analog_programmer, are you sure? ESCD are for sure stored in the flash chip. Late socket 7 boards don't have a dedicated RTC chip - on this board there is a peripheral controller (VIA VT82C586B) which is capable of providing RTC and CMOS memory functions (see page 26 of the datasheet, it describes the corresponding registers and page 12 for RTC pin functions). An external RTC chip is still supported but almost no vendor had one to optimize the production costs.

Cyanopsis. you can query PIPC if the voltage is good enough. You can do this in DOS using DEBUG.COM, the command is "-o 70 0d -i 71" (type lowercase 'O', space, number 70, press enter, then type lowercase 'I', space, number 71, press enter), it should reply with 80 if the battery is good or 00 otherwise.

Another thing to check is the RTC crystal (32.768KHz) - check the connections between the crystal and pin 104 of 82C586B and between the battery and pin 102. Then check if the crystal frequency is stable. And finally the voltage on pin 102 and the current draw from the battery.

There's no RTC module/chip on this board for CMOS settings to be stored, but... Yeah, I'm wrong. The BIOS settings are not stored in the EEPROM chip.

kmeaw: My board is currently disconnected from the PSU and everything else, so I'll try that DOS command in a bit. I have located the VIA chip and I've found the documentation. I'm a little out of my comfort zone using a multimeter but there's a stabe 3.3V on the battery pins at least. There's also continuity between the chrystal and pin 104.

So to be clear, if I measure voltage between battery ground pin and RTC pin 102 on the VIA chip, I should be getting 3.3v also? Because I don't get anything at all. Am I doing it right? There should be a way to trace voltage from the battery to the pin right?

Debug command gives me 80, as you wrote.

Cyanopsis wrote on 2024-10-21, 20:06:

there's a stabe 3.3V on the battery pins at least

thats bad, it isnt a rechargeable cell and should read 3V. Weird considering all P5MVP3 pictures on google have a coin cell holder there.

Cyanopsis wrote on 2024-10-21, 20:06:

So to be clear, if I measure voltage between battery ground pin and RTC pin 102 on the VIA chip, I should be getting 3.3v also? Because I don't get anything at all. Am I doing it right? There should be a way to trace voltage from the battery to the pin right?

Yes there should be voltage there, around 3V minus diode drop = ~2.5V with unplugged computer.
Trace battery + terminal as far as you can go, from pictures I could see trace on the back going near the BIOS chip and popping back to front near D3 and another diode. This is where most likely place of the defect. Both diodes should read as some resistance in one direction but no connection when measured other way, or just use Diode mode in the meter 😀

For brand new CR2032 battery a voltage in range of 3.2 to 3.35 V is quite normal.

OK, so let's trace it! 😀

Following the + trace on the battery pin ends up in the circled joint. Measuring voltage between battery -minus and this joint gives me 3.3v. This tells me that the initial trace from the battery holder to the joint is OK. Right? The trace then continues on the other side of the board and I'm losing track. It's somewhere below the GD75232 chip. Is it connected to it or where is it headed? Should I be getting voltage reading on some of the Clear CMOS pins (JCC) using the same method? I mean, the jumper is like a on/off switch for providing CMOS voltage by closing the circuit, right?

Cyanopsis, don't use your multimeter in voltage measuring mode to trace on which via/pad/leg you'll get 3.3 V from the battery. Switch the multimeter to continuity test mode ("beep" on short/connection), power off the board, remove the battery and check if batteryholder "+" terminal is connected to one of the two diodes near the BIOS chip (I can't clearly see, but seems like there are some SMD resistors near these diode - check them too for their connections). Then check the diodes (and nearby resistors) if they're fine (rasz_pl explained how). Also you may trace back if there's connection between JCC jumper and leg 102 on southbridge chip, JCC and one of the diodes, etc.

Some new findings: There's also 3.3v on this joint, super close to the CMOS pin 3. But there's no reading directly on pin 3. Pin 2-3 should be closed in normal operation and in my very narrow understanding of things means that voltage is provided to pin 2 for further transport to the CMOS portion of the VIA chip... Am I getting close?

analog_programmer: yeah, let's switch to continuity! So far, I've only found a trace between battery + and those two circled joints. They are connected on the top side beneath the GD75232 chip. I don't know if its just a bridge between joints or if it's supposed to connect to the diodes in the vicinity because I get no reading on any of them (circled here). Maybe they are covered with coating, I'll do some research on that, but since the trace keeps on going towards the JCC jumpers, I'm wondering if anything really could be wrong here?

