I would confirm your suspicions with another person who has performed the upgrade before desoldering.

Hi guys, sorry for reviving this thread, but I have a question regarding 386sx upgrade. It is not related to the Cyrix deiscussion, but to 386sx upgrade topic in general. I have here a nice mainboard, the same one as this Looking for 386 motherboard ID (3SIUD), but with i386sx-16. Just for fun I was experimenting with overclocking and had apparently Am386sx-33 around, so I replaced the CPU. The old 16MHz CPU worked absolutely stable at 20MHz, but refused to work at 25MHz completely. As I can see, the same mainboard was used with Am386sx-25, so I thought, I'll give it a try. Well, I ended up with working, but unstable system at 25MHz and it doesn't work at 33MHz now. At 16MHz however it works just fine. Do you have any suggestions, if there is something I can do? Is it possible, that the chipset doesn't support anything above 25MHz? Why do I get then instability issues with 25MHz in my case? There are obviously 25MHz versions of the same mainboard in the wild?

If the CPU is a really old one, then maybe there just isn't enough overclocking headroom.
Do you know the production date of the CPU? Does it have "C-Step" printed on it?

The old CPU marking is NG80386SX-16 (L1344284), however, the old CPU is not the problem, I can even overclock it to 20MHz. The thing is, that the "new" CPU is actually Am386SX/SXL-33 with markings NG80386SXL-33, NG80386SX-33 (C 224BA8E), however I can't get it booting with 33MHz and even 25MHz seem to be instable. The question is, if that's motherboard issue, or not? I mean, can all 386sx chipsets handle 25/33 MHz CPUs?

scorp wrote on 2020-07-16, 14:00:

The old CPU marking is NG80386SX-16 (L1344284), however, the old CPU is not the problem, I can even overclock it to 20MHz. The thing is, that the "new" CPU is actually Am386SX/SXL-33 with markings NG80386SXL-33, NG80386SX-33 (C 224BA8E), however I can't get it booting with 33MHz and even 25MHz seem to be instable. The question is, if that's motherboard issue, or not? I mean, can all 386sx chipsets handle 25/33 MHz CPUs?

What's the crystal for the Isa bus or it's divider.

I thought about that and, unfortunately, I'm not sure. For the CPU I tried four crystals 32, 40, 50 and 66 MHz. For the old 16MHz CPU I could boot with 32 and 40MHz crystals. With the 33MHz CPU I can boot with 32,40 and 50 MHz. For 33MHz CPU I'd actually need 66MHz crystal. That wouldn't even be overclocking yet. Anyway, regarding the divider, there are no jumpers for it and in BIOS I can't set anything either. However, I measured the frequency on ISA B20 pin and got 7,16MHz, which should be ok, I guess. The chipset obviously tries to keep the ISA Bus at that frequency, doesn't matter which crystal I'm using. Or do I understand it wrong and B20 is completely unrelated?

Hi, this mainboard has the chipset UM82C366F by UMC. Does anybody know, where I can get the datasheet for it? Unfortunately google gives me nothing, but may be there are some "secret" places whatsoever 😀

If ISA is 7.16MHz it sounds like it's probably derived from the OSC signal. The other two methods being derivative of system clock, and asynchronous mode. So it seems ISA clock is not your problem. Is there no way to adjust memory timings? What kind of RAM is currently installed? If any of the RAM is less than 70ns it might be a problem. Especially check the DIP RAM if installed.
I'm not familiar with the UMC 286 and 386SX Chipsets, but you wouldn't think 25MHz would be much of a challenge for those.

Anonymous Coward wrote on 2020-07-17, 12:24:

If ISA is 7.16MHz it sounds like it's probably derived from the OSC signal. The other two methods being derivative of system clock, and asynchronous mode. So it seems ISA clock is not your problem. Is there no way to adjust memory timings? What kind of RAM is currently installed? If any of the RAM is less than 70ns it might be a problem. Especially check the DIP RAM if installed.
I'm not familiar with the UMC 286 and 386SX Chipsets, but you wouldn't think 25MHz would be much of a challenge for those.

A good point, I'll try to install 60ns memory and remove DIP RAM chips. Currently I have 80ns SIMMs and 70ns DIPs in there.

