LCD Display Replacement Head-scratcher - Page 1
Forum section: electronics / repair

author: al_m
author_rank: Contributor
timestamp: February 19, 2021, 07:18:30 pm
content: 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.

author: TheMG
author_rank: Frequent Contributor
timestamp: February 20, 2021, 12:22:21 am
content: 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.

author: wraper
author_rank: Unknown
timestamp: February 20, 2021, 12:34:12 am
content: Quote from: al_m on February 19, 2021, 07:18:30 pmWho 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.

author: al_m
author_rank: Contributor
timestamp: February 20, 2021, 12:47:17 am
content: Quote from: TheMG on February 20, 2021, 12:22:21 amWhen 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.Quote from: wraper on February 20, 2021, 12:34:12 amYou'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.

author: wraper
author_rank: Unknown
timestamp: February 20, 2021, 12:53:14 am
content: Quote from: al_m on February 20, 2021, 12:47:17 amWhere 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 themonlywhen IC is powered.

author: al_m
author_rank: Contributor
timestamp: February 20, 2021, 01:11:22 am
content: 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.

author: coromonadalix
author_rank: Super Contributor
timestamp: February 20, 2021, 01:20:32 am
content: hiYou 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 voltshttps://forum.arduino.cc/index.php?topic=129203.0Eevblog thread who may helphttps://www.eevblog.com/forum/projects/winstar-wg12232o-lcd/and having the datasheet sometimes may get you furtherhttps://www.winstar.com.tw/uploads/files/18bcfc4f8b87774ab69949731b01adc0.pdf

author: al_m
author_rank: Contributor
timestamp: February 20, 2021, 01:41:24 am
content: 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.

author: amyk
author_rank: Super Contributor
timestamp: February 20, 2021, 02:57:50 am
content: Suspect ESD for sudden component death.

author: al_m
author_rank: Contributor
timestamp: February 20, 2021, 11:11:21 am
content: Quote from: amyk on February 20, 2021, 02:57:50 amTwice 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.

author: tooki
author_rank: Super Contributor
timestamp: February 20, 2021, 02:28:29 pm
content: Quote from: al_m on February 20, 2021, 12:47:17 amThe 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.

author: tooki
author_rank: Super Contributor
timestamp: February 20, 2021, 02:29:36 pm
content: Quote from: wraper on February 20, 2021, 12:34:12 amWhile you are technically correct on the content, your tone here iswildlyinappropriate. There’s absolutely no need to be so rude to OP.

author: Ian.M
author_rank: Super Contributor
timestamp: February 20, 2021, 03:03:03 pm
content: Quote from: tooki on February 20, 2021, 02:29:36 pmIt 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 causelatch-upand device destruction.Quote from: al_m on February 20, 2021, 11:11:21 amESD is unlikely but leakage current from poorly grounded soldering irons/stations not designed for floating use is a significant risk. Seehttps://www.eevblog.com/forum/chat/does-you-soldering-iron-have-grounded-tip/msg2206860/#msg2206860

author: al_m
author_rank: Contributor
timestamp: February 20, 2021, 05:06:55 pm
content: Quote from: tooki on February 20, 2021, 02:28:29 pmHi, 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.Quote from: Ian.M on February 20, 2021, 03:03:03 pmThe 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.Quote from: Ian.M on February 20, 2021, 03:03:03 pmBut 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.

author: al_m
author_rank: Contributor
timestamp: February 20, 2021, 05:22:31 pm
content: Blast, that's not the answer. Just measured resistances on the iron:tip to ground lug - 0 ohmsground lug to plug ground pin - 0 ohmstip to plug ground pin - 0 ohmsI 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.

author: al_m
author_rank: Contributor
timestamp: February 20, 2021, 09:48:03 pm
content: 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...

author: jmelson
author_rank: Super Contributor
timestamp: February 20, 2021, 11:31:34 pm
content: Quote from: al_m on February 20, 2021, 09:48:03 pmOh 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

author: Audiorepair
author_rank: Frequent Contributor
timestamp: February 21, 2021, 12:21:13 am
content: Every time I solder, I spray with flux remover and brush off.Takes 10 seconds.In retrospect, why would you not do that?Quote from: jmelson on February 20, 2021, 11:31:34 pm

author: coromonadalix
author_rank: Super Contributor
timestamp: February 21, 2021, 02:06:53 pm
content: Some flux cleaner(s) will leave residues even if the brand say the contrary, i could call it contaminationTo 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 lolllEven 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 lollFor 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 ?

author: al_m
author_rank: Contributor
timestamp: February 21, 2021, 10:19:08 pm
content: 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!

author: helius
author_rank: Super Contributor
timestamp: February 21, 2021, 11:05:46 pm
content: 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.Quote from: al_m on February 19, 2021, 07:18:30 pmSo 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.

