Samsung Galaxy S20 can't connect to GSM networks but LTE works

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Samsung Galaxy S20

Repair and disassembly information for Samsung's flagship S20 Android smartphone, released in March of 2020.

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Novgorod @novgorod

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Gepost: 29 okt. 2023

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Samsung Galaxy S20 can't connect to GSM networks but LTE works

I have a strange network connection issue on a Galaxy S20 (G980F): When LTE reception becomes bad, the fallback to GSM (2G network) never works despite good GSM signal coverage. The phone reports close to full bars and tools like Network Cell Info detect the different GSM base stations in the area with good RSSI values, but the phone can't connect to the network (no voice or data). It's not a network provider issue because GSM works fine on a different phone at the same location with the same SIM card (tested with LTE manually switched off) and the cell tower has similar RSSI values on the good phone, so it seems not to be an antenna problem either.

Is it possible that only the 2G transmitter in the phone is broken but the receiver and antenna are fine, so it can scan for cell towers but never connect? Is it even a thing that only the transmitter can get fried and not the whole baseband chip or at least the band's receiver as well? Or does the phone use two different antennas for 2G transmission and reception and one has a bad contact or something like that? 4G always works fine on the phone if it's available. What else can I check?

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Novgorod @novgorod

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Gepost: 30 okt. 2023

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Ok, I solved the mystery. For the desparate future reader, here's the essence: Stay away from ebay spare parts!

Prior to this ordeal, I changed the USB-C board because USB data didn't work, only charging. The USB worked fine again with the replacement board, so I didn't think it was related to the issue. The reception was always bad at this location as reported by the owner (it's not my phone, so I couldn't judge whether it became much worse after the port change), but I noticed that weird 2G issue while 4G seemed to work normally (just bad reception at the location). The spare part from ebay had the correct part number for the region-specific model printed on the PCB, exactly the same one as on the original PCB (SM-G981B), only some components and the printed text font looked a little different, maybe a different revision, no big deal, right? Wrong! Although USB worked fine again with the new port, under closer inspection, there were some tiny components (capacitors or filters) missing compared to the original board next to what I assume is an antenna pre-amp IC. Putting back the original USB-C board restored the 2G functionality, so the GSM fallback works again!

The irony is: Turns out, the original USB-C port wasn't even broken, just full of compacted black lint and fluff and whatnot, which apparently prevented the USB plug to go in all the way to make contact with the data pins, but the power pins are a little bit longer which was just enough to make contact. After proper cleaning, the original USB-C port works again, so I put the original board back and called it a fix. Thanks, knock-off ebay sellers, for saving 0.07 cents on SMD components to render a $10 part useless.

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YUSUF DWI KURNIAWAN @yusufdwi010199 Rep: 179 4 · Answer 1 · 2023-10-29T21:34:26-07:00

If restarting your phone normally doesn't help and you seriously suspect that a software glitch is the cause of your network problems, you can go a step further by performing a factory reset on the phone. This will completely restart the phone's operating system and eliminate any glitches it may have, but it will also erase any settings or data you have, so it's important to back up any data you want to keep before performing a factory reset. To factory reset your Galaxy S20:

If nothing else solves your connection problem, the problem may lie with one of the chips used to create or manage network connections. If your phone experienced a heavy impact or water ingress shortly before you noticed connectivity issues, your connectivity issues are most likely stemming from hardware damage. Luckily, all the chips that handle network connections are on the same motherboard, so you don't have to narrow the problem down to just one chip. Instead you can replace the motherboard to fix the problem.

jayeff @jayeff Rep: 450,2k 11 367 1,4k · Answer 2 · 2023-10-30T06:55:47-07:00

Hi @novgorod

Here is the service manual that may help.

It's a bit hard to use online but it can be done, just scroll down the page. To download it you need to subscribe to the website.

It may depend on what frequency the 2G network uses in your location so scroll down to sections 8-4-33. GSM 850/900 TX or 8-4-34. GSM 1800/1900 TX to view the troubleshooting flowcharts if there's a problem with the 2G transmit signal from the phone.

Earlier in the manual you will find the board layout diagrams which shows the location of the components mentioned in the flowcharts and also an electrical parts list which shows the Samsung part numbers for each component.

If a component is found to be faulty, search online using the part number only to find suppliers that suit you best.

Note: It is better if you can download the service manual because it most probably is a .pdf file and you can then zoom in on the board layout diagrams which makes it a lot easier to find the components mentioned in the flowchart. You can use the search function in your computer to do this, e.g. Ctrl + F keys if using a Windows PC. You can use this method to find the component in the parts list as well. Just search for the component's board designation e.g. L1017 (1st component mentioned in the flowchart 8-4-33)

Thanks, that's a good start. The scribd website is horrible, but there are downloaders for it, so no issue here. The service manual is for the S20 FE (G780F) though, not for the S20 (G980F). I've looked for the S20 service manual but it's paywalled everywhere (for-profit piracy? What has become of society?)... I hope the GSM module is similar enough.. — Novgorod