Maybe some of the address lines from the board are broken and it can only address a certain amount? Busted caps I wouldn't think do this, but you can try replacing them if you want to. Does it do this with less than 2/3MB SIMMs?

I got some more information from the guy who gave it to me - could it be a bad BIOS? He said that when he first got it it gave him a bad BIOS beep code, and then started half-working as it is now. Is there somewhere I can obtain another BIOS to test?

What board is it? It may be possible to chase down another BIOS, or someone may have one lying around.

Did you check for corrosion on the BIOS DIP socket and try different densities and configurations of RAM? Although, I have my money on obtaining a freshly flashed BIOS.

feipoa wrote:

Did you check for corrosion on the BIOS DIP socket and try different densities and configurations of RAM? Although, I have my money on obtaining a freshly flashed BIOS.

Sorry for the late replies! I just looked and everything seems fine. I've tried different RAM configurations.
I have no idea as to the model of the board unfortunately, I found something kind of close on TH99 that had all the jumpers in the same places and named the same, but it was listed as an unknown model.

The BIOS chip "AMIBIOS American Megatrends 486DX ISA BIOS (c) 1993", and also says "AB6118613" on it. The chipset looks to be made by "PC Chips" or "IPC Chips" - the logo is kind of ambiguous.

This is the closest board I can find: http://arvutimuuseum.ee/th99/m/U-Z/32303.htm

Of some note is that I had this board originally and, if I recall right, it failed to work if a TAG ram module or cache was installed.
I think, I'm actually unsure but that might help find a solution.

New information gained from tonight's tinkering session!

Sometimes, when I turn the system off, it refuses to come back on (no video) for ~5 minutes no matter what I do, then after letting it sit it magically starts working again.

Could this be bad caps?

Kinda smells like it, yeah.

486 VLB sounds like electrolytics don't play much of a role (the high-capacity electrolytic caps came later).

What you can do:
- re-seat any socketed chips
- clean all sockets, connectors, plug ISA cards several times
- have a thorough look at traces and chip pins for damage (resolder them if you have the equipment)
- in case the board is rather dirty, brush it with soap and water, after that dry in the oven around 80°C
- bend the board or ISA cards while switching on, check if that makes a difference, like freezes immediately when put under mechanical stress

AppleDash wrote:

New information gained from tonight's tinkering session!

Sometimes, when I turn the system off, it refuses to come back on (no video) for ~5 minutes no matter what I do, then after letting it sit it magically starts working again.

Could this be bad caps?

That is a symptom of bad caps, yes, though that's not the only potential cause.

Visually inspected the system, cleaned everything I could, nothing is damaged or dirty. Replaced every capacitor on the thing and that did not help (did not hurt either).

Maybe the bad caps aren't located on the board but rather inside the PSU ?

I remeber that my PCChips 486 had stability issues. Once I recaped the PSU, it worked flawlessly. Mu 8088 crashed after a few minutes of usage. Once the PSU was recaped, again, problems gone !

To find out which board you have, you should rather look to the POST string. The POST string is located at the bottom of the screen where the BIOS counts the ram.

I've tried with 2 different known-good PSUs. There's no useful information on the POST screen I'm afraid 🙁

I've just had a strange RAM behavior on 486 VLB motherboard.

It's ECS UM486V rev. 2.0 motherboard that has 8 30-pin connectors for memory. Specially for that case I bought 8 sticks of 1MB SIMM30. 4MB SIMMs are hard to find and expensive. All of these RAM modules have 3 ICs on them but from different manufactures.

The seller gave me 16 modules in total to check all of them.

8x1 MB Fujitsu chips (2x70 ns, 1x60 ns parity)
4x1 MB TI Chips (2x70 ns, 1x70 ns parity)
4x1 MB TI Chips with third parity chip having AM marking (all three are 70ns).

TI/AM chips caused Parity errors on different stages of loading process, seems like all of them caused problems as I tried them in different combinations.

Fujitsu and TI modules are working great BUT only 4 of them. When I install 8 modules the computers sees only 4 MBs of RAM. I tried all possible combinations and that's definitely not a RAM problem.

I found a solution for that. I have a 386 m/b with 4x1MB GoldStar modules installed. They all are 9 chips and 80 ns. I took these 4 modules and put them to the second bank and it worked! I'll play with it more, now I have plenty of combinations as I don't understand what causes that - amount of ICs of IC timings but at least I don't have to search for other RAM chips.

rkurbatov wrote on 2022-01-23, 16:27:

I've just had a strange RAM behavior on 486 VLB motherboard. […]
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I've just had a strange RAM behavior on 486 VLB motherboard.

It's ECS UM486V rev. 2.0 motherboard that has 8 30-pin connectors for memory. Specially for that case I bought 8 sticks of 1MB SIMM30. 4MB SIMMs are hard to find and expensive. All of these RAM modules have 3 ICs on them but from different manufactures.

The seller gave me 16 modules in total to check all of them.

8x1 MB Fujitsu chips (2x70 ns, 1x60 ns parity)
4x1 MB TI Chips (2x70 ns, 1x70 ns parity)
4x1 MB TI Chips with third parity chip having AM marking (all three are 70ns).

TI/AM chips caused Parity errors on different stages of loading process, seems like all of them caused problems as I tried them in different combinations.

Fujitsu and TI modules are working great BUT only 4 of them. When I install 8 modules the computers sees only 4 MBs of RAM. I tried all possible combinations and that's definitely not a RAM problem.

I found a solution for that. I have a 386 m/b with 4x1MB GoldStar modules installed. They all are 9 chips and 80 ns. I took these 4 modules and put them to the second bank and it worked! I'll play with it more, now I have plenty of combinations as I don't understand what causes that - amount of ICs of IC timings but at least I don't have to search for other RAM chips.

Is it possible that 4 of the simm slots are for FPM simms ? and the other 4 are for EDO ?

Its also possible you have some simms with parity chips and some without, its possible they wont run together.

TrashPanda wrote on 2022-01-23, 17:24:

Is it possible that 4 of the simm slots are for FPM simms ? and the other 4 are for EDO ?

I have no idea if you can run both types together.

Doubt there were 30 pin EDO modules. At least m/b manual doesn't mention that. Anyway, all modules I have are FPM ones, I checked. Also, all modules have parity bits.

The problem is mobo doesn't see 8 modules of the same type (and even the same party), seeing only first 4MB. But it sees 8MB if I use two sets of chips while second set is 9 chip FPM. That's unexpected.

I don't have another set of 9 chip modules, I would have checked it too.

rkurbatov wrote on 2022-01-23, 17:34:

TrashPanda wrote on 2022-01-23, 17:24:

Is it possible that 4 of the simm slots are for FPM simms ? and the other 4 are for EDO ?

I have no idea if you can run both types together.

Doubt there were 30 pin EDO modules. At least m/b manual doesn't mention that. Anyway, all modules I have are FPM ones, I checked.

You can find scads of them on eBay, could be they are mislabelled but they seem pretty legit (4mb 30 pin EDO simms without parity), the seller im looking at has sold 288 of them and still has more than 10 available.

How about 16mb EDO 30 pin simms for 15 USD ?

I'm guessing this seller makes them or has access to someone who does.

Thats weird as heck, sounds like there are a few dodgy IC's on them simms the seller gave you.

If it's about Keystron that looks like modern replacement. That's good it's possible to find such a thing, but for my 486 I am completely satisfied with 8MB. I wanted 16MB for my late 486DX4-100 so badly but now I just try to recreate something balanced. Don't see much sense in 32MB DX2-66, there is nothing to fill it with 😀