author: al_m
author_rank: Contributor
timestamp: February 22, 2021, 01:27:41 am
content: Quote from: helius on February 21, 2021, 11:05:46 pmI 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.Quote from: helius on February 21, 2021, 11:05:46 pmNot 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:

author: helius
author_rank: Super Contributor
timestamp: February 22, 2021, 01:38:47 am
content: 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?Quote from: al_m on February 22, 2021, 01:27:41 amMy 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.

author: al_m
author_rank: Contributor
timestamp: February 22, 2021, 02:41:30 am
content: Quote from: helius on February 22, 2021, 01:38:47 amApparently 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.Quote from: helius on February 22, 2021, 01:38:47 amThank 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).

author: al_m
author_rank: Contributor
timestamp: February 22, 2021, 04:19:50 pm
content: 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..

author: helius
author_rank: Super Contributor
timestamp: February 22, 2021, 11:03:15 pm
content: I think if you don't have a good desoldering tool, you should probably invest in a set of hollow stainless needles. The idea is that you melt the joint, then push the needle through the hole around the pin. Solder won't wet to stainless, so what this does is separate the pin from the hole. These sets were available for $1 delivered a few years ago, and might still be sold cheaply from some vendors.(The latest trick the ebay/aliexpress sellers use to get subsidized shipping to the West is to use freight forwarding from central Asia, which still has the lowest rates in the International Postal Union.)

author: tooki
author_rank: Super Contributor
timestamp: February 22, 2021, 11:09:28 pm
content: Quote from: al_m on February 22, 2021, 04:19:50 pmMany LCD modules have an unpopulated footprint for an ICL7660 charge pump circuit for exactly this purpose. See if yours does.FYI, i really suggest against “forceful rocking” — you could easily rip out a plated through hole. If you don’t have the tools to desolder a pin header (or other multi-pin device) properly, you’re much better off cutting the header pins so you can desolder them individually.

author: gnuarm
author_rank: Super Contributor
timestamp: February 22, 2021, 11:52:09 pm
content: Quote from: al_m on February 21, 2021, 10:19:08 pmI discovered this myself recently. The contrast control on multiplexed, passive LCDs is the end of a chain of resistors that set the drive voltages for multiplexing the LCD segments. The other end of this resistor chains is tied to Vdd. In 5V modules the end of this resistor pin, I'll call Vo, is left for the user to connect to a pot between 5V and ground. However, it is just as effective to just use a pot between this pin and ground without a 5V connection.On 3.3V modules there is not enough drive to bias the display for optimum contrast. So they provide a negative voltage generator which can be used in place of ground to use with the adjustment pot. Otherwise the 5V and 3.3V modules are identical. In fact, the 5V modules have the negative voltage generator on the design, just not populated. I expect they change the value of the current limiting resistor for the backlight.I'm working on a project that is using a 5V display because they started with an Arduino. Now everything else is 3.3V so the 5V level shift uses two more chips. Rather than that, I want to run the module from 3.3V. So I'm adding a 7660 chip to the board in place of the two level shifters.

author: gnuarm
author_rank: Super Contributor
timestamp: February 22, 2021, 11:59:16 pm
content: One thing I didn't see when reading the posts is what the original problem was that prevented the original display from working? I read that you discovered the negative voltage circuit on the display which is missing from the replacement displays. But what was wrong that prevented the original unit from working?

author: al_m
author_rank: Contributor
timestamp: February 23, 2021, 12:44:03 pm
content: Quote from: helius on February 22, 2021, 11:03:15 pmWow, thanks for the tip. I had never heard of such a thing before! I see i can get a set for less than 5 pounds on ebay. So will buy a couple of types and see how i get on. Hopefully they could be useful for future work (i have a large 4x DIP DRAM desoldering job ahead that i have been putting off because i know it might not go well with the tools i have to hand...)Quote from: tooki on February 22, 2021, 11:09:28 pmYes, the replacement i bought has an unpopulated area with pads for a SOIC, which is filled in on the original with the charge pump. I'm going to order the components from digikey and see if i can work out the correct jumper positions to get it working. Will also contact winstar to see if they can advise, but not holding out much hope for that...Quote from: tooki on February 22, 2021, 11:09:28 pmYeah, i'm against using force to remove components, but in this case i had no better option, and the PCB i was rocking was the LCD one trying to get the header pins up off the extremely tight through-holes. It worked out ok, but i do know better, despite all my seeming naïveté. I'm going to look into the needles that helius recommended, as that might save any rough stuff in future.Quote from: gnuarm on February 22, 2021, 11:52:09 pmIt was making the connection between there being no voltage 'put' on the pin and the connection to ground working well, that made me realise it's not about voltage going in, but the difference between the two ends. A tricky point, and not obvious at all from first glance at the datasheets i've been working from (don't get me started on the winstar one, the word datasheet is a compliment to that POS).Quote from: gnuarm on February 22, 2021, 11:59:16 pmThe short answer is: i don't know. It could have been something as simple as a bad solder joint. The fact the contrast trimmer had no effect made me panic and suspect LCD failure, but that turns out not to have been the case. I contacted the manufacturer and they told me to look at an SMD ferrite bead in series with the power line as a point of failure, but when i told them what voltages i'm reading they said it looks good and is unlikely to be that component. So, I'm waiting on some dupont jumpers to arrive after which i can do a solderless install of the original LCD and see if it is in fact working in circuit or not... From my readings and being able to trace the circuit, it looks hopeful. Will report back.

