• We recently switched our forum platform. If you experience any issues please email support@crystalfontz.com

CFA-634-TFB-KU Contrast problem

muppetgeoff

New member
Hi,

I have a CFA-634 (the TFB-KU). I had to pull it from a box, where it was used to display IP addresses and such.

I have hooked it up to a WIndows box, and run the WinTest app.

I can adjust the backlight brightness, but cannot adjust the contrast.

The problem is - the contrast seems to be set to 0 or something - because I can *only just* make out the text on the screen - if I angle myself in relation to the display. Making changes to the contrast setting makes no difference - in fact it makes no adjustment at all (as my angled-view doesn't change at all).

I have this unit connected via USB. I have not changed anything (jumpers etc) but it is a good few years old, and so I wouldn't rule out a dry joing.

Any suggestions? I have tried the WinTest, CC2 and LCD4Linux, and none of them will set/change the contrast - but all will adjust the backlight OK.

Many thanks,

Geoff
Looking for additional LCD resources? Check out our LCD blog for the latest developments in LCD technology.
 

CF Tech

Administrator
The only reason I can think that a CFA-634 USB would not have correct contrast (assuming you can send it the correct commands with WinTest) would be if one of the components that set the contrast was broken. The odd thing is that those components are kind of protected by the USB board.

Since you can barely see the text on the display, can you verify that you can change that text (and therefore verify that communications is active)?

Well, I guess you know that because the backlight changes.

It would have to be that the display somehow got damaged. :(
 

muppetgeoff

New member
Thanks for the response,

Yes, I can change the text - both using Wintest (running any of the automated tests) and using CC2 (defining a screen etc). I can just about make out the text, looking at an angle of about 10 degrees, and slightly from the side lol. The brightness also adjusts. So it appears the control logic is working?

There is no visible physical damage - as I said, this display had been inside a well used box, untouched since it was built. I pulled it out so as to test it using the windows apps (the box it came from runs Linux using lcd4linux and I suspected that was causing the problem - but alas no).

Any ideas? Maybe the contrast control has a dry joint or bad solder (or something) if I knew what to check...

Many thanks for your help!

Geoff
 

CF Tech

Administrator
I think the components involved are R8 and C7 (here is a post on an older revision of the CFA-634 contrast circuit).

R8 and C7 have moved in the current rev, but still have the same function.

There is a trace from the processor also.

If the LCD module voltage was low (use a volt meter to measure from +5v(LCD) to GROUND on J2) that could cause similar problems, but I cannot imagine it making the screen invisible.

I guess if you have a volt meter, measure the voltage from ground to the bottom of R7. That should change with different contrast settings.
 

muppetgeoff

New member
Hi,

I guess I should have mentioned I have the v2.3 board, and can see R8 and R7.

The joints look OK - I'm wondering if there is a way to reset the stored contrast level? I'll grab a multi meter and measure the voltage across R7 - see if it changes.

I'll post back with my results, and if you have any other suggestions in the mean time, they would be gratefully received :)

Many thanks,

Geoff
 

CF Tech

Administrator
It is the voltage of one end of R7 to ground that matters. One end is filtered by the capacitor, the other is raw PWM from the processor.
 

muppetgeoff

New member
Am I correct in assuming I should see the voltage change as I adjust the settings in CC2? or a constant reading (5v?)?

Many thanks,

Geoff
 

muppetgeoff

New member
Well this is strange :)

I got hold of a Multi Meter, and measure the voltage of R7 (bottom contact) and ground - and using WinTest to adjust the contrast - the voltage DOES change! So its getting the signal etc.

The range I see on the reading is between 0 and 9.9 - it changes smoothly as I adjust the control slider. But still nothing on the display itself.

Any thoughts?

Many thanks,

Geoff
 
USB LCD Displays - Graphic and Character LCDs with a Keypad

CF Tech

Administrator
Hmmm. No. I guess you could send it in and we could see what we could do. You can write support@crystalfontz.com and refer to this thread.

Since the contrast circuit is working, It is probably not good news. The circuit past that is under the epoxy and not replaceable.
 
Top