AMD Geode/Video 3/Script

This is a work in progress textual description of the video.

Accessibility notice

Narrator: As a quick notice I want to establish that a full textual description of this video is linked in the description below. Enjoy!

(This message is displayed on screen as well.)

Recount

Narrator: Welcome back to my AMD Geode repair video series. If you weren't here for the first few videos, let me give a quick recount.

(TODO: Display something here)

Narrator: In video 1 I bought and did some basic troubleshooting on an AMD Geode computer board.

I drew some wrong conclusions and at the end damaged the board with bad soldering.

(TODO: Display something here)

Narrator: In video 2 I spent 6 hours trying to repair the board. I was successful in the end but damaged another part later.

(TODO: Display something here)

Mistakes

Narrator: Okay we're done. Let's talk about mistakes.

(This message is displayed on screen as well.)

Scene 2

Narrator: To get it out of the way: Practice on test boards, take breaks, make a plan. Do not brute force. Definitely do not try and focus on soldering for 6 hours without breaks. Obvious stuff.

(TODO: Display this as a point list)

Scene 3

Narrator: Secondly, understand the circuit properly! Had I done this I wouldn't have even had to solder the board. Let me explain.

(TODO: Some clip art maybe?)

Narrator: In the video I drew this diagram:

Diagram description: A diagram showing how the components fit together is shown. DAVdd connects to the LM4041AIM3-1.2 voltage reference. From the 1.22 volt reference a 0.25 nanofarad capacitor and 10 kiloohm resistor in parallel run to DVREF. The 1.21 kiloohm resistor follows from the DVREF capacitor and resistor to the DRSET pin.

Narrator: I also measured these voltages:

Diagram description: The above diagram, but DAVdd is labelled 3.3 volts, DVREF is 1.22 volts and DRSET is 40 millivolts.

Narrator: How is DAVdd 3.3 volts if it's connected to a 1.2 volt reference? How is DVREF 1.22 volts if there's a 10k resistor? What is this capacitor doing here? It makes no sense.

(The video zooms at these various parts of the diagram)

Narrator: It's trash!

(The diagram is dragged to the recycle bin and emptied)

Scene 4

Narrator: When it comes to the part of the board I spend time troubleshooting and soldering, it gets worse!

Pictured: A webcam screenshot of the VGA inductor section.: It contains various surface mount chips, but in the center is a set of 3 small capacitors, 7 inductors, 3 ESD diodes and 7 resistors.

Narrator: I thought this circuit had the video signals travelling through a capacitor, through inductors, through these ESD diodes, and out through these termination resistors.

Diagram description: The above picture but three sets of lines are drawn for three sets of components. Each starting at a capacitor, going through a middle inductor, through an ESD diode and through a final termination resistor. Each of these lines represents red, green and blue signals.

Narrator: I figured that an ESD diode was faulty, pulling a signal line down to GND. This would be responsible for the low voltage.

(The video draws a line from a signal to a GND symbol)

Narrator: Had I just checked the ESD resistors with my multimeter for resistance to GND and diode directions I could've ruled this out.

(A multimeter appears maybe? Not sure)

Narrator: Even then, I decided to remove the capacitors. But these are connected to GND, not the signal. Removing them would not show the fault at all!

(The video draws a line from the top of the capacitors to GND)

Narrator: I was doomed from the start

(The diagram is dragged to the recycle bin and emptied)

Writing code

• Entering test mode gives a blank screen
• Datasheet seems wrong: "MBD_MSR_DIAG[18:16] = 101h" should be "MSR_DIAG_VP[18:16] = 0b101"
• DAC status registers always say the DAC is below 0.35V
• Mention memory mapping and how to investigate this

Editing the DVREF required source code hacking? DVREF stuff

What's interesting in the data sheet?

Measurement

I did what I should have done in the first place: Measured when the display WORKS!

• The DVREF shunt looks okay and produces the correct voltage
• The DVREF shunt has a 12nF cap in parallel
• The DVREF shunt has 13k to the 3V3 rail
• The DRSET resistor is 1.21K to GND
• The VGA resistors seem fine and correct values
• When external DVREF is used, the shunt drops to 800mV, the rail somehow drops to 960mV

Shorting between an existing 3.3V rail nearby caused the screen to suddenly display in full brightness.

Repair

I followed the rail and found a blown resistor, but I also wondered if the capacitor was somehow draining. So I removed both. This wasn't a completely correct fix: The bypass capacitor is in parallel to the circuit, it couldn't drop voltage like this normally. Had I measured it I could've confirmed it was 3.3V. The voltage drop here is 2.34V! So there's some in series resistor that must be dropping voltage based on current or use. The burned resistor looks like a good candidate.

Conclusion

At this point I believe the DAC is working, so the problem might be related to clocking.

I added in a fresh clock battery and it had no effect.

Things to try:

- BIOS flashing

- Fix network