Settings are stored in the RTC I believe.. but some bioses I think had the ability to rewrite themselves, there is a thread around here about one that kept corrupting its self during that if I recall.

Anyway, battery! Sweet!!! Great job dude!

Yeah, definitely wanna remove that 5v power source. Measure the socket powered on to see (sure you already checked though 🤣)

Btw,
I bridged a bad trace under there, wasn’t too happy about the adhesion. Stupid alkaline damage.. Did it look okay to you?

tony359 wrote on 2024-01-11, 21:45:

Remind me: where are the bios settings stored??

The UMC UM82C206L chip has a small amount of memory and can switch between PSU and battery power when you turn off the system, so that's where the CMOS settings are stored. That chip also provides a real-time clock (there's probably a 32.768KHz chip nearby connected to it, the little can near the keyboard controller).

Other boards have a dedicated RTC chip with integrated battery like the Dallas DS12887, which also provide some memory for storing the settings.

Interestingly, I also have one 286 board where only the RTC chip is powered by the battery, and the CMOS settings are stored in a flash ROM, but that's not a very common design I think.

By the way, that UMC chip is a clone of the CHIPS 82C206, which was commonly used as an integrated IRQ/DMA controller + RTC on many 286, 386 and 486 boards. This datasheet can help you troubleshoot problems with the CMOS settings and other stuff, and has diagrams for the battery backup circuit:

Sphere478 wrote on 2024-01-12, 00:58:

Yeah, definitely wanna remove that 5v power source. Measure the socket powered on to see (sure you already checked though 🤣)

That's the removed diode D2 😀

tony359 wrote on 2024-01-11, 21:45:

wooo, maxi spoiler here! You ruined the surprise! 😀

Hi, Tony! Should we expect a video with the repair job of this motherboard? I really like your videos with the repairs of old PC hardware.

Hi - thank you and yes, I'm planning this to be the next one after the one that - hopefully - goes live today!

Video is live 😀

https://www.youtube.com/watch?v=yVg2pvyYbn0

tony359 wrote on 2024-01-26, 17:35:

Video is live 😀

https://www.youtube.com/watch?v=yVg2pvyYbn0

Thank you, Tony! I've already watched it and as always it was fun 😀

Good job. Sphere478 must be pleased with this repair.

One suggestion for the two diodes in the circuit for external and rechargeable battery: You rightly removed the first diode to break the +5 V trace for original battery charging, but also you could have easily bridge (with some very short piece of wire) the second one, thus avoiding the voltage drop form freshly installed substitute 3.3 V non-rechargeable "coin" battery.

I thought about that just yesterday: what is the purpose of the diode? It prevents voltage coming from the rest of the board from reaching the battery I guess. In case someone plugs something on the external battery connector I guess?
I mean, it's unlikely but probably a "good to have".
If the battery lasts what it's supposed to last, I wouldn't worry, otherwise it's definitely something to try!

tony359 wrote on 2024-01-26, 20:06:

I thought about that just yesterday: what is the purpose of the diode? It prevents voltage coming from the rest of the board from reaching the battery I guess. In case someone plugs something on the external battery connector I guess?
I mean, it's unlikely but probably a "good to have".
If the battery lasts what it's supposed to last, I wouldn't worry, otherwise it's definitely something to try!

Theorethically I agree with you. But since the external battery 4-pin connector will be unused (as it always was), there's no problem to short the pads for the second diode in this circuit. I thing this will prolong a bit the coin-cell battery life.

I agree, those diodes are there for "just in case". Sphere will let us know 😀

Board should still be "in transit" 😀

Well, I guess the question is at what point does the rtc stop functioning (at what voltage)

And then, need to examine the discharge curve of the coin cell in a data sheet) if we are talking about a overlap of .5v after the cell’s voltage is in freefall in the last few percent of its capacity. Then it isn’t worth worrying about. But if it is cutting out 50% of the usable battery capacity… then we should short it. So I guess I should try it out with some almost dead batteries and see how it does

Just watched the video and I am so happy I sent it to you, you did a perfect job on it and it could not have been in better hands! You had exactly the tools needed also! I think I will be saving up for a microscope soon and those tweezers! OMG I need those haha!

When those traces started moving I tried my best but with two moving targets, without the right tools it was a loosing battle. Let alone without a microscope, or your skills 😉

Tony, you are amazing and a master of your craft and I salute you.

I totally forgot about those very-fine-tip tweezers... Guys, where I can get one of these?

tweezers good enough for phone repair (0201 smd parts)
https://ipadrehab.store/products/our-favorite … -rehab-tweezers
https://ipadrehab.store/products/custom-hooked-tweezers
https://ipadrehab.store/products/gold-noble-superb-tweezers

Thank you Sir - I don't think I did anything special, it's "easy" with the scope and the right tweezers!

I was recommended those tweezers by another youtubers, Bits und Bolts - the link is in the description of the video. Yes, the listing looks bad (features: they don't explode!) but they seem good quality. I haven't tested them long enough though.

I hope everything is still working by the time you get it back! 😀

I was sceptical about buying a scope - but it's now a primary tool in my shop. They're not expensive. I would avoid spending money on expensive ones (from China) as I don't think you're getting any better quality. Either you buy a cheap one - which IMHO are good enough - or you buy something proper from a proper seller.

Oops, I missed this since I rarely read any descriptions for the videos on YT 😀 Thanks for the link, Tony, I found the same twizers even cheaper from another seller on AE.

For those chineese digital microscopes I can only confirm your opinion.

tony359 wrote on 2024-01-26, 20:06:

I thought about that just yesterday: what is the purpose of the diode? It prevents voltage coming from the rest of the board from reaching the battery I guess. In case someone plugs something on the external battery connector I guess?
I mean, it's unlikely but probably a "good to have".
If the battery lasts what it's supposed to last, I wouldn't worry, otherwise it's definitely something to try!

I would be careful and measure the voltage at the cathode ("output side" of the diode) while the board is turned on before removing it.
This point typically gets connected to +5V by the famous two transistors once the board is powered up.
Here's the explanation: https://www.eevblog.com/forum/projects/ibm-pc … rcuit-question/

Excellent video (as always), I am always looking forward to the next one!

watson wrote on 2024-01-27, 15:47:

I would be careful and measure the voltage at the cathode ("output side" of the diode) while the board is turned on before removing it.
This point typically gets connected to +5V by the famous two transistors once the board is powered up.

You didn't watch the video carefully, the +5 VDC battery-charging line was broken by removing of one of the diodes.

analog_programmer wrote on 2024-01-27, 15:53:

You didn't watch the video carefully, the +5 VDC battery-charging line was broken by removing of one of the diodes.

I was referring to the other diode that has not been removed.
Removing the one for the charging circuit is fine.