[Samsung Galaxy S7 Questions and Answers] phreich:
Hi,

The question is: Can a Live Display Unit (LDU) version of the S7 be made a fully functional S7 by swapping out it's non-useable motherboard with a good motherboard from a broken donor S7, and what would be involved to make this work?

I am posting and answering my own question here in the hope it will provide answers to various other posts I have seen in the S7 and the S7 Edge sub-forums. The questions and the answers are relevant to both versions of the S7.

Background:

I am a semi-retired technically oriented handy fellow who is in his early 60's, living on a modest income. As of this last summer (2017), I was still using the Galaxy S4 that I had gotten new in early 2014. The S4 has been remarkably durable, and is still working like the day I bought it, but it's just slowly becoming more and more obsolescent. The only thing I have had to replace over the almost 4 years I have had it was a tempered glass screen protector that did it's job and sacrificially protected my screen, a few cases that wore out, a frayed charging cord, and a few replaceable Li-Ion batteries that no longer were keeping their charge long enough.

However, the S4's limitations of only 16GB of onboard storage, only 2GB RAM, the increasing (relative) slowness of it's 32 bit quad-core processor when running multiple current apps, and the fact it is stuck at Android 5.1 were becoming increasingly an irritant.

I determined the time had come to upgrade my phone. I didn't mind signing a new contract and getting a new phone as I've always had access to reasonably priced phone plans, but I don't like the idea of having to pay a monthly additional fee of $30-40 to lease a phone that has to be returned after 2 years or purchase a phone at full retail cost with even higher monthly payments and/or a big down payment. So I determined that somehow I would have to find a "deal" on a like-new good fairly current but used phone.

I looked around at the cost of the then new S8s and could not justify paying $800 or $900 USD on a phone. I then looked at the 2016 flagship, the S7, but they were still selling then for $300 or more used. I didn't want to get anything older because I wanted to be on the most current version of Android and have at least 32GB of on-board storage, 4GB of memory, and a good 64bit quad-core processor. I also wanted to stay with Samsung, because I was familiar with their "version" of Android and knew them to usually have a fantastic build quality (except for their debacle with the self-igniting Note in 2016).

So I started out looking at how I could acquire a like-new used S7 for about 1/2 of the going rate -- or about $150 USD. I have repaired my own "smart" cell phones in the past -- but that was back in the days when they were held together by screws. I hadn't needed to open up my S4, and had not personally seen the insides of the ever-more-glued-together phones that have been produced since, although I had kept somewhat current by watching various YouTube videos of tear-downs of new phones as they came out. As I said earlier, I am a technically oriented handy fellow, so the thought of learning to deal with the hardware and assembly methods used in the newer versions of smart phones didn't scare me (at least not very much). If someone else can work on them, then I can educate myself and do it too. Thus I determined that the way to get my new phone at a reduced price would be to find one and repair it. So I started my research of the S7.

Research:

Through my research, saw that there are a lot of really busted up S7 phones out there that do show solid signs of life when plugged in (although the screen is broken and non-functional). These phones power up, hold a charge, make the appropriate booting sounds, and the LEDs at the top and bottom light up during the boot process. All of this indicates a functioning motherboard. Additionally, many of these badly broken phones with functioning motherboards also have readable ESN/IMEI numbers on their backs, and so their ESN/IMEI numbers can be verified as non-blacklisted and non-leased. Basically, you can tell if the broken phone you are buying has an activatable motherboard in it or not before you purchase it.

I saw that S7 phones with blacklisted ESN/IMEI numbers or that were blocked because of being on-lease but that were in like-new condition were still selling at prices in the mid-to-high $200s, which is more than I wanted to pay. For ethical reasons I also didn't like the possibility of potentially supporting thieves by buying stolen stuff they were selling. So it was looking like I was going to have to look at paying for a broken phone and a $150 replacement screen assembly and miscellaneous other needed parts, at a total cost of $200-$225. This still didn't fit in my desired budget.