author: Ian.M
author_rank: Super Contributor
timestamp: February 23, 2021, 01:03:00 pm
content: If the holes are tight enough that square pins are effectively a press fit, with their corners biting into the hole plating, stainless hollow needles wont help. Short of cutting the pins, there aren't many good options. If you *MUST* preserve both the pins and board, probably the best option is one ofChipQuik's low melting point desoldering alloys, which in typical usage will lower the melting point of the solder joint to lower than a fresh cup of tea!Remove as much solder as is reasonably possible with flux+braid or a sucker, flux and apply the desoldering alloy with an iron, then ideally keep all the pin joints above melting point with a board preheater, hot air pencil or hotplate, all at or near minimum temperature while you gently work the pins out of the holes. Its more of a PITA if you have to run an iron up and down the row of pins to keep the joints molten, unless you can enlist a second pair of hands to assist.

author: al_m
author_rank: Contributor
timestamp: February 23, 2021, 03:15:08 pm
content: Quote from: Ian.M on February 23, 2021, 01:03:00 pmThanks, Ian. Yes, i was wondering about that. This unit does seem to want you to use snippers to get the thing disassembled, it's bloody hard otherwise, everything is really tight, including the self fastening standoffs. I can see how the needles could be of use in other projects though, so thought it worth a punt for the low cost.I had considered using ChipQuik, but it's quite expensive, i've never used it before, and i couldn't find a good place to source the correct type fast enough. It would probably help a lot though, as would a fresh cup of tea!

author: Ian.M
author_rank: Super Contributor
timestamp: February 23, 2021, 05:41:43 pm
content: The trouble with using dykes to snip the pin, especially on straight pinned multi-way connectors is they can deliver a severe mechanical shock to the board. Also, sooner or later, you'll encounter a tin or gold plated hard steel pin and wreck your *good* precision dykes. Depending on access you may be better off grinding through the pins or the body where the pins emerge with a Dremel cutting disk. Some care is needed to avoid the cutting disk wrenching unsupported pin remnants - solder bridging them all so they gain mutual support may be advisable for SMD pins.Yes, ChipQuik desoldering alloy is expensive. You certainly wouldn't want to use it for every part you need to remove, but you only need enough to go along all the pins of the chip, so 2" will do a medium size TQFP, and it actually works out under $0.60/inch. Also, as has previously been discussed here, you can save the 'blob' of used ChipQuik alloy and reuse it for less critical jobs several times, dropping the price per chip removed even lower. Not having to scour EBAY for a donor unit because youve wrecked the PCB is priceless . . .

author: gnuarm
author_rank: Super Contributor
timestamp: February 23, 2021, 05:50:41 pm
content: Quote from: Ian.M on February 23, 2021, 01:03:00 pmI would wonder about the residual contamination. But I suppose that's not much of an issue and a noop for home use. Interesting. I've never seen this before.

author: Ian.M
author_rank: Super Contributor
timestamp: February 23, 2021, 06:23:46 pm
content: The melting point is so low you can scrub most of the used ChipQuik off the pads with brush followed by a cotton bud, and easily suck through holes or clear them with a cocktail stick. SMD pads should be clean enough to immediately resolder for all except the most demanding applications. As PTH pads will trap more residual ChipQuik, its advisable to 'wash' them with the correct solder alloy for the board, removing it by suction or with flux & braid, to reduce the contamination to acceptable levels before fitting a new part.

author: al_m
author_rank: Contributor
timestamp: February 23, 2021, 06:36:11 pm
content: I'm glad this has turned into a productive discussion rather than just me grumbling on about my wayward handiworkIf there is still residual ChipQuik remaining, is there a risk that it will lower the melting point of solder joints to the point where normal thermal transfer from electron flow would remelt the joints?

author: gnuarm
author_rank: Super Contributor
timestamp: February 23, 2021, 07:40:08 pm
content: Quote from: al_m on February 23, 2021, 06:36:11 pmI think for professional work that might be a concern, but as Ian says, good practices can make it workable. I would not be concerned about normal use. Letting your devices run at the temperature of boiling tea is a bit more than expected for most devices.

