[Help] PCB suddenly stopped working

[theomaniacal]

Backstory: I have a Vortex ViBE that I desoldered and millmax'd. The keyboard was fully functional for a couple of days following the millmax socket installation.

Issue Details:

• One day while AFK, I suddenly began hearing USB connect and disconnect sounds, alternating rapidly and repeatedly. I noticed my keyboard was the cause of this event.
• After replugging, the board no longer registered at all (no more USB connect/disconnect sounds). The LEDs on the board fail to come on as well.
• I hear a buzzing noise coming from around the USB connector, when it is plugged in.

What I tried:

• Using keyboard in BIOS
• Using different cable
• Plugging into different PC
• Cleaned PCB with 91% IPA and a toothbrush
• Visually inspected for broken traces and solder bridges

Pics:

• PCB Back: https://i.imgur.com/XtlfYD5.jpg / https://i.imgur.com/TEQ1UT9.jpg
• PCB Front: https://i.imgur.com/XULObXj.jpg / https://i.imgur.com/StxIumz.jpg
• USB Connector: https://i.imgur.com/hhGZ5rN.jpg
• Holtek HT32F1654 Microcontroller: https://i.imgur.com/Rh7kMpx.jpg

Additional:

• Previously, I had installed a jumper wire that can be seen in the "PCB Back" pics. This was because of a small scratch that broke a trace. This scratch can be seen by the num00 key (labeled DOWN in this pic: https://i.imgur.com/JRj0aG3.jpg). This was installed around the same time I millmax'd the board, so everything had been working fine with the jumper wire.
• I suspect a short is causing this issue somewhere. I have a multimeter to test continuity with - however, my knowledge of circuitry is lacking, so I'm unsure of what exactly to test. Hopefully nothing is fried.

I'm at my technical limits in troubleshooting this problem, so any help would be GREATLY appreciated. Please let me know if anyone has any questions or needs additional pictures. Thanks in advance!

[theomaniacal]

Update: I was able to get the board fully working again... somewhat. Here was my process:



I plugged the board in and tested for voltage along the VCC trace. Everything reported ~5V through D120, C10, and C11. However, there was some strangeness when I got to U2 (insert corny band joke here). I labeled the solder pads in the pic above for clearer explanation. When I tested Pin 1, it reported ~5V. When I tested Pin 3, it reported ~3.5V. When I tested Pin 2, some light sparks arose before I quickly removed the leads. Suddenly, all the LEDs on the board lit up and the board became fully operational. When I retested Pin 3, it now reported ~5V. However, the voltage seemed to decrease slowly over time.

This leads to some further questions:

• What exactly is U2? I believe it is labeled 20RC. My guess is some failing capacitor, considering the decreasing voltage at Pin 3.
• Why did shorting Pin 2 at U2 bring the board back to life?

Edit: After further testing, it seems like U2 is most likely a voltage regulator chip. I think it's supposed to change 5V to 3.3V to meet the 3.3V needed to power the microcontroller (as stated by its datasheet). My best bet seems to be to replace this chip, as its output voltage seems inconsistent. Currently it's sitting at ~1V, and the USB connected noise occurs but is non-functional and shows a "Port Reset Failed" error in the Device Manager. Hopefully, this info helps someone in the future.