dominik0801 wrote on 2020-05-14, 18:12:

Then I started measuring voltages, and noticed, that the -5V line has -6.80V coming out of the PSU.. so I switched to another, which had the right output voltage, but that just didn't help.

-5V level is unlikely to cause any problems as it is only used by less common expansion cards. But might be a sign that the PSU isn't healthy.

dominik0801 wrote on 2020-05-14, 18:12:

This is where I found the configuration: https://stason.org/TULARC/pc/motherboards/S/S … SA-VER-4-2.html

How should I jumper the board, and which RAM slot should I use, if I only want to use a single 16MB 72pin memory?

Looks like you need DRAM setting 1 and the module in the first slot.

As I side note. Having performed major traces and component level repairs and not knowing if your RAM, CPU or PSU is working seems to me like you are trying to hit way too many birds with a single stone. There are so many moving parts. I'd suggest to simplify the problem and troubleshoot board with components that are known to be working. Otherwise it can be be one big frustration to find out what is wrong.

Thank you. Yes, I know that I am not making my job easier, but unfortunately this is the only CPU I have atm. Maybe I can get another one soon.
I have more RAM, and they are known working ones. However, the next PSU I'm gonna try will be an ATX to AT Codegen 300W one, it worked last time, so hopefully there won't be any problems with it now either.

Tomorrow I'm gonna try the DRAM configuration you mentioned, and also the PSU.

If I can get some beeping, I will be already happy.

I am a bit afraid btw, that the BIOS could be bad, since I don't have a board which I could use to hotflash this chip in.

BIOS chips from that era are usually UV-EPROM, so not hotflash-able anyway.
You need to jumper the board for Cyrix (ST, TI and IBM manufactured their chips). I think M7 is the Cyrix DX2, so yours should be the M6.

quicknick wrote on 2020-05-14, 19:56:

BIOS chips from that era are usually UV-EPROM, so not hotflash-able anyway.
You need to jumper the board for Cyrix (ST, TI and IBM manufactured their chips). I think M7 is the Cyrix DX2, so yours should be the M6.

Well, then I definitely jumpered it the bad way, thanks for the information!

By the way, my BIOS chip: mx27c512pc-15

That's a UV-EPROM. I think you can read it on another board by hot-swapping, but I think you must use a board from the same generation, i.e. not one using a 32-pin ROM.

quicknick wrote on 2020-05-14, 20:19:

That's a UV-EPROM. I think you can read it on another board by hot-swapping, but I think you must use a board from the same generation, i.e. not one using a 32-pin ROM.

I understand, thank you. Tomorrow I will check the board again with the new jumper settings on the CPU and DRAM side.
Hopefully it will be working now.

I will post about the results.

Switched PSUs, RAMs, tried all the mentioned jumper settings, the problem stayed, so I guess it's a bad motherboard or CPU.
I try to find a 486 CPU in my area, so I can further test the board.

Today the new CPU arrived, re-jumpered the board for this particular processor, tried to boot, but unfortunately, everything is the same. The CPU gets warm (this one gets warmer, but I guess because it's a faster one), but that's really all. Nothing else gets warm or show any sign of life.

Switched CPU, RAM, PSU, jumper settings, today I also tried other cache settings, but that didn't help at all. So at this point I think it is the motherboard.. but what could cause this? Bad BIOS? Missed traces that I didn't fix yet? Another chips?

One thing, that I forgot to mention in the first post (but I think it's not even an important information), that the speaker clicks when I turn the machine on/off, also, the 3 keyboard LED flash once when I turn it on.

dominik0801 wrote on 2020-05-20, 11:39:

Today the new CPU arrived, re-jumpered the board for this particular processor, tried to boot, but unfortunately, everything is the same. The CPU gets warm (this one gets warmer, but I guess because it's a faster one), but that's really all. Nothing else gets warm or show any sign of life.

Switched CPU, RAM, PSU, jumper settings, today I also tried other cache settings, but that didn't help at all. So at this point I think it is the motherboard.. but what could cause this? Bad BIOS? Missed traces that I didn't fix yet? Another chips?

One thing, that I forgot to mention in the first post (but I think it's not even an important information), that the speaker clicks when I turn the machine on/off, also, the 3 keyboard LED flash once when I turn it on.

Also rule out any shorts, use a multimeter on the AT pins. Take the battery off the motherboard (I don't know if you have a dallas type battery or coin cell). Put the meter in continuity mode and test the middle AT ground pins on the motherboard (hold the black probe against one ) and with the red probe touch the +5v, +12v, etc voltage pins for continuity. It shouldn't beep on any of the voltage to ground pins. If it does you have a short.
If you haven't already, try a POST card, they are less than $5.

And there is still the possibility of an empty bios EPROM. They can break. Do you have another board or programmer to read the contents of that chip?