I then learned of the existence of the Live Display Units (LDUs) that come up for sale on Ebay from time-to-time. These LDUs have special motherboards in them that were manufactured without the cellular radios that fully functional S7s have. This was done by Samsung to reduce the thefts of display models at the stores that sell their phones. Other than the lack of cellular capability, the LDU models are fully functional -- working screens, cameras, speakers, wi-fi, apps, they just can't connect to cellular towers and so can't be used for phone calls or for mobile data purposes. They also have a "special" firmware version installed on them that prevents them from being turned off, and automatically wipes their memory and restarts the "demo" programs after a certain amount of time has elapsed. Most of these LDUs are in like-new condition -- never having been in pockets or purses getting scratched up, never dropped, never gotten wet, etc. They might have some screen burn-in issues -- but that is tolerable to me. I saw them selling for around $110 USD. The LDU models have an "X" suffix. In the case of the S7 models, the LDU model numbers are G930X (S7) and G935X (S7 Edge).

So I got to wondering if it might be possible to combine the two otherwise useless phones and make a like-new S7 out of them by swapping a fully functional motherboard from a donor S7 into an LDU like-new "body" with a good screen. I determined to try it out, and promised to document the results of my experiment on the various posts I had seen asking similar questions -- but didn't have answers. (I'll be going back to those posts and posting a link to this one.)

Preparation:

I watched a number of tear-down and repair videos on the S7, and purchased a collection of the appropriate specialized tools from various sellers on Ebay (some directly from the manufacturers in China, some from US sellers). These specialized hand tools are fairly inexpensive -- for example one of the most useful tools when ungluing glass from the frame is a set of plastic guitar picks -- some use a sacrificial deck of playing cards (plastic guitar picks can be wiped off and reused). I knew that I was going to have to make do without some of the equipment the "pro"s use -- like vacuum tables, heating ovens, UV curing stations, etc. I would have to make do with other methods to hold the phones, to heat them, and use LED UV flashlights or UV fluorescent bulbs in existing fixtures for UV glue curing.

On to actually "doing it":

I purchased a couple of functioning G930X LDUs on Ebay in good cosmetic condition. One of them had some minor screen burn-in, the other was in a boot-loop. I didn't care about either issue as I wasn't going to keep the motherboard, and minor screen burn-in doesn't bother me, as it is really only noticeable when the screen is mostly white, which isn't often in real-world usage. I got these for about $107 USD each. Far less than just a replacement screen assembly would have cost. For some reason, the Edge versions of the LDU's were going for much more, some close to $200, but I wanted the non-edge version anyway.

I then purchased a couple of badly busted up S7 donor phones with functioning motherboards and clean ESN/IMEI numbers, which I got from the sellers and verified prior to the purchase. I was able to get these "parts-only" phones for around $45-60 USD each.

I then got to the actual work of opening up the phones and attempting the motherboard swap. Here's what I found out:

Well, my first attempt didn't work out the way I had hoped, but I learned A LOT.

Firstly, to replace the motherboard on the S7, you pretty much have to completely disassemble all the parts contained on the back of the metal middle-frame. An exception to this is the bottom daughterboard which normally can be left in place when replacing a motherboard. This part of the work turned out to be fairly easy to do once the back glass was removed. I cracked the first back glass I removed, but was able to get them off in one piece after that. Luckily, the replacement glass backs are not very expensive (even the ones that come with the camera lens and adhesive pre-installed are less than $10 USD.)

In addition to the motherboard, I found the following parts are different between the North American (NA) S7 models and the demo G930X LDU model. This will likely be true for the G935 Edge models too:

1. The plastic covers at the top and bottom of the phone that include the molded-in antennas are different between the LDU and the NA models. I think that they might be the same as the European versions though. This was not a problem, because I could simply use the parts from the non-LDU donor phone that had the bashed in screen and back.

2. The phone speaker (not to be confused with the earpiece speaker) is different than the NA versions. The LDU contained the European version. I don't know if this is really so different that it wouldn't work, but, if you ever want to send it to an official "factory" service place for some reason, they might reject the phone because it doesn't contain the "proper" parts. Again, this is not a problem, because I could use the speaker assembly from the North American non-LDU donor phone.

