Does this Display require a 6.8 ohm instead of a 68 Ohm resistor? its totally black [FIXED]

Hi all

built up a polivoks using the BOM from mouser and exchanged the display listed for the one Bleo suggested

it lights up and everything works but teh screen is always totally black, no matter how high I adjust the brightness then I remembered this from the Yellow Magic instructions:

“Note that a 6.8 ohm resistor should be used instead of 68 (R20 in the schematics) to increase the brightness of the display. And don’t forget to solder the 1x6 “expansion connector” on the bottom left of the control board!”

Am I correct in assuming I need a 6.8 ohm also ? :frowning:

I swear Ill get one of these to work first time yet!

oh , the filter board works flawlessly with a YM control board BTW !

It should display something anyway. You should first reconsider checking your solder joints before proceeding to lowering the current limiting resistor to this extent.

the trimmer is for the contrast not the brightness do you see blocks on your screen? … you say it lights up and is totally black!? which is it?

the screen lights up you can see light seeping out of the edges, but the screen is totally black, its a red on black display

i dont see any blocks

Then it’s a problem with the contrast trimmer. What is the voltage on the middle pin of the trimmer? Should be less than 1.0V. If not, trim trim trim, and check the solder joint on the 3 pads of the trimmer.

ok will do

Keep turning the contrast trimmer… You’re probably just out of range.

do the trimmers have a stop at each end?

also the BOM I used the trimmer has the screw on the wrong side, so I have to keep taking the boards apart :frowning:

its this trimmer

I used one of those for my Polivoks build. Quite apart from any contrast trimmer issues I would recommend replacing the display resistor with a wire link-the display has its own internal series resistor and even with the wire link the backlight is really dim…

centre pin is + 4.16V

curious as i was measuring the centre pin i actually touched the inner pin also and the lcd lights up but all white blocks , going to resolder the trimpot

ok resoldered

just to verify

i power up

put com lead on grnd and red on middle pin to get V reading ?


ok from top to bottom i get


on the trmmer

and if i touch middle and bottom together the screen works and after half a second it goes to all brite blocks

bad trimpot?

Have you tried turning the trimmer in the other direction?

yes i did , doesnt do anything , the only time it works is if I short out the middle and bottom pins by accident ( or on purpose, I tried it) gah , you must be hating the day I get my anushri!

the trimmer needs 20 turns to go from one side to the other…

i just did 41 half turns one way and 41 the other, still nogo , ill order a new one as I need to order some caps anyway

does anyone have the mouser part number for the best 5k trimmer?

should there be a voltage on the 3rd pin of the trimmer? because there isnt any and it feels loose, like I can jiggle it about 5-10 degrees

By the way I REALLY appreciate all this help and assistance , cheers

Pad 1 must be at +5V and continuous with the +5V pin on the control board. Pad 3 must be at ground potential (and continuous with the GND pin on the control board). Grounded pads are often sources of worries since the ground plane absorbs the heat during soldering, and the joints can be weaker.