ASUS RT-AC88U hardware/software defect identification

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intruder82

New Around Here
Hello everyone,

first of all incredible forum! Was reading through it half a day finding a lot of relevant information.

I'm a repair enthusiast, wanted to upgrade my home network so decided to play around with router repairs.. got myself a defective RT-AC88U - really nice build quality. Was hoping for a firmware problem but it seams I got myself a hardware problem (not a problem though, as I like to fix these too)..

When I power it up, it responds quite interestingly haven't found it here in forum. It lights up very weak PWR, WPS1, USB3 and USB2 led's, LAN1 and WAN2 lights up strongly. Right after that USB2 and WAN2 led's turn off and I'm left with strongly lit LAN1 and weakly lit WPS1, USB3 and PWR led's, that's it, no blinking, they just stay the same all the time..

As far as my diagnostics (reading this forum) got me:
- router looks like new, original power supply is missing, but I have a power supply that is giving 19,5v and 4,6 amps (original is for 2-2,6amps) so power supply shouldn't be an issue.
- router can't get in to recovery mode. Pressing and holding reset and powering it up to bring it in to recovery mode or pressing WPS button and powering it up to clear NVRAM did nothing, it stays the same.
- Tried to connect an FTDI cable to the serial connection (GND to GND, TX to RX, RX to TX) with PuTTY, didn't get from descriptions at what point I need to press ctrl+c when powering it up, but it seams to be dead too.. I'm not getting any response in the screen.
- after some time with power I spotted some warmth on the CPU heatsink so it's either shorted or stuck under boot loop..

I'm still going to go through power supply circuitry to check if maybe there is some voltages missing, but maybe some one can see if this is a dead CPU indication (static led's) or am I doing something wrong with the serial connection..

thanks for any hints where too look further!
 

thecheapseats

Regular Contributor
just guessing - that somebody at sometime before you got your hands on it, powered it up with a dc supply with a reversed polarity plug...
 

intruder82

New Around Here
Well the router looks as if it was new, no scratches, no fingerprints and it was not opened on the inside..
As I checked voltages I found a short on some of the supply lines. Sadly tracing down I have 2 transistors/mosfets that are heating up right next the CPU.. I'll try to check them but it seams it's the CPU that draws to much/is shorted as if the mosfets were bad.. need to find a datasheet for them, to check..
So it seams I got myself a paperweight this time..
 

thecheapseats

Regular Contributor
those ac-88u boxes always ran hot - you could heat coffee on them... in particular I had one that cooked/roasted a radio circuit, but the rest of it survived... however, there's no fixin' a dc-fried pcb... good luck. running it to ground (pun intended)..
 

ToeStub

New Around Here
Exact same situation. Bought it hoping it was just a firmware issue, but it won't boot at all. Pristine case, no marks or scratches whatsoever. Tamper seal was still present and unbroken. No lights, nothing over the serial port.

Swapped power supplies; no dice. Measuring 19V solid at the inputs to all of the DC/DC converters. One has 0V output, but the enable line is also 0, so I'm guessing that one is turned on during the boot process by the firmware.

One of the other DC/DC converters shows ~1.1V output.... seems low, but I don't feel like opening my good router to compare.

All the 0-Ohm resistors (fuses) are good.

Also I'm seeing 3.3V on the RTL8365MB, so I don't think it's a power supply issue. Maybe it got fried by a voltage spike or something on one of the ethernet ports. Appears to be a blown capacitor on port 1, although an open cap shouldn't keep the device from powering on, especially not that far down the line. Maybe someone plugged in PoE or something... no idea b/c I've never played with PoE, but I'm guessing these routers weren't designed for that kind of voltage on the ethernet ports.
 

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