Ok, the first marked via connected to batteryholder's positive terminal on the "top" side of the board is under the GD75232 chip and this very same battery terminal has no connection to any SMD diode or resistor, right. Is the second marked via connected to JCC pin 3? Is there a connection between those two vias?

analog_programmer: Let's have a better overview:
1. Battery holder pin + goes to...
2. trace joint and continues on the other side
3. comes around to another joint and goes back to the backside...
4. to another joint just beside Clear CMOS pin 3.

There's continuity between 1 and 4, but nothing on CMOS pin 3.
Then I lose track.

The trace between 2 and 3 is below the GD chip. I can't guarantee there are no other hidden traces between 2 and 3 but I get no reaction from my multimeter in this area. I interpret this "flip flop" bridge as a PCB path design decision. But I will do further testing on the diodes.

The top side of joint 4 is pictured just beside the CMOS pin/black plastic. I don't know where it is going. I will do further measurements. Nobody is thinking there should be continuity on CMOS pin 3?

Voltage from battery always goes thru a diode, and 3.3V from motherboard goes thru another diode, then joined they go to the chipset. Diode is a one way valve, lets power pass one way, those diodes are tehre so that power from motherboard wont flow back to the battery and so battery doesnt try to power whole board when there is no power.

Cyanopsis wrote on 2024-10-22, 08:29:

chip. Is it connected to it or where is it headed? Should I be getting voltage reading on some of the Clear CMOS pins (JCC) using the same method?

those red glass cylinders with a stripe are diodes, there are at least two near the clear cmos jumper in the corner, D10 and D5? D6? test those in Diode/continuity test mode of your multimeter. Test = putting probes one way and thene in reverse, meter should only beep in one orientation.

Cyanopsis, you have connection from battery positive terminal to via 4 (you call it "joint", but it's "via" - a conductive through hole). Check for connection from via 4 to two diodes (D8?? and D10) and resistors (R72, R14, etc.) next to it, and also from JCC pins to same diodes and resistors.

Did you check for connectivity from SB chip leg 102 to one of the JCC pins (probably pin 2)?

I guess that JCC pin 1 is ground (or unlikely not connected).

Thank you both! I did some very quick check for continuity on 2x2 diodes (located near bios and near CMOS) and yes, I get a signal one way but not the other on all of them. My multimeter doesnt beep, but gives me a value in the hundereds. I will have to wait a little while until I check VIA 4 as per your suggestion, analog_programmer. I will also get better close up pictures of these areas for those helping me. Thanks again! Much appreciated!

Ok, seems like all these four diodes are fine and healthy. There's no need to waste more time in checking those SMD elements near BIOS chip - obviously they're out of the battery circuit. Focus on connections around "via 4", JCC pins, D8, D10, R72... and check for connection from JCC pins to southbridge chip leg 102.

Ok, let's see if you agree with me. The VIA that I'm pointing at is number 4. There's connection between this and battery +. The (funky looking) trace is connected to R72. There's NO connection between these two. Do you agree that it should? Are we seeing a broken trace?

Following the trace on the other end of R72, it passes underneath D6 (there's a VIA here as well, I'll come back to it) and also underneath C25. It then turns right, slightly down and then disappears under the JCC pins. Measuring JCC PIN 3 on this trace gives me continuity from R72!

The VIA near the middle of D6 mentioned above connects to D6 on both ends. I get continuity between JCC PIN 3 and the right ends on both D6 and C25 from this.

As far as I can tell, JCC PIN 1 is ground. I get continuity between this pin and board ground as well as PIN 101. I can't however get continuity between PIN 102 and any of the circuitry mentioned above. That pin is hard to trace backwards.

The picture is still incomplete, but I think the main problem is missing connection from SB chip leg 102 to JCC pin 2. As I supposed JCC pin 1 is ground and JCC pin 3 is +3 V.

Leg 101 on SB chip is USB Differential Output Ground and it's out of the scope.

This is what I've managed to understand from your explanations for connections:

Yes, that's exactly how I'm interpreting this trace. However, there's no connection on the first path to R72. It is and it looks cut. If you don't oppose to the idea, I am going to solder and fix this connection.

That should fix the connection to PIN 3. And with the jumper set on 2-3 there should be continuity from battery + to PIN 2. But I don't know how this trace continues towards PIN 102 on VT82C586B . I don't get a reaction when I try that on my multimeter. But I have to fix what is obviously a broken trace first.

The yellow line represents the broken/cut trace by the way, not some new connection to another resistor.