JayAlien wrote on 2024-11-21, 23:08:

1. Upon boot there is a memory check, and reports '1664 kb ok' - I have 2 x 1mb simms in there, so this seems off

This is probably OK, sometimes the BIOS does not report the 384KB of memory between 640KB and 1MB (High Memory Area) depending on how certain settings are configured (such as ROM shadow and the like). If the BIOS settings on your PC got reset to defaults, that's probably the cause.

JayAlien wrote on 2024-11-21, 23:08:

2. After the memory check I get 'CMOS battery state low' - this is a barrel battery I replaced about 18 months ago - multimeter gives a stable 4 volts (battery is a 3.6v 80mAh)

Since you mention that the settings don't stick even after a soft reboot, the RTC chip may not be getting power at all. Does the clock tick properly when you are in the BIOS setup? A photo of the motherboard would be useful to identify the chip and troubleshoot further.

Thanks for the reply. I've attached some images below.
Yes, the clock ticks in the BIOS settings. Interestingly the clock settings are retained on soft-reboot, but if I add my disk B: as the 1.44mb 3.5 inch, that isn't retained. Nor are any settings for the HD that is in there. My 5.25 inch A: drive settings are retained though.

Odd, normally you see something like a CR2032 - non-rechargeable and they do "go bad" every few years...
But I don't see one here ... I guess the green 3-cell rechargeable is the RTC/CMOS offline power.

Assuming the systems been in regular use, this pack should be charged.
There may be a jumper to disable charging, but if you've had the system for some time, you should have seen problems...

If you have a multimeter, check the pack (each end should be a pole) with both power-on and off...
It should be around 3.6v (each cell is 1.2 x3 = 3.6)

If not, and you've left the system power-on for 5-10 mins (should be enough to
charge close to rated voltage) the pack itself could have gone bad.

If you are ok working on stuff like this, there are some techniques which
sometimes recover nicad or nmih packs ... let me know if you want more details.

Dave ::: https://dunfield.themindfactory.com ::: "Daves Old Computers"->Personal

JayAlien wrote on 2024-11-22, 00:43:

My 5.25 inch A: drive settings are retained though.

I think that 5.25 inch setting for the drive A is just the default, so it's definitely not remembering any settings.

I found your board on TheRetroWeb. It's a Biostar MB-1316VST-SX:

https://theretroweb.com/motherboards/s/biostar-mb-1316vst-sx

I have marked in red the chip that has the RTC and the memory where the CMOS settings are stored. Can you check if yours is similar to the one in this picture? I have a suspicion your board may be using a Dallas/Benchmarq RTC chip with a built-in battery, in that case the barrel battery isn't even being used, instead you must replace or mod the RTC chip itself.

Also, I see some corrosion from the old battery under the keyboard connector area. I don't think that is causing the problem but it will have to be checked out at some point or it may continue to eat the PCB.

Thanks for the replies so far!

Looks like I have a different rom in there

Poor battery is from 1992

How dare it die now after 32 years of loyal service!
So if this is the issue, is it repairable, or just new new old stock from the 90s 😂

If it's compatible with one of the DS12[C]887[A]+ variants, it's still being made
Alternative is to modify the chip by drilling into it to expose the contacts and disconnect the internal battery, then connect an external one directly to the chip

The first thing to check is if it is in a dip socket. After that you can google for "Dallas RTC mod" for a DIY fix or get an old stock (not recomended) or aftermarket variant (e.g. monotech). If thats the fix, I recommend ripping this awefull green barrel off the board again (notice how I get emotional at this point)

Thanks all, I’ll take another look tomorrow.
One thing I’m not clear on - if this chip has its own battery, what is the purpose of the barrel battery on the motherboard?

If that's an "internal battery" chip (like the Dallas) then the question is..
whats the green rechargeable doing there?
Did it die "years ago" and if so is there a jumper to remove it from the
circuit?

I've repaired a deal Dallas chip - ground the top down to expose the internal
CR2032.. fortunately it was positive side up (full battery width) - There was
no evidence of it having leaked or otherwise corroded...

So I left it in and put a good CR-2032 on top (positive side down to contact).
I also dug down the side to find the negative connection coming out where I
could access it - added a short wire which went to a small plastic disk, passing
though the center and ending in a "solder blow".

Placed that on top (negative side) of the new battery and used some flex
plastic tape (black electrical tape) to hold it in place and give some
pressure to keep everything connected.

