Hi, good evening!
Does the board have got a "clear CMOS" jumper, perhaps ?
If it is missing, or in the wrong position, your CMOS will last only as long as the caps can hold it.

I'd say the CMOS jumper as well, but it may be also either there's a diode gone bad for the CMOS (I had COUNTLESS of ASUS boards suffer from this.) or it may be as simple as a bad battery holder in which case harvesting another from a dead motherboard should get it back up and running.

Also, recap that thing ASAP. I had one and Hermei's green little buggers were the cause of its death.

Jo22 wrote:

Hi, good evening!
Does the board have got a "clear CMOS" jumper, perhaps ?
If it is missing, or in the wrong position, your CMOS will last only as long as the caps can hold it.

Hi! Yes, it does have a clear CMOS jumper and it is set correctly.

PcBytes wrote:

I'd say the CMOS jumper as well, but it may be also either there's a diode gone bad for the CMOS (I had COUNTLESS of ASUS boards suffer from this.) or it may be as simple as a bad battery holder in which case harvesting another from a dead motherboard should get it back up and running.

Also, recap that thing ASAP. I had one and Hermei's green little buggers were the cause of its death.

Regarding the caps, unless I'm missing something my board does not use Hermei caps, it uses a mix of Sanyo and Tayeh, which are somewhat okay I guess, but after 20 long years of service definitely warrant a replacement. Unfortunately, like I said, I lack the equipment to do so.

Could you elaborate a bit more on the diode thing? How did you figure that one out on the ASUS boards? I'll check the battery holder, although it seems to be fine. My bigger concern is why the system isn't holding its settings while plugged in, the battery should only be making a difference whilst it's unplugged.

F2bnp wrote:

Jo22 wrote:

Regarding the caps, unless I'm missing something my board does not use Hermei caps, it uses a mix of Sanyo and Tayeh, which are somewhat okay I guess, but after 20 long years of service definitely warrant a replacement. Unfortunately, like I said, I lack the equipment to do so.

Could you elaborate a bit more on the diode thing? How did you figure that one out on the ASUS boards? I'll check the battery holder, although it seems to be fine. My bigger concern is why the system isn't holding its settings while plugged in, the battery should only be making a difference whilst it's unplugged.

Tayeh should be replaced, Sanyo is good.

The CMOS circuit usually has a 1N4001 (or SMD equivalent) that keeps the battery's settings alive EVEN after killing power from the PC. Although from what it sounds, maybe the RTC circuit has gone bad, since usually even if the diode is dead, the system will act like there's no CMOS battery and keep settings until 5vSB is killed by unplugging the PSU. Not sure what can you do if it's the RTC that's dud.

I had this issue with a Socket 7 board. I presumed it was a battery issue and replaced the coin battery, but did not resolve it. I then went through troubleshooting every possible cause including even the BIOS chip. It turned out to be the battery; it had leaked ever so slightly and corroded the contacts on the battery holder. A little bit of filing and rubbing alcohol later it worked flawlessly.

Thanks, I'll have the above in mind. Perhaps I can get somewhere now, I'll look into it and report back with results.

Update: I felt like playing with this a little bit and here's what I found out:

- Diode is fine

- Battery holder on the other hand produces odd results. I'm not getting any voltage between the two poles, measured from rear of the board. Individual points however seem to be fine!? Anyway, I went ahead and soldered two wires on the back of the board directly to the battery holder poles and then attached a battery there. Afterwards, I checked for voltage on the front side of the board and battery holder and voila, we have some progress.
Unfortunately, this made 0 difference in the end, as I still can't save settings.

-Next up, I figured that the 5V EEPROM should be receiving about 5V during normal operation and around 3V when the power is off. Measuring between the VCC and Ground pins, I get ~5V (4.65V) during normal operation a.k.a. when the system is powered on, but I'm getting only 0.08V when it's powered off, so no power at all. This is even with the PSU connected.

Any thoughts, other than the obvious recap that must be done here 😜?

Your real-time clock and CMOS RAM is incorporated in the VT82C586 (southbridge?) chip, you can find the datasheet here. It should receive battery power on pin 102 (VBAT), but if i understood correctly and your board isn't keeping settings even between "warm" restarts (without being powered off), then it means the problem lies elsewhere.

Proceed with extreme caution if you are going to measure voltages on that chip, pins are extremely close, you should use a needle probe which can slip very easily between two pins and that could render your board useless (been there, done that 😁 ).

Many thanks for the datasheet and info!

Okay. I looked at some photos of a DFI P5BV3+ -- and I'm surprised no one has asked...
The board takes either/or AT or ATX PSUs.
- Which one are you using?
What you describe (the voltages) would be normal for an AT PSU and no (or a dead) CMOS battery.

An AT PSU has no standby rail so when it's off it's totally powered down. There is no charging going on when it's off.

Most ATX boards (most then and maybe all today) don't charge the battery either.
Rechargable batteries on mobos slowly faded away about the S7/SS7 era. (Except Laptops maybe.)
They don't even use rechargable batteries anymore. CR2032 are not rechargable.
You need LIR2032 if you want the rechargable version.
C=Lithium, LI=Lithium-Ion, R=Round, 20=20mm diameter, 32=I dunno

Rather than charge the battery the strategy became to unload the battery with circuitry that shifts to taking CMOS power from the standby rail when it is powered up. Probably by a simple resistor and diode auctioneer. Since the batteries rarely ever power anything they last a long time.