author: al_m
author_rank: Contributor
timestamp: February 23, 2021, 09:19:54 pm
content: I suppose so. And we're also talking about a case in which that specific product is only being used in a rework/repair scenario. If it was that critical it may just be better to do a full board replacement, where possible at least.

author: helius
author_rank: Super Contributor
timestamp: February 24, 2021, 07:25:56 am
content: The lead-containing ChipQuik is Cerrolow 136, at a markup of around 12x for turning the bulk alloy into wire solder. The "other metals" in 136 are bismuth and indium, which will not ruin a joint if present as impurities (indium is compatible with most alloys, while too much bismuth can make joints brittle). So while the used ChipQuik should be cleaned away, it won't prevent components or pads from being re-used. A good practice for surface-mount pads is to put a drop of normal solder on the iron and "scrub" lightly over the pads to re-coat them with the correct alloy.The same alloy is also sold as LowMelt by Zephyrtronics. As Ian.M alluded to, there are ways to re-use the low-melting alloy as well. If kept warm, it can be pushed aside with a swab onto an area of soldermask, and then lifted away.

author: al_m
author_rank: Contributor
timestamp: February 25, 2021, 04:25:32 pm
content: Well chaps, my dupont cables arrived today, and i hooked up the original LCD as planned. All is working fine. So what the heck went wrong is pretty much an unanswerable question now. Possibly a bad solder joint? Or something else. Really not easy to say, and since i did try the contrast trim pot at the time and nothing happened, i would guess at a bad connection being likely somewhere. Unfortunately i was in a bit of a panic and decided that the worst must have happened rather than taking it slowly and methodically tracking down the fault.Anyway, thank you all for your help. My next step will be installing ultra-low profile pin headers to the LCD and control pcb so that i don't have to go through that desoldering madness again when i want to install another display, and then eventually converting the replacement i bought to work on 3.3v and putting that in there. All the parts have been ordered and are on their way. Hope i can get it done next week.

author: YetAnotherTechie
author_rank: Regular Contributor
timestamp: March 01, 2021, 11:55:08 pm
content: Quote from: helius on February 21, 2021, 11:05:46 pmCan you please elaborate on how to fix the "ant tracks"? it's the first time i read about it being possible.

author: helius
author_rank: Super Contributor
timestamp: March 02, 2021, 03:49:33 am
content: It may not be possible in every case. The idea is to pulse the polarity of the display at around 1 Hz while heating to about 50°C. Liquid crystals are by nature amphoteric, and if opposite charges get too close together they can "solidify" into an ionic bonded solid. What you want is to loosen those bonds by adding kinetic energy (heating) as well as pulling the charges around to separate them (pulsing).

author: YetAnotherTechie
author_rank: Regular Contributor
timestamp: March 02, 2021, 05:06:20 am
content: Would it also require a drive pattern that "moves" the cristals across the panel away from the rat-track, or would they naturally disperse after de-bonding ?

author: helius
author_rank: Super Contributor
timestamp: March 02, 2021, 10:07:03 pm
content: I did not experiment with that, but from what I understand it would not help. The crystals are normally adhered to the (electret charged) inner surfaces of the glass and should not be migrated horizontally or they will not work properly.

author: al_m
author_rank: Contributor
timestamp: March 04, 2021, 05:59:17 pm
content: Just a final note to say i got the original display reinstalled using low profile sockets and header strips, and it's working as well as it did before i started the work on the unit. I have no idea what went wrong originally. But anyway, i have been sent schematics from Winstar as to how i should do the conversion work to add the negative voltage rail for the contrast offset to my replacement display, so can get on with that when time permits, and all that needs be done is to pull out the old LCD and put the new one in, solderless install (except for soldering the header to the LCD board).I also worked out a very effective way to remove the standard header strips by snipping the plastic body of the header at the last pin and heating the joint on the underside while pulling the pin with pliers. Worked down pin by pin from either end, and then all that was left to do was to remove old solder from the empty mounting holes, which was fairly easy with my limited tools. It seems a much gentler approach, and it was certainly easier, if more destructive of the header strip.Thanks for all the advice and help guys.

author: tooki
author_rank: Super Contributor
timestamp: March 04, 2021, 07:46:03 pm
content: Glad to hear it’s sorted out!As for cutting up the header to desolder it: so, like, what I told you to do back in reply #26?

author: al_m
author_rank: Contributor
timestamp: March 05, 2021, 12:51:45 am
content: Quote from: tooki on March 04, 2021, 07:46:03 pmAh, yes, heh.I don't think i quite got what you meant, or wasn't really paying attention when i read that reply. It was a very helpful technique though, so you're welcome to get the credit for the idea!