(I suspect one of the reasons for the differences in the antenna and speaker parts has to do with tighter European RF (radio frequency) emission standards)

3. Here's where the big problem occurred -- the LDU also has a modified bottom daughterboard (this is the daughterboard that has the charging port and headphone jack on it). This daughterboard on a regular S7 contains antenna connections and circuitry that is missing on the LDU model. That wouldn't be an issue except It turns out that, on the S7, this bottom daughterboard includes the leads, sensors, and LEDs for the "soft" buttons to the left and right of the Home button. What this means is that, in order to swap this bottom daughterboard out, you have to remove the screen assembly from the front of the phone so you can expose and release the softkeys that are part of this daughterboard and that cannot be detached from it. I understand that this is true for both models of the S7.

Getting the screen assembly loose from the adhesive that bonds it to the metal middle-frame without damaging it, even with using heat, is a very tricky and failure-prone operation. The adhesive is incredibly strong and the slightest wrong amount of pressure will fracture the fragile LCD that is on the bottom of the screen assembly (the LCD is actually what is glued to the middle-frame). Break the LCD and you've got a dead $150 USD front screen assembly, basically making the LDU a useless investment. The LDU motherboard isn't worth much. Basically what you have left is the battery, the frame, the Qi and NFC antenna assembly, the cameras, and possibly the back glass if you managed to get it off without breaking it. None of these parts are very valuable.

I failed at my attempt to do this, and must have cracked the LCD -- although I was very careful and worked slowly. I couldn't see a crack, but it wouldn't light up after I reassembled the phone. I was careful not to damage the leads on the display when loosening the glue on the screen edges (you have to be extra careful on one side where the leads are routed through the middle-frame close to one of the edges) -- so that wasn't the cause of the failure. :(

As the euphemism goes -- "hindsight is 20/20". I now think the correct approach is to leave the LCD and digitizer glued down to the middle-frame, and instead (with heat) separate the front glass from the digitizer and LCD using the wire separation method you can see being done on YouTube in videos where just the front glass is cracked and being replaced (the LCD and digitizer are still functional). Once the glass is separated, the soft keys are exposed and the daughterboard can be swapped out. You would then have to use the specialized UV activated glue to reattach the glass to the digitizer and LCD. I seem to recall that there is a plastic film polarizing filter that often gets damaged during the separation process and has to be replaced, along with the seals for the top and bottom, and the seals for the sides of the glass. The glass, seals and the filter are not that expensive. However, doing this screen disassembly process is tricky too (but not as bad as trying to preserve the LCD when attempting to remove the entire assembly).

I don't recommend others go this route (attempting to use an LDU for it's frame and display assembly) unless they have experience successfully being able to remove and install the front glass on these phones.

With it's fully curved sides, the S7 Edge (G935) version would be more difficult to get the front glass removed and replaced. I know I would want to practice on a couple of S7 Edge phones with fully broken displays before I attempted it on a good display..... It is certainly not a project for the faint-of-heart.

The glass separation for the regular G930 S7 doesn't look too difficult, as long as a person uses (moderate) heat, is careful, and has patience. I have one more G930X LDU that I purchased, so I am going to find out. I'll try to update this post with the results of that effort.

International/European versions of daughterboard are different than North American

Something I did learn that will be of interest to anyone attempting to swap motherboards between an international/European version of the phone and a North American version of the phone is that there IS a difference in that same troublesome bottom daughterboard between the two versions of the phones. I know this because I asked a parts supplier, and they informed me that there are two different versions of this daughterboard -- one international/European and the other North American. (I don't know which version is used in the Chinese domestic version of the S7, or whether or not there is a difference between the versions used in the various Asian markets. Hopefully one of the experienced Asian posters will know and reply to this thread with the answers.)

UPDATE (12/16/2017):

I don't know exactly what the differences are in the daughterboard, but they are slight. I recently got a G930FD parts phone with a bad motherboard, and discovered that I could use the European daughterboard with a North American motherboard and antenna set -- you should switch out the antenna wires that go from the motherboard to the daughterboard (the colors are reversed), and it works just fine

Well, it turns out I was wrong about the European daughterboard being compatible with the North American motherboards. Everything on the phone works EXCEPT the wireless radio can't get a signal lock, and eventually shows an "x" on the signal bars indicating that the radio has turned off. I tried this with multiple T-Mobile SIM cards, and worked with a tier 2 T-mobile phone technician trying every possible thing to work around this. Bottom line, it looks like the European version of the charging port daughterboard is only compatible with the G930F and G930FD motherboards. Oh, well. It was worth a try. I am now looking for a working G930F or G930FD motherboard to get this phone working again. I am also looking for a good North American G930A, P, T, V frame and good display (or just a good complete screen full assembly) to make a whole phone with the good T-mobile motherboard I have.

