LCD Display Replacement Head-scratcher

[al_m]

Hello all. Bit of a long read this:

A couple of days ago i undertook a repair to a synth of mine that had a fading LCD with signs of persistent burn in, so ordered a replacement part (Winstar WG12232A). This is a 122x32 LCD driven by the Avant SBN1661G. It proved to be a total bugger to get out of the unit, the pcb of the display and main control board being joined by four clippy standoffs, a 20 pin header row soldered directly between the LCD connector and the control pcb, and two more header pins soldered in to provide the power to the backlight. My desoldering tool was too wide to get in between the standard 2.54mm pitch pins, so i had to try using solder braid to get it out. Anyway, i gave up because it was seeming like i'd end up having to use too much force to remove the thing. So i resoldered all the pins, and put the unit back together.

When i turned it on the display was blank. The backlight came on, but there was nothing on the display, which had previously been ok other than the burn-in. Ah, now i've done it! So, i get back in and after another hour or so with the addition of some snips manage to remove the old unit and put the new one in. Installed, unit reassembled. And... blank display! The machine still functions perfectly, just no display at all.

So, out with the meter and scope, and after probing about i find that continuity is good from the pins right through to the vias on the control board which lead directly into the micro that drives the LCD. Correct signals are present at all pins, pressing buttons causes activity to occur on data lines and CS, etc. So i am pretty certain the LCD is getting fed the right stuff, but it's dead.

Next up, today i fixed a little test rig with an arduino to try and drive the old LCD i had removed, and i get exactly the same, nothing on the display, even though the data is getting through ok. So i probed around the back of the LCD on the driver chips, and found that all the vias which lead out of the chip into the display itself are pinned high! There's no life at all in the bugger. And to top it off, when the chip is powered all the data lines get shorted to each other, i can buzz them all and get a through connection that isn't there when the power is removed.

My question is: is it likely i have destroyed the LCD driver from excessive heat? The data sheet gives 70c max operating temp and 125c max storage temp, but nothing on soldering temps or time. I've never heard of this being likely, but the fact that two displays now show the same behaviour (i checked the installed one and it also shows a short on the pins when power is applied) must mean either i'm missing something or my soldering has wrecked a delicate component.

Any tips or advice? Thanks.

[TheMG]

I didn't see this mentioned in your post, did you verify the contrast adjustment pin for correct voltage? If the contrast is adjustable, does the voltage change when adjusting contrast? If the contrast voltage is incorrect you could get essentially a blank display.

[wraper]

And to top it off, when the chip is powered all the data lines get shorted to each other, i can buzz them all and get a through connection that isn't there when the power is removed.

Who in the hell measures resistance between pins of a powered chip? Most likely they are simply in the same state but you for some dumb reason decided to measure resistance between them .

[al_m]

I didn't see this mentioned in your post, did you verify the contrast adjustment pin for correct voltage? If the contrast is adjustable, does the voltage change when adjusting contrast? If the contrast voltage is incorrect you could get essentially a blank display.

When i initially turned the machine back on for the first time i did think the contrast may have gone out of alignment, and adjusted the trimmer on the main board, but it made no difference. Also, the contrast pin on my test rig sits ~Vcc even with nothing plugged into it, which is a bit strange, and thus makes me think something must be damaged, as i can't tell where that voltage would be coming from otherwise. I have since tried just using a simple voltage divider from Vcc to put 2.5v on the contrast pin, but it makes no difference, it still reads near 5v. Quite odd.

Who in the hell measures resistance between pins of a powered chip? Most likely they are simply in the same state but you for some dumb reason decided to measure resistance between them .

You're wrong. It seems i'm not the only dumb one around here. If you haven't got anything useful to contribute please do the decent thing and shut up.

[wraper]

You're wrong. It seems i'm not the only dumb one around here. If you haven't got anything useful to contribute please do the decent thing and shut up.

Where I'm wrong? Most likely they are tied to the same power rail through opened MOSFETs inside the IC (because it is set to read mode), thus providing low resistance path to the same power rail (either VDD or GND) and thus between them too. Please explain the mechanism how otherwise there can appear low resistance path between them only when IC is powered.

