dennisE wrote on 2025-07-13, 12:05:

How do I check the voltage at pin 24 of the HD146818? I understand that I connect pin 24 on one side of the tester, and the other side (battery or power supply)?

You need to measure between pin 24 and GND (ground plane). For GND you can pick pin 12 (opposite corner of 24) or perhaps one of the black wires in any spare PSU connectors. Using the pins directly will give you more accurate result but with decent PSU wires even using a plug far away from the chip the difference shouldn't be more than 0.1V anyway.

dennisE wrote on 2025-07-13, 12:05:

If I put in an external battery, for example, 3.6V, will it be enough to work?

It should be enough, but some older mobos required 6V external pack, and had more than 1 diode that caused voltage drop. These are rare though. The 3xAA pack will give you 4.5V which is a perfect value, high enough to ensure good chip power supply, low enough to not be drained if the mobo has just 1 diode (unlike 6V pack would be). Cheap, comes with wires, preferably look for one that already has the pin-compatible connectors.
Note you can't just connect a battery in place of the original rechargeable NiCD without modding the power supply path there. If you just do a 1:1 swap the mobo will force current through the new battery, trying to recharge it, and it will eventually cause the battery to fail - bloat and/or leak. That's another reason why I don't like doing CR2032 mods to these old mobos, it's not as simple as it looks.

Deunan wrote on 2025-07-13, 12:48:

If you just do a 1:1 swap the mobo will force current through the new battery, trying to recharge it, and it will eventually cause the battery to fail - bloat and/or leak.

And that's the reason why I claim, that his CR2032 battery "modification" is inappropriately done and I gave him a link and a picture to see how it should be done... but the last question about the chip voltage measuring explained everything.

I noticed the battery is a 3.6V LIR2032. It's rechargeable! What do you think? I checked the voltage at pin 24 and it's 3V with the PC off. If I turn it on, it goes to 0V! Is this normal?

Rechargeable is good, it'll probably work OK. There is no charge control or termination on the mobo, it'll just supply about 4.5V but through a resistor to make the current small. The question is if it's small enough for the coin battery, but I would give it a go. With 3V on the chip it should keep the settings, and time, just fine. But the 0V is not normal at all, who did the mod? Maybe they did remove some diodes and now there is no charging but also no power from PSU. The same resistor I mentioned will drop the voltage very low when the chip is accessed by CPU, it might be even connected to the ALS245 bus driver, which is TTL. That will draw a lot of power through the I/O pins, sagging the battery-only power rail.

I think before messing with this any further you need to know exactly what the battery mod does here.

Before mounting the motherboard in an AT case, it worked fine. It wasn't until after mounting it that the message appeared. The RTC is probably faulty.

I was wrong when measuring the RTC voltages! The voltages are correct:
When it is on, there is 4.8V between pin 12 and 24.
When it is off, about 3.2V.
So the RTC chip seems to be correct. The problem must be internal!

Other question, I have used the JP20 pins (power LED and Keylock) and the LED lights up by connecting pins 1 and 3. Will the Keylock be the two remaining pins 4 and 5? Best regards.
Best regards and Thanks very much

dennisE wrote on 2025-07-14, 04:33:

Before mounting the motherboard in an AT case, it worked fine. It wasn't until after mounting it that the message appeared. The RTC is probably faulty.

It could be, but usually the problem is the corrosion from the original NiCd that damaged the mobo. Did you clean it all up? Sometimes it requires both chemical and mechanical cleaning to even properly see the extent of the damage. Could be a weak trace or via that got completly broken when the mobo was flexed during installation in the case. A broken data line to the RTC chip will make it look like the chip is glitching for example.

dennisE wrote on 2025-07-14, 04:33:

Other question, I have used the JP20 pins (power LED and Keylock) and the LED lights up by connecting pins 1 and 3. Will the Keylock be the two remaining pins 4 and 5?

That's how it is usually wired. The last pin should be GND, you can check that with ohm meter vs the black wire in the PSU cable, should be very low resistance between the two. If so the very next pin is indeed KB LOCK input. You can just put a jumper there if your case doesn't have a key lock.

Will any disaster happen to the motherboard if I connect the key lock to the two pins if they are not those JP20?

Solved!: I checked with an ohmmeter on pin 5 and a black wire from the power supply, and the resistance was very low. I connected the Key Lock and successfully tested it. Thanks very much Deunan!

Regarding the RTC failure, I'm giving up on it. I can still use the PC. Thank you so much for your help. I appreciate it.

Other question, Can the RTC chip be replaced with a new one? Or is it too complicated?

That depends on your skills, can you desolder the old chip without causing any PCB damage? If so it's not complicated at all, except there are no "new" RTCs of that type. Best you can do is buy a NOS part, or even used/desoldered one, on ebay. Also you'd want 146818A if possible. Replacing what you already have with non-A variant from mid-'80 is not a great idea. I will work but will drain the battery even faster.