LCD Lights up but theres no text

Hi There,
I’ve just finished assembling my Shruthi. I was following the steps on the mutable website, which are extremely helpful. I turned it on and the LCD lit up but there was no text. I tried adjusting the R21 trimpot but this had no effect. In fact the trimpot I have doesn’t have an end, as it states in the guide. Connected it up to a keyboard and it seems to be generating sound.
Does anyone have any ideas?

Do the LEDs lite up and change when you press a button?

In fact the pot has an end, you will hear (and feel) a click and thats the end. Try the whole range up and down, if nothing happens try resoldering the Trimpots GND-Connection (the one farest from the edge of the pcb…#1 cause of failure).

If you get sound you’re on the good track, it’s likely to be a soldering problem on the contrast trimpot - or the contrast trimpot turned in the wrong direction.

Yes the LED lite up and change when I press the buttons.
I turned the trimpot counterclockwise for about 5 mins and it did not appear to have an end. Should I keep going?
Just to be sure the trimpot is a H502I P1009.
I’ll try and resolder it now

It has a end, not as in “it suddenly stops turning”, more as it “it’ll click a bit and turning will become a tad harder” - that’s how trimmers end.

Also, explore a bit, try turning the trimpot in the other direction :slight_smile:

There are two possible orientations for the trimmer, maybe it wasn’t in the right one while I wrote the instructions.

Just tried resoldered it. I’m afraid it didn’t make any difference. In fact the the trimpot doesn’t effect the brightness of the screen at all.
But would this be the reason why there is no text?

I had exactly the same problem, spent ages trying to de-solder with limited success then realised that the trimpot went in the opposite direction. :slight_smile:

The trimpot doesn’t control the brightness of the backlight LED, it controls the contrast. If the contrast is wrongly set, you see no text at all - if the contrast is wrongly set, but in the other direction, all the pixels on the screen appear filled. You have to find the point in between.

Note that even if the MCU and the logical part of the LCD is kaput (which doesn’t appear to be the case on your board) you should still see the first line of the LCD “filled” with pixels if the contrast is set. If the display is just blank it’s really a problem with the contrast.

Which voltage do you measure at the middle pin of the trimmer by the way?

I’m having the same problem as moog69. In my case, I accidentally built the digital board without C1 first. I tried dilaing in the trimpot with the digital board plugged in, and the display would briefly be readable before going back to blank (no pixels lit). Next, I noticed C1 was missing and soldered it in. Now the display is completely blank, adjusting the trimpot seems to do nothing. My trimpot is installed with the trim screw along the same edge as the MIDI ports. I measure roughly 5V on the middle pin, though my multimeter isn’t very accurate.

I also get the expected button behavior, and the rotary encoder changes the lit LEDs. I assume audio works, but haven’t tested it yet.

The voltage at the middle pin of the trimmer ensuring a good contrast is more in the 1V-2V range.

My LCD works perfectly now. Root cause: poor solder joints on the trimpot pins.

It turns out that my little radio shack multimeter is more accurate than I expected. I tested it on some dead (1v) and charged (1.2v) batteries to confirm. Then I hooked up some test clips, one to the middle pin of the trimpot and another to ground. I found that with power on, my reading was 5v, but power would drop down to the 2v range while turning the screw. Next step was to re-solder the pins, and now all is good.

It is never the component.


^ Never? Nonsense…
Unlikely perhaps, granted. But “Never” just isn’t true at all.

ok, almost never :slight_smile:

/puts on a sweater, shuffles over the carpet to pick up a stick of RAM…