80ns RAM is generally good up to 33MHz and 70ns up to 50MHz, but it depends. 386SX based mainboards were low end back in the day, so don't expect many good capacitors there. Even if there are no electrolytic ones, I'd suggest to replace some on the +5V rail near the CPU, chipset and RAM slots with 100uF or 47uF tantalum capacitors 10V+ rated. It helped me to bring many mainboards back in the business.

ph4nt0m wrote on 2020-07-18, 12:24:

80ns RAM is generally good up to 33MHz and 70ns up to 50MHz, but it depends. 386SX based mainboards were low end back in the day, so don't expect many good capacitors there. Even if there are no electrolytic ones, I'd suggest to replace some on the +5V rail near the CPU, chipset and RAM slots with 100uF or 47uF tantalum capacitors 10V+ rated. It helped me to bring many mainboards back in the business.

The suggestion with the capacitors sounds plausible. I'll try this out too, I guess 😀

It is also worth to mention that chipsets also have their overclocking limits. Many 386SX chipsets were 20MHz or 25MHz rated. There is no guarantee they will do 33MHz or 40MHz even if everything else is in a great shape.

ph4nt0m wrote on 2020-07-18, 12:29:

It is also worth to mention that chipsets also have their overclocking limits. Many 386SX chipsets were 20MHz or 25MHz rated. There is no guarantee they will do 33MHz or 40MHz even if everything else is in a great shape.

Yes, this is what I assume. That's why I was asking for a datasheet, but nobody seems to have an answer. I guess this chipset is ok for up to 25MHz. Probably, it is near the limit even at that frequency, because it is not even stable at 25MHz. However, may be this instability is related to bad capacitors, just as you mentioned....

I tried everything, another RAM down to 60ns, SIMM or DIP only, checked and replaced caps. No difference. Interesting enough, here is written, that the mainboard supports 16,20 and 24 MHz. https://stason.org/TULARC/pc/motherboards/U/U … 6-UM-386SX.html

The last frequency is kind of weird, there were no 386SX CPUs with 24MHz 😀 And at 25MHz this mainboards gets instable. I guess, the chipset is the bottleneck and UM82C366A is not made for anything faster. Still, it was a nice experiment 😀

Ok here is my 386SX upgrade attempt...
https://www.youtube.com/watch?v=sgXdiCPwGUA

ohhh i realize i didn't put my contribution here.
i successfully upgraded 2 PS/2's with SXLC2 50 and 40 Mhz chip.

first is a model 35 (isa) originally with a SX20, successfully upgraded with the SXLC2 50 G50 (without 3,3v mod) , overclocked to 33 MHz. and for bonus, cache is enabled by bios.
and clock doubling work with software control, and 66 MHz is possible totally reliably. but i added a heatsink, clock doubling also mean heat doubling 😀.

second is a model 55SX (mca) originally SX16, successfully upgraded with the 40 Mhz (so real 5V part) , tried overclock to 20 MHz but fail. cache is enabled by bios.
clock doubling doesn't work, the machine crash. don't know why.

some pictures :

- model 35

- model 55SX

galanopu wrote on 2022-05-02, 16:36:

Ok here is my 386SX upgrade attempt...
https://www.youtube.com/watch?v=sgXdiCPwGUA

Somebody actually went out of their way to use the CPU solder pads on the M396? I thought about doing that years ago, but figured it would be easier to just cut off the CPU.
I guess this upgrade module is only useful on this one particular board, but at least it's a common one. Also, the addition of VRM was a nice touch.

scorp wrote on 2020-07-17, 11:25:

Hi, this mainboard has the chipset UM82C366F by UMC. Does anybody know, where I can get the datasheet for it? Unfortunately google gives me nothing, but may be there are some "secret" places whatsoever 😀

This is long ago but I had a 386sx-20 board with that chipset and the board was labeled 24mhz max.

I believe that identical chipset came in different speed ratings and it’s very possible your rev or board max out at 24mhz

You could try refreshing the board with new caps and replacing commonly available ICs and ram with ones more likely to operate at 33mhz and see if it makes a difference

Other option used in ye olde times on a non-regulated motherboard was to get a better PSU and internally turn the 5v up to 5.5volts and heat sync the chipset, trying your hand at overclocking

24MHz is odd. Was it a silk screen typo, or was this board in development before the 80386-25 was released?

Anonymous Coward wrote on 2022-05-02, 23:48:

galanopu wrote on 2022-05-02, 16:36:

Ok here is my 386SX upgrade attempt...
https://www.youtube.com/watch?v=sgXdiCPwGUA

Somebody actually went out of their way to use the CPU solder pads on the M396? I thought about doing that years ago, but figured it would be easier to just cut off the CPU.
I guess this upgrade module is only useful on this one particular board, but at least it's a common one. Also, the addition of VRM was a nice touch.

Yeah, I did this in hurry. It is not that hard to design.
It took me something like 2-3 days to do so.
M396 I think is my second board, the pins look different there.
Most M/Bs have the pins as on my adapter, so I hope I will use this again.