Flakey wifi router; oscilloscope screencaps of psu - Page 1
Forum section: electronics / repair

author: brew918
author_rank: Newbie
timestamp: July 05, 2021, 02:04:02 pm
content: I've got an wifi router that has become unreliable over time. I've attached screencaps of my DS1054Z probing the 12vDC power supply: is this normal? Could these voltage spikes be causing stability problems?New to ee, eevblog and oscilloscopes.

author: tunk
author_rank: Super Contributor
timestamp: July 05, 2021, 02:38:00 pm
content: Difficult to say.Is it measured with a load?Don't think many (any?) routers run on 12V internally,they will have internal DC-DC converter(s) that deliverse.g. 3.3V etc. Some time ago I had to replace somecapacitors in a 5 port switch.Edit:Maybe you could open the router and post some photos?

author: peteru
author_rank: Regular Contributor
timestamp: July 05, 2021, 02:41:38 pm
content: Where are you measuring? On the DC connector? With load?You'll probably get a better idea of whether this is a problem if you can find the first DC-DC converter in the router and examine the output of that.

author: wraper
author_rank: Unknown
timestamp: July 05, 2021, 02:49:12 pm
content: IME often this happens because electrolytic capacitors in the router died. I don't think those pulses are nearly enough to hinder normal operation of router.

author: brew918
author_rank: Newbie
timestamp: July 05, 2021, 02:56:37 pm
content: Okay I think this is the internal converted voltage ~3.3v. It's a test probe point right after a large inductor (says 150) close to the dc supply input.Does this look healthy? The second image is an alternate 12v switching power supply (wall wart) with no known problems.

author: peteru
author_rank: Regular Contributor
timestamp: July 05, 2021, 05:27:58 pm
content: Seems OK. In fact, the second power supply seems to have more noise/ripple than the first.Have you tried completely reflashing the firmware. I've seen a number of embedded devices that end up with flipped bits in flash memory over the course of a few years. The usual fix is to take note of existing configuration (screenshots work well), restore factory defaults, reflash the firmware image and then restore factory defaults again. After that, reconfigure the device from scratch - do not restore backed up configuration.

author: MathWizard
author_rank: Super Contributor
timestamp: July 07, 2021, 04:39:31 am
content: That looks pretty normal to me, the spikes on the 12V from a standard 12V wallpack, such as a forward converter with probably a DIP-8 off-mains/line controller IC

author: brew918
author_rank: Newbie
timestamp: July 07, 2021, 12:45:30 pm
content: I'll try the factory reset. The flipped bits theory makes sense. These things are basically computers with operating systems running on super low end hardware, a lot more complex than microcontrollers.

author: peteru
author_rank: Regular Contributor
timestamp: July 07, 2021, 01:21:20 pm
content: A factory reset alone is unlikely to be sufficient as it will only erase the user settings most of the time. Any corrupted code and system data will stay as it is.You need to get a hold of the firmware image, erase the flash and apply the new firmware. Think of this process as a format of the HDD and a reinstall of the operating system.

author: wraper
author_rank: Unknown
timestamp: July 07, 2021, 04:51:32 pm
content: Quote from: peteru on July 07, 2021, 01:21:20 pmJust update the firmware via normal process. Usually barely anything from the old data will be left (mentioned below).QuoteBad idea since flash also contains serial number, model/hardware revision, MAC address and may contain WIFI modem calibration data.

author: peteru
author_rank: Regular Contributor
timestamp: July 08, 2021, 04:50:04 pm
content: Quote from: wraper on July 07, 2021, 04:51:32 pmOf course you would not erase the entire chip. Doing so on most consumer devices is tricky, even if you get access to the bootloader. We often go into a significant amount of effort to prevent end users from bricking their devices. Even the "factory mode" is often a one shot affair, where we erase the software that the factory uses during the manufacturing process as the device reaches the end of that process. Once that is done, the only way to alter the factory information/flash partitions is using an in-circuit flash programmer or similar "drastic" measures.What I was talking about is the fact that in order to reprogram those flash partitions that contain the firmware image, each block of flash will typically be erased, tested, remapped if the erase fails, programmed, tested, remapped if the programming failed. Thus, reflashing firmware on a router will typically not only "re-install" the software, but also go through an erase and verification cycle to ensure that the new image is stored in good blocks.

author: wraper
author_rank: Unknown
timestamp: July 09, 2021, 10:03:20 am
content: Quote from: peteru on July 08, 2021, 04:50:04 pmI don't think that any sort of remapping is possible on the vast majority of consumer routers since they use SPI NOR flash.

author: peteru
author_rank: Regular Contributor
timestamp: July 09, 2021, 04:13:00 pm
content: The devices that I work on have a bootloader that uses UBI. As long as the first couple of erase blocks where the bootloader lives are functional, the rest of the flash blocks can be remapped as required.