Contrast problem Optrec 20171 LCD Unit

[Dialtone]

Hello:
I am having a problem viewing the display on an Optrex 20171 LCD unit. Not sure of the model, but markings on back are "PWB20171-CEM" and date code is "8955M4". I have data sheet for DMC20171 that seems correct but not sure this is the same unit. Has 3 chips on board 1- HD44780A and 2 HD44100H that match data sheet.
Problem is I can only see display if I view it by tilting and look into it from the bottom edge. My contrast control is a 10K center wiper volume control pot tied between the +5V and ground supply to board. I see voltage vary at pin 3 when I rotate pot and display is only visible at max contrast setting. Board does not appear to have either LED or EL backlighting.
Question: do I need to increase the contrast more? I am using a PC power suply for my project, so I have -5V available. Can I wire the contrast control to -5 thru the pot? Would that help?
Any suggestions appreciated.
Dialtone

 

[Exo]

Have you got a datasheet for your LCD?
Some LCD's require a negative voltage for contrast control.
Find a datasheet and check it first.

 

[Dialtone]

I have the data sheet for DMC20171 but not sure my unit is EXACTLY that one (see above post markings). Data sheet for DMC states that Vee (contrast voltate) can range maximun of 13 volts from Vcc (logic voltage). I am assuming I can supply a variable negative 5V to increase contrast setting of display beyond what I am seeing by setting the contrast voltage to ground versus the logic supply voltage. The data sheet I have is not very clear on this matter at all. What do you think, is my logic correct?
Thanks for the reply.
Dialtone

 

[Dialtone]

I got it. The negative voltage to the contrast did the trick. All is well.
Thanks for the reply EXO.
Dialtone