[al_m]

Well, why didn't you just say that instead of insulting me the first time? I'm no genius, and still learning all the intricacies of EE. Having some random person insult me for explaining my methodology and asking a question about my findings is not helpful. Didn't expect that sort of treatment here tbh. I only noticed the 'continuity' because i had left my meter in that mode before i picked it up to probe again after powering on, i forgot to change it, but got that result and was confused.

So, now i can discount that as being an issue, and ignore what i did there. But it still doesn't explain why the driver chips are unresponsive. Could i have damaged them by applying excessive heat? That's what i want to know, because it seems to be the only reasonable answer.

I should also note that i was wearing an ESD strap, and had the machine open on an ESD mat, so i think static discharge is unlikely to be a factor, therefore i'm looking for other causes.

[coromonadalix]

hi 
You have this arduino library to help,  sometimes you have to adjust the pinouts definitions, i used an arduino to tests glcds with an small dc-dc converter to generate the contrast voltages on some glcd's, sometimes you could need up to minus 20 volts

https://forum.arduino.cc/index.php?topic=129203.0

Eevblog thread  who may help
https://www.eevblog.com/forum/projects/winstar-wg12232o-lcd/

and having the datasheet sometimes may get you further
https://www.winstar.com.tw/uploads/files/18bcfc4f8b87774ab69949731b01adc0.pdf

[al_m]

Hi, thanks for the links.

I'm using the later version of the library, u8g2, and have it all set up right as far as i can tell. The data coming in on the LCD pins from the arduino looks good to me, similar to what was being sent in when installed in the machine.

I've got a new WG12232 on the way so that i can be sure i've got the test circuit working correctly. Hopefully i'll be able to plug it in and get some response from the display.

[amyk]

Suspect ESD for sudden component death.

[al_m]

Suspect ESD for sudden component death.

Twice though? Despite wearing an ESD strap and working on an ESD mat? Both of which i checked before starting work on the machine? Seems somewhat unlikely... could i really be so unlucky?

Also the LCD driver chips are potted.

[tooki]

I didn't see this mentioned in your post, did you verify the contrast adjustment pin for correct voltage? If the contrast is adjustable, does the voltage change when adjusting contrast? If the contrast voltage is incorrect you could get essentially a blank display.

When i initially turned the machine back on for the first time i did think the contrast may have gone out of alignment, and adjusted the trimmer on the main board, but it made no difference. Also, the contrast pin on my test rig sits ~Vcc even with nothing plugged into it, which is a bit strange, and thus makes me think something must be damaged, as i can't tell where that voltage would be coming from otherwise. I have since tried just using a simple voltage divider from Vcc to put 2.5v on the contrast pin, but it makes no difference, it still reads near 5v. Quite odd.

The contrast pin is itself part of a big resistor divider network, so it’s eminently possible for it to have a voltage when floating.

Some LCDs need a contrast voltage close to zero (in fact, some even need a negative voltage, though that is not likely the case here), so you can’t just assume that 2.5V will produce a visible image. On your Arduino setup, use a 10K pot between 5V and ground, with the wiper on the contrast pin, and test the entire range. You should have some point where all segments are darkened, and another where they all disappear. In between that is a point where turned-on segments are dark and the rest are light.


As for your hypothesis of heat damage, I doubt it. I can’t see how you could overheat the IC without having burned the board to a crisp.

[tooki]

And to top it off, when the chip is powered all the data lines get shorted to each other, i can buzz them all and get a through connection that isn't there when the power is removed.

Who in the hell measures resistance between pins of a powered chip? Most likely they are simply in the same state but you for some dumb reason decided to measure resistance between them .

While you are technically correct on the content, your tone here is wildly inappropriate. There’s absolutely no need to be so rude to OP.

[Ian.M]

And to top it off, when the chip is powered all the data lines get shorted to each other, i can buzz them all and get a through connection that isn't there when the power is removed.

Who in the hell measures resistance between pins of a powered chip? Most likely they are simply in the same state but you for some dumb reason decided to measure resistance between them .

While you are technically correct on the content, your tone here is wildly inappropriate. There’s absolutely no need to be so rude to OP.