Samsung learned their lesson regarding this daughterboard

One other point I learned from a cell phone technician -- the issue of the front softkeys being part of this bottom daughterboard has been fixed in the S8. It seems Samsung learned "the error of their way" with this design. Whenever this daughterboard needs to be replaced on an S7 due to a charging port issue or headphone jack issue it requires removing the front glass to get the job done -- making the work a lot more involved and costly for Samsung's own repair staff, as well as all the authorized in-warranty carrier cell phone repair centers.

A lot of grumbling has occurred. due to this design. I am sure the post-warranty non-authorized repair centers are also pleased that this design ended with the S7. Their customers can't have been pleased at the elevated post-warranty charging port repair costs this design has caused....

Success in a non-LDU motherboard swap:

After failing to be able to use the first LDU to build a useable S7 phone by swapping it's motherboard with a good motherboard (with a clean ESN/IMEI number) from an S7 with a badly broken display and back (and scarred up middle-frame), I was able to use another North American S7 that I found with a good display, but a bad motherboard. The swap was pretty easy to do, and I wound up with a functional S7 at the end, for much less than I would have paid for a refurbished or a good used S7. I did have to deal with bypassing the FRP lock on the good motherboard because, of course, when the screen was broken, the previous owner couldn't log out of their Google account, so the FRP lock came into play once the phone was factory reset. That "adventure" is covered in another post I made a few weeks ago. Doing this first S7 motherboard swap has been an invaluable learning process as well -- one that I hope to be able to leverage and turn into a profitable sideline/hobby in doing future swaps and repairs. I find I really enjoy working on these intricate pieces of electronic equipment.....

A cautionary tale:

BTW, there's an interesting story (at least to me) as to why the motherboard on the S7 I used as the "screen donor" was damaged. I think it's a cautionary tale of what

NOT

to do to your mobile phone....

The previous owner had not used the "sim tray removal tool" to eject the S7s SIM tray, and had snapped the top off of the sim tray attempting to brute-force it out. They then went on to use what must have been a screwdriver, knife and/or nail file to try and pry the rest of the tray out, and in the process broke off most of the electrical connecting "fingers" on the motherboard that contact and connect to the SIM and MicroSD cards. I only discovered the extent of the damage once I had removed the motherboard and gently pushed the remnants of the broken SIM tray out and was able to see what had been done. In fact, I found the SIM card was still in place within the bottom 3/4 remnant of the broken SIM tray, which was also still very much in place. The SIM card, which is located at the bottom end of the SIM tray, also showed evidence of how much digging and gouging the owner or the seller had done trying to get the broken tray out -- they had dug around that deeply within the slot on the phone that they scratched and deeply scarred up the SIM card.

Otherwise this motherboard was functional -- unfortunately since it couldn't connect to the SIM card, it rendered the motherboard almost as useless as an LDU motherboard. I think the Ebay seller who sold me the phone had done some further damage trying to get the phone to work. The story he told me was that the original owner's kid had jammed something down inside the slot -- this proved obviously not to be the work of a small child.

A last thought regarding using LDU and other semi-functional motherboards

One last thought regarding LDU and other semi-functional motherboards -- these boards can be reflashed with functional firmware, and the motherboards can be probably be used in phones with broken displays as always-on smart home controllers, possibly as media players, or whatever else a person can imagine with a wi-fi capable Android device. I am planning on using my old S4 in this fashion -- connected to a USB OTG and MHL enabled docking station so it can be connected to an external display and sound system. Samsung made a special docking station just for the S3 and S4 (and probably the Note 2) that makes this possible, and can be found used online for much less than it originally cost. I know that the S7 is also OTG enabled, so with it's included adapter or a generic OTG docking station or cable you can do similar stuff.

My hope is that this post is both useful and interesting.

Good luck with your adventures with the Galaxy S7!

Philip