going on besides display issues. R20 is 68 ohm. The display is Newhaven NHD-0216K1Z-NSW-BBW-L-ND (white). I’m using my other SMR4 II control board to test the Mission filter. No obvious soldering issues that I can see. I’ve been using a Hakko FX888-23BY which is way better than any crap from Radio Shack I’ve used in the past. Can you be in love with a soldering iron? I think so. :slight_smile:
Any ideas?

Check the voltages on the outer pins of the trimmer R21, they should be 0V and +5V. If that’s OK, check the voltage on the middle pin with the trimmer adjusted to the clockwise and counterclockwise extremes (remember, it takes a LOT of turns from one end to the other) - it should vary between 0V and +5V. If it doesn’t, redo the soldering of the pins on the trimmer, if it still doesn’t, replace the trimmer.

Hi Roberth909.
ByteFrenzy is right. I had the same problem 2 days ago, when I built my 1st Shruthi control board. First make sure you turn about 10 turns +/-, something should happen, move the trimmer around (this was my problem: it didn’t have contact) with your finger and keep a view on the LCD. Resolder the trimmer. Make yourself familiar with the trimmer and a multimeter. Try different values of R21…

Trimmer is suspect #1. Low voltages (0.3 to 1.0V) on the middle pin of the trimmer are necessary to get the right contrast.

You can also bridge the 68R resistor.

I got it to work. I had to turn the trimmer all the way clockwise and then start going counter clockwise for 5 or so turns. I’m not sure what I was doing with the first display I tried? All is well.