It is however a *VERY* bad idea to measure resistance around a powered CMOS device unless you are 100% certain that your meter's Ohms range open-circuit voltage is under half a volt and short-circuit current is at most a few mA.  Although modern CMOS devices usually have reasonably robust inputs, if you drive a pin below ground hard enough to forward bias the isolation well to substrate junction and pass significant current, it can cause latch-up and device destruction.

Suspect ESD for sudden component death.

Twice though? Despite wearing an ESD strap and working on an ESD mat? Both of which i checked before starting work on the machine? Seems somewhat unlikely... could i really be so unlucky?

ESD is unlikely but leakage current from poorly grounded soldering irons/stations not designed for floating use is a significant risk. See https://www.eevblog.com/forum/chat/does-you-soldering-iron-have-grounded-tip/msg2206860/#msg2206860

[al_m]

You should have some point where all segments are darkened, and another where they all disappear. In between that is a point where turned-on segments are dark and the rest are light.

Hi, thanks for the reply. The thing about the contrast is what made me initially suspect i had damaged the LCD as when i gave up at first and resoldered the pins, the contrast trimmer in the unit had zero effect when i turned it back on. I had previously tweaked it before attempting the repair, so know that it was working, and yes, the pixels went through the whole range from black to blank. After the botched repair it has zero effect, the display remains blank whatever position the trimmer is in. This is the same for the replacement display i installed, and also for the old one which is hooked up to the arduino test rig. I get zero change at all on the display, not even a smidge of a pixel anywhere, the display is totally blank.

It is however a *VERY* bad idea to measure resistance around a powered CMOS device unless you are 100% certain that your meter's Ohms range open-circuit voltage is under half a volt and short-circuit current is at most a few mA.  Although modern CMOS devices usually have reasonably robust inputs, if you drive a pin below ground hard enough to forward bias the isolation well to substrate junction and pass significant current, it can cause latch-up and device destruction.

The chip appeared to be dead before i started probing, so it's unlikely in this instance to have been the cause, but your words of caution are noted and will be in mind going on from this point, as i have a replacement display on the way now, to use as a verification of my test rig, just to eliminate that as an element of error in discerning the fault.

ESD is unlikely but leakage current from poorly grounded soldering irons/stations not designed for floating use is a significant risk. See https://www.eevblog.com/forum/chat/does-you-soldering-iron-have-grounded-tip/msg2206860/#msg2206860

But here i think you might be on to something. I thought my iron was grounded, but actually it appears that the tip is not! And there is in fact a grounding lug on the back. It's not something i had considered as an element of suspicion before, as i've been using it for years with all sorts of board layouts and topologies, and never once has it damaged anything by ESD shock. But i'm staying at my mum's house at present, and the wiring here is quite dodgy, there are lots of RF issues and i suspect perhaps if the iron was floating while everything else was grounded (or near enough), there could be a cause there. I'll have to investigate further, but will be sure to make use of that lug from now on.

[al_m]

Blast, that's not the answer. Just measured resistances on the iron:

tip to ground lug - 0 ohms
ground lug to plug ground pin - 0 ohms
tip to plug ground pin - 0 ohms

I see from the linked thread that temperature can change this. So it's probably still worth it to use the ground lug, but i'm doubtful now that it could be the cause of the chips demise. I can try again next time i turn the iron on and see what the readings are.

[al_m]

Encouraging development! I thought i'd fiddle with the test rig and old LCD a bit more, and when i was putting a jumper between gnd and the contrast pin i got a display! It's fairly clear until the whole wire makes good contact in the socket and then i get very dark display. Makes me think it would actually prefer a negative voltage. Hmm. But at least i know the old LCD is not busted, and thus the one i installed is most likely also working, so what have i broken that's causing the contrast adjust on the unit to have no effect? Will have to open her up again and see if i can trace out a circuit for that pin...

Thanks for all the help guys. If you have any further thoughts as to what i can do from here, please let me know. I'll also update with any more developments.

Edit: how confusing. There's proper activity on all the vias leading into the LCD panel from the driver chips now that wasn't there last time i had it turned on. It's just been sat on my table for a day or two with no alterations and now it works...

