LCD module display on a Voltech PM300

[jpk42]

Hi there (first post on EEVBlog forums, hooray!),

Attempting to repair a broken LCD 240x64 display on a Voltech PM300 three phase power analyser/meter I purchased from ebay. The existing LCD got smashed in transit and unfortunately that was a long time ago, so its too late to get a refund or send it back.

The original module with controller and LED drivers plus the display itself is a Varitronix (http://media.digikey.com/pdf/Data%20Sheets/Varitronix%20PDFs/MGLS-24064-C-HV-G-LED3G.pdf). I purchased a replacement from digikey (http://www.newhavendisplay.com/specs/NHD-24064WG-AYYH-VZ.pdf), however when I plug it in it won't work. The backlight will come on but instead of the main interface coming up with the appropriate pixels being on, all the pixels turn on and the screen goes fully black as a result.

I've tried most solutions to fix it, including swapping over the original controller IC (a T6963) and the SRAM module, to no avail. Has anyone experienced a similar problem and would know the solution?

Thanks.

[Andy Watson]

Is there an adjustment potentiometer for the LCD contrast (should connect to pin 4)?

[jpk42]

Is there an adjustment potentiometer for the LCD contrast (should connect to pin 4)?

I don't have it in front of me right now but I don't remember seeing one. Here's the board itself: http://www.newhavendisplay.com/images/large/nhd_24064wg_a_yyh_vz-front_LRG.jpg

Adjusting the contrast through the Voltech does nothing (confirmed this works with a good unit).

[Andy Watson]

Adjusting the contrast through the Voltech does nothing (confirmed this works with a good unit).

That should have covered the problem.

What about pin 9 on the module - the Varitronix unit says this is not connected but the Newhaven unit has -10V output on this pin. Is this pin connected on at the power meter end?

[jpk42]

Adjusting the contrast through the Voltech does nothing (confirmed this works with a good unit).

That should have covered the problem.

What about pin 9 on the module - the Varitronix unit says this is not connected but the Newhaven unit has -10V output on this pin. Is this pin connected on at the power meter end?

It is connected to the power meter PCB because this is via a ribbon cable, I haven't traced where it goes yet though. How would that affect operation of the module? I have probed the pinout on the module end and there is definitely -10V on that pin, generated by the onboard PSU (+5V to -10V).

[Andy Watson]

How would that affect operation of the module?

I was going to suggest that pin 9 might be shorted to ground by the power meter, however, if you've already found -10V on the pin, that puts the kibosh on the theory.

[bktemp]

-10V could be a bit on the high side:
According to the Varitronix datasheet the old display required -9.6V, that's why you measure -10V on pin 4.
The new display wants only -7.5V. If it gets -10V, the contrast setting may be too high, therefore all pixels get black.

[Andy Watson]

The new display wants only -7.5V. If it gets -10V, the contrast setting may be too high, therefore all pixels get black.

The -10V is an output from the Newhaven device - the Varitronix does not have it.
But yes - there is a difference in the expected contrast voltage (pin 4?) which could account for a blank display.