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878: 1 digit doesn't want to cooperate
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878: 1 digit doesn't want to cooperate

by zpj9t5e2fx on Thu Jul 30, 2020 5:53 am

Product 878: 0.56" 7-segment HT16K33 Backpack with red digits

Display module: Luckylight KW4-56NCLB-P datasheet

One digit (digit 3 - second from the right, looking from the front) never lights any segments.

For troubleshooting, I reflowed all of the display and IC pins. Then, I tried a jumper wire from display pin 10 to IC pin 5 per the backpack's schematic thinking it might've been a PCB or pad issue. That was a hard no dice. Next, I manually applied 5V through a 330 ohm resistor from pin 10 to pins 1-5, 9, 12, and 13 to verify the LCD was intact. No third digit segments ever lit up while all of the others did. So, the display seems hooped. For grins, I scrubbed the flux and gunk mostly off the PCB with technical grade isopropyl.

Attached are some pics under the trinocular's camera. They're chocked-full of filthy, nasty, metallic specular reflections because the polarized light source hasn't arrived yet. And no soldering station has yet been invented to quiet my hand tremors, therefore the soldering "quality" (if you can call it that) looks straight out of a cheap, 1950's mass-produced radio assembled by a sleepless jr. parts-stuffer with a bad hangover. The scratches are a bonus feature, not a bug, which I though I could get away not using magnification removing bridges... I wasn't trying to recreate Mix Master Mike's demo tape, honest.
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Posts: 2
Joined: Thu Jul 30, 2020 3:47 am

Re: 878: 1 digit doesn't want to cooperate

by mikeysklar on Thu Jul 30, 2020 4:17 pm

The shaky soldering does come through in the pics. Thank you for the detailed photos and description.

When did you purchase this display?

Posts: 1677
Joined: Mon Aug 01, 2016 8:10 pm

Re: 878: 1 digit doesn't want to cooperate

by zpj9t5e2fx on Fri Jul 31, 2020 11:37 pm

Haha. Thanks arot! I can bump it up to 270x to make it look even worse if you want. ;-)

Yeah, so I did some research just after posting but slept before replying back.

The display itself is no longer manufactured and no one I can find has a 14-pin-compatible substitute part, because all display manufacturers love to do what seems like assign LCD display pinouts with a random number generator.. perhaps to ensure customer EE and buyer job security doing board revisions, vendor part lock-in and/or build-in some good old-fashioned planned-obsolescence.

I suspect that pin 10 on the display had a thermal excursion beyond what the datasheet recommended (3 seconds with a 25W iron, 3 seconds between pins). The damage would then be most likely heat conducted down the pin and melting the fine bonding wires buried inside the epoxy potting. Fixing it would require dremelling them out like a rocket neurosurgeon doing microneurosurgery, running a/many jumper wires with the small tip of a very low-power precision soldering station, and repotting. Only the few would do that for a personal repair challenge. Who wants to volunteer to do that? :-))

The conclusion I take away is that it's uneconomical for anyone to fix it. There are so many low-cost I2C LCD displays from (they know who they are, but let's say) Adafruit (anyhow ;-) ), it doesn't make sense to replace it. I'll just have to use my shiny used JBC CD & T210 and hope my milli-Parkinson's doesn't get any worse on the next one. :)


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Joined: Thu Jul 30, 2020 3:47 am

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