[jmelson]

Edit: how confusing. There's proper activity on all the vias leading into the LCD panel from the driver chips now that wasn't there last time i had it turned on. It's just been sat on my table for a day or two with no alterations and now it works...

Oh yes!  How wonderful.  Well, LCDs are fairly high impedance circuits, and solder flux can get conductive at times.  Then, after soldering, the conductivity changes.  Usually, the flux residue is quite insulating right after soldering, and then absorbs moisture and conducts more.  But, I've seen enough REALLY WIERD stuff in my years of tinkering, anything is possible.

Jon

[Audiorepair]

Every time I solder, I spray with flux remover and brush off.

Takes 10 seconds.


In retrospect, why would you not do that?

Oh yes!  How wonderful.  Well, LCDs are fairly high impedance circuits, and solder flux can get conductive at times.  Then, after soldering, the conductivity changes.  Usually, the flux residue is quite insulating right after soldering, and then absorbs moisture and conducts more.  But, I've seen enough REALLY WIERD stuff in my years of tinkering, anything is possible.

Jon

[coromonadalix]

Some flux cleaner(s)  will leave residues  even if the brand say the contrary,  i could call it  contamination

To clean boards, i use an aqueous solution   Atron ac205  we use it into our board production, this boy get rid of everything, even clean floors tiles  lolll

Even for fun i used some brake cleaners brands here in canada. On some tests boards, i did find a cheap brand who get rid of almost everything in one go and doesn't damage plastics or electrolytic heat shrink marks   loll

For the lcd,   in some cases you could snatch  any cheap  laying around lcd's say 16x2 20x2  etc...    based on 44780 chipset  and you could see if some data could be seen, assuming the contrast pin voltage is compatible  before doing so.

For the solder "station" grounding, i saw some models had a protection toasted inside ?? the ac grounding was open,   some kind of short or over-voltage damaged it ?

[al_m]

Well, it turns out there's a negative voltage being generated on the LCD board from a small innocuous charge pump sitting there, and routed out to the pin on the main header marked as backlight anode on the datasheet. There's no mention of this pin being for that purpose, though there is a note 'negative voltage optional for 3v supply' on the datasheet, and that's all. Turns out it's perfect for setting the contrast, in particular when running the LCD on 3.3v, the datasheet that says there should be 4.45v typical between Vdd and V0. It took me ages to realise this is a differential measurement, and i was getting very confused that contrast was working even when i fed no voltage to the pin directly. It seems that it's referenced to Vdd and obviously that requires V- when running on 3.3v. So i now have an idea that the main PCB could be drawing this negative voltage from the LCD and putting it on the trimmer. I'm going to see where the pin leads off to, and see if i could have cracked and traces or something like that which could have killed the contrast circuit. Fingers crossed.

The old LCD which i pulled from the machine is definitely not damaged, in fact it's working within spec pretty well. Glad i made this test rig and stuck with it. Seems to have paid off, at least, partly!

[helius]

For future reference, LCD displays do not have "burn in". You can display the same image on an LCD for 100,000 hours without permanently impressing it in any way. When the pattern is changed, the old image can be left behind for some period of time, but this is not a permanent burn, just slowness of changing the shape of the crystals. There are other ways that LCDs can wear out (the worst are leaking liquid, or separation of the liquid crystal from the charged inner glass surface), but they have nothing to do with static images. This is the reason that screen savers dropped sharply in popularity when most computers switched from CRT to LCD displays.

Some other effects that are confused with "burn-in" are tunnel vision, caused by degrading polarizer film; and "ant tracks" which are solid agglomerations of crystals. Both can be fixed with the proper tools and techniques.

My desoldering tool was too wide to get in between the standard 2.54mm pitch pins, so i had to try using solder braid to get it out.

So I guess your desoldering tool was useless for the most common application of a desoldering tool? Or more likely, PEBCAK. There are desoldering tips with extended reach for tight spaces but this doesn't even sound especially tight.

[al_m]

For future reference, LCD displays do not have "burn in".