AvalonH wrote on 2020-05-20, 13:59:

Also rule out any shorts, use a multimeter on the AT pins. Take the battery off the motherboard (I don't know if you have a dallas type battery or coin cell). Put the meter in continuity mode and test the middle AT ground pins on the motherboard (hold the black probe against one ) and with the red probe touch the +5v, +12v, etc voltage pins for continuity. It shouldn't beep on any of the voltage to ground pins. If it does you have a short.
If you haven't already, try a POST card, they are less than $5.

I measured shorts, there aren't any. The battery was a barrel one, which I already removed before started working on the bad traces.
I now also measured the #RESET pin on the CPU, but it's at 0V, which means it is not held reset.

I didn't try a POST card yet since I don't have one, but I will definitely try to buy one - however, in my country it's like $20 + shipping..

About the empty EPROM: i couldn't find any suitable board yet to read or write this one. Can you recommend me some kind of programmer that could read this BIOS? (mx27c512pc-15)

You need to get one that can read 27 series EPROMs. I have seen some floating around on the interwebs but it's even possible to build one yourself:
http://www.compendiumarcana.com/eprom/

evasive wrote on 2020-05-20, 20:24:

You need to get one that can read 27 series EPROMs. I have seen some floating around on the interwebs but it's even possible to build one yourself:
http://www.compendiumarcana.com/eprom/

Thank you for your answer! I will check it out, but before I build/buy one, I'm gonna try using flashrom with my TNT2 M64, because it has a pin compatible BIOS chip in it. I know I can't hotflash it, since it's an OTP EPROM, but if I get something read out of it, it's already enough for me to begin with.

By the way, the chip on the board, as I already mentioned is a MX27C512. The one on my TNT2 is a NM27C512.
They seem pin compatible, and also, both are 512K. Do you think the NM could be a suitable replacement for the MX?

Because I have another of the NM chip, and since it's erasable with UV, I would try that chip in the motherboard.

Yes they are compatible. I'm almost sure 27C512 EPROMs from all manufacturers are compatible.

Reading it on the TNT2 should work, you can also use a network card. Also, if flashrom fails you can try uniflash.

Building an EPROM programmer certainly has its charm, but I took the shortcut and got the very common TL-866CS a few years ago. Dunno if it's still easily available now, and from what I've heard the new version (CS-II) doesn't support some of the older chips anymore, so it's best to check the compatibility list before buying.

Found an article here:
https://www.classic-computers.org.nz/blog/201 … 0-21-eproms.htm
The "Willem" programmer is still for sale:
https://www.aliexpress.com/item/32809137211.h … searchweb201603_
NOTE: You need some additional hardware, an USB-to-parallel converter or a parallel port add-in card (they come in PCI-Express variety as well) + an additional power supply (I guess this thing cannot reach 21V or something).

Well, today I opened my closet, and built a suitable system that could handle an Ubuntu for flashrom but old enough to have an AGP slot. Sooo it was a P4 2.4GHz with 768MB RAM, ubuntu 11, aaaand its slow as hell. It was fairly hard and long to install ubuntu on it, but it's done. Also installed flashrom, but unfortunately, even tho it sees the graphics card, and it won't recognize the BIOS chip, guess it's not a compatible model -- too bad.

Uniflash? I am unfortunately not familiar with it. I just read about it, and found out that it's compatible with 27C512 chips, but I couldn't find anything about using a network card or gfx card as a programmer. Is it possible with it?
If yes, what version would you recommend me, and what's the suitable OS for it?

And yes, I know, "too much effort for a 486", but I like to fix things and fiddle around with them.

"too much effort for a 486"

BS.

Uniflash seems to support networks cards based on the RTL8139 chip that have a bootrom socket. I guess I'll have to dig up the documentation to see what is true about that. Sounds like it will work, after all it is a matter of finding the right memory address to write on.

I'm not familiar with flashrom, but I guess it is a Linux tool with pretty much the same functionality as uniflash, which is a DOS tool made by Rainbow Software. There's also another uniflash by Texas Instruments, in fact most of the results returned by Google are related to this one, but you'll need the Rainbow Software one.

But this will allow you at most to read the chip (maybe with -FORCE switch, because it will try to detect a Flash chip, and your 27C512 is not one). I don't think there is a PCI card or even a motherboard that is able to program a EPROM as they require 21V or even higher voltage for programming. It wouldn't make much sense either, because the chip cannot be electrically erased.

Ah, now I see the problem, I am sure I found and read about the TI version, that's why I couldn't find too much interesting information about it. 😁
I will read about the Rainbow Software one.

To be honest, I don't try to program the chip, just try to read some information out of it - just to make it sure, that the BIOS is in fact the problem. Because in case the BIOS is alright, I wouldn't like to spend $30+ on equipment that won't take me any further.

Btw, sorry for too much questions and rather unneccesary posts.