Yes. look at your caps.
PcBytes is correct. Tayeh was a crap brand with no website and no published datasheets.
No one has been able to figure out who the company was or even the country they were from.
I have seen a few Tayeh with Teapo vents so maybe they were rebranded Teapo or even rebranded "whoever is cheap this week".

Capacitors will become a big deal for anyone into retro from here forward.
The right end of the bathtub curve (of reliability) for Aluminum Electrolytics starts rising sharply at about 15 years old.
I got that info from multiple industry sources including Nichicon and or Chemicon. (Don't remember which. Maybe both.)
It's not like they hit 15 years and all pop. That's just when the failure rates start going up rapidly.
With SS7 boards (around 20 years old) even quality Japanese caps are past the reliable part of their lives.
So... IMHO...
If you are going to be into retro PC's then you need to learn all about caps. (And replacing them.)
Antique radio buffs have to learn all about tubes. Caps will become the retro PC equivalent.
.

First off, a small update, I took a look into the datasheet and indeed found the VBAT pin to be pin 102. I measured the voltage on that pin and... there's nothing there. Zero. Double checked that the battery was still on and I was able to measure 3.10V on the battery holder so it was working.

Now, to answer some of PCBytes' questions:

I'm using an ATX PSU and have also set the jumper appropriately. Out of curiosity, I have tried this motherboard with an AT PSU as well, no dice, not that I expected there to be any difference.
You then proceed to talk in length about rechargeable batteries, I'm not sure why, perhaps I didn't phrase something properly? Anyway, I'm using CR2032 batteries for all my motherboards.

I agree about the caps, I never said otherwise. I just don't have the equipment necessary to do proper recapping and I don't have any money to spend on that at the moment. If I had a desoldering gun and proper soldering station, this would be a piece of cake.

I'm Bonez, Bytes is the other PC. 🤣

Okay so,,, with it powered down you have voltage at the holder but nothing at the pin.
Have a DMM? [Edit] Actually an analog meter would do.
You need to manually trace the circuit between the + side of the holder and the pin.

DMM == Digital Multimeter? If so yes, this is how I've been getting the readings. I'll give it a god I guess, I'll see if I can start from the VBAT pin and see where it ends up.

You'll probably need a combination of both of these techniques going back and forth.
1] Start at the battery checking voltage to ground until it's not there anymore.
2] To figure out where traces go use resistance (continuity) checks from point to point at component pads or vias.
-
You may see some traces that go to a little dot and disappear. Those dots are vias.
Vias are more or less traces that go through the board to the other side.
Sometimes they go to traces between the board layers. Those are a pita to trace out. Hope you don't find any.
-
It helps if you draw a little picture of the circuit as you trace it out.
.
Oh, and don't forget to verify the ground side of the battery holder has continuity to the mobo ground plane.
The PSU connector ground pins or the solder rings around mounting holes are usually good points to check continuity to the ground plane.

Ok, out of curiosity i've checked this on two S7 boards from my stash that use the same chipset, Azza PT-5VMD and LuckyTech P5MVP3. On both of them i have measured around 2.85v at pin 102 (board not powered up, not connected to anything, only the battery fitted), and in both cases there is a SMD diode in series, which drops only around 250mV - so maybe a Schottky diode, or maybe a standard one and the voltage drop is so small due to the very small current flow. Find this diode on your board, it should have the full battery voltage on the anode, and a little less on the cathode.

Edit: looking at a picture of your board, i believe it's the diode that sits right between JP2 (clear cmos?) and the cpu socket. Check that diode, the resistor next to it, and i would also check for voltage on pin 102 with the cmos jumper in the other position (with board powered off!).

Wow, that's immensely helpful! Thank you for taking the time to do this, I'll give it a go tomorrow.

Hello again, I tried to find the diode you mentioned, but I can't seem to. The only obvious one that I see is next to the battery and CMOS jumper, which seems to be working fine.

You mentioned that you saw a photo of the board online and spotted the diode in question between JP2 and the CPU socket, but since the board had a few revisions, perhaps you saw a later one. JP2 sits right between two PCI slots in my revision and configures whether or not you're using an ATX or AT power supply. It's quite a ways from the CPU socket and I can't see anything blatantly obvious here.
I'm uploading a close-up of my own board perhaps you could spot it for me?
This revision is also pretty close to mine ( I have Rev. B3)

Many thanks in advance and for all the help so far!

I'm sorry, for reasons i cannot even imagine it seems that yesterday i was looking at a completely different motherboard (Shuttle HOT591P).

Now, being certain that i'm looking at your board, i guess that D1 is the diode in question. With everything disconnected, only the battery inserted, and keeping the negative lead of the multimeter on a ground pad, can you check if you have the battery's full voltage on the anode of D1, and somewhat less, around 2.8v on it's cathode? If not, try this with the clr_cmos jumper in the other position.

That's quite alright.

Unfortunately, I'm getting the full voltage (3.1V) on both the anode and on the cathode, with the jumper making zero difference on either position (and completely removed). What am I to make of this?