I didn't know a better way of putting it. The LCD has an area that goes from normal to almost full-on black in a smidge of a turn of the trimmer, much darker than the rest of the display, and that area also has worse overall contrast. The contrast is bad in general, very uneven. And the update rate is sluggish. The manufacturer acknowledged a poor batch of displays were installed in early runs of the machine, and this one is from that set.

So I guess your desoldering tool was useless for the most common application of a desoldering tool? Or more likely, PEBCAK. There are desoldering tips with extended reach for tight spaces but this doesn't even sound especially tight.

Not PEBCAK thanks, that might be more applicable to you... Don't know why you would say that. Yeah, it's a cheap piece of shit. It's not like i'm proud of the thing. The nozzle is a kind of volcano shape, it's hopeless for getting a tight vacuum on close pins, especially so in this case where the construction of the unit has the longer part of the standard header pins mounted poking up through the PCB i wanted to desolder.. The nozzle would have to have clearance of over a cm to work. I just don't know where to source a 'decent' gun style desoldering tool for less than a hundred quid. Not keen on buying more china made landfill candidates.

It's this one:

[helius]

Isn't the update rate under control of the processor/main electronics? Indeed many older boards have very slow CPUs, <1 MHz 6809s and so on. I wouldn't expect miracles.

Uneven contrast can be caused by a poorly manufactured panel, which may be the case here. It can also be caused by inadequate local power reservoir for the LCD drivers. Are there any surface-mount can capacitors on the LCD board?

It's this one:

My condolences. With a tip as wide as that, it could only work on radial-leaded components and not on rectangular ICs. Non-temperature-controlled is also risky.

[al_m]

Isn't the update rate under control of the processor/main electronics? Indeed many older boards have very slow CPUs, <1 MHz 6809s and so on. I wouldn't expect miracles.

Uneven contrast can be caused by a poorly manufactured panel, which may be the case here. It can also be caused by inadequate local power reservoir for the LCD drivers. Are there any surface-mount can capacitors on the LCD board?

Apparently the poor update rate is just a product of the poor part. In the test mode of the machine i'm working on there is a function to alternate the screen between all cells on and off by pressing a button for each setting respectively, so it's fully manual and i'm guessing unconnected to the clocking in the unit. Going from one to the other shows a very noticeable 'filling-in' of the screen from white to black and vice versa, where certain areas take much longer to transition between the two states. As for reservoir caps, the LCD PCB is surprisingly sparse, there are a couple of links, a couple of resistors, a few decoupling caps on the driver chips, and then the charge pump circuit for the negative voltage, which has two SMD tantalum caps. And that's it. Lots of unpopulated pads marked for resistors and caps, i'm guessing for different versions of the board.

My condolences. With a tip as wide as that, it could only work on radial-leaded components and not on rectangular ICs. Non-temperature-controlled is also risky.

Thank you . I'm not a fan of the thing. For standard large component through hole stuff it used to be adequate, though nowadays i'm much happier with simpler methods like solder braid or good old heat 'n' pull. Most of my work consists of recapping PSUs, so it's not been a big problem until now. I will one day get a proper gun, but it is an investment i need time to save for, and i'm not in a position to use it properly where i'm living at the moment (space constraints).

[al_m]

For anyone still following, it looks like my assumption about the charge pump produced negative voltage was correct. I have reopened the unit and removed the LCD, and there are traces taking the pin 19 output directly to the trim pot, then from there back over to the V0 contrast adjust pin. Not only that, but the replacement display i bought does not have the same configuration as the original one, and is missing the charge pump and capacitors necessary to provide that voltage. So what was being sent out of the LCD i put in was in fact 5v, which i think is coming from the backlight V+ on the main PCB. This means the contrast can't ever get low enough for the difference to be enough. In fact, it's worse, as the LCD is being powered from Vcc of 3.3v, which means that the potential 5v going to the contrast adjust pin is out of spec by more than a volt over the max expected. I can't get the replacement display working in my test rig either. Perhaps i busted it. Will see.

Next i'm going to hook the original LCD back in to the unit and see if it actually works or not. Just waiting for some jumper wires so i don't have to solder it in again. The thing is a total nightmare to get out. Took me almost 2 hours to remove the LCD using solder braid and some quite forceful rocking back and forth..