(no, I didn't cut the old battery circuit - I have 4.5 digit DVM with lowest current scale 2ma
and I can reliably detect currents of 2ua ... I saw nothing "reverse charging" the old battery!)

This was 32k Dallas RAM inside a home-built 6809 based piece of test equipment
to which I download various bits of software to run various test scenarios.
Often leaving the same test "installed" for days/weeks at a time.

It's been going fine for more than 3 year now!

-- Another options ... replace the "internal battery" chip with a regular RTC,
remove the "green" battery, disconnect whatever charged it (prob just remove a
resistor) and wire in a CR-2032 to power the RTC ... yes, you will have to
replace it every few years (might be worth using a socket).

Dave ::: https://dunfield.themindfactory.com ::: "Daves Old Computers"->Personal

JayAlien wrote on 2024-11-22, 03:51:

One thing I’m not clear on - if this chip has its own battery, what is the purpose of the barrel battery on the motherboard?

The pic posted by "TheMobRules" which is the same board shows a non-internal-battery RTC ... I'd guess that the
barrel on your board died years ago, and someone fixed it by replacing the RTC.

Dave ::: https://dunfield.themindfactory.com ::: "Daves Old Computers"->Personal

Those chips with internal batteries are pin compatible with regular RTCs, so it is possible someone did use one as a replacement when the original barrel battery expired as DaveDDS said.

Also, I have a Biostar 486 board from that era that came from factory with a Dallas RTC but the board still has an empty spot for a barrel battery, so Biostar may have provided both options back then.

Thanks all. So if I'm understanding correctly, I should be able to pull the RTC (I'm assuming this is a socket rather than soldered to the board) and find one of these non-battery powered chips:

and I'll be back to running?

JayAlien wrote on 2024-11-22, 21:31:

Thanks all. So if I'm understanding correctly, I should be able to pull the RTC (I'm assuming this is a socket rather than soldered to the board) and find one of these non-battery powered chips:

and I'll be back to running?

Looks like a socket to me (note how pins go into a raised carrier with copper fingers at the edged rather than into solder
directly on the board).

Assuming your battery is dead (measure and see if it's 3+ volts - if not, make sure there is some voltage when power is ON .. if not
it might be disconnected) - you will need to replace it as well, or use a CR2032 like I described earlier.

Dave ::: https://dunfield.themindfactory.com ::: "Daves Old Computers"->Personal

JayAlien wrote on 2024-11-22, 21:31:

Thanks all. So if I'm understanding correctly, I should be able to pull the RTC (I'm assuming this is a socket rather than solde […]
Show full quote

Thanks all. So if I'm understanding correctly, I should be able to pull the RTC (I'm assuming this is a socket rather than soldered to the board) and find one of these non-battery powered chips:

and I'll be back to running?

For perspective, Digi-Key has new, genuine DS12C887+ (battery inside) for $16.73 plus shipping. They're in Minn. and you're in Wis...

JayAlien wrote on 2024-11-21, 23:08:

2. There is some mild marking around the battery area from a previous battery that I removed when I received the motherboard. It was cleaned up and I don't see any damaged traces.

thats a very peculiar definition of mild 😁 you have corroded pads, corroded components (D1 R8 R9 legs), corroded tracks, and probably even corroded vias under slots.
Start with D1 diode and most likely +5V track (track with R8 printed on it). Edit: oh wait, you got RTC with battery in there 😮
You can cut it open and connect battery https://www.ardent-tool.com/misc/Dallas_Rework.html
https://www.youtube.com/watch?v=IbiczYoVncQ
https://www.youtube.com/watch?v=NdlSfqto_0o
https://www.youtube.com/watch?v=oyNp-u5nl-8

jakethompson1 wrote on 2024-11-22, 22:19:

JayAlien wrote on 2024-11-22, 21:31:

Thanks all. So if I'm understanding correctly, I should be able to pull the RTC (I'm assuming this is a socket rather than solde […]
Show full quote

Thanks all. So if I'm understanding correctly, I should be able to pull the RTC (I'm assuming this is a socket rather than soldered to the board) and find one of these non-battery powered chips:

and I'll be back to running?

For perspective, Digi-Key has new, genuine DS12C887+ (battery inside) for $16.73 plus shipping. They're in Minn. and you're in Wis...

Thanks, is this the correct one?

https://www.digikey.com/en/products/detail/an … DS12C887/956875

thats a very peculiar definition of mild 😁

😀 yes I'm a little optimistic