You never know, I had many 16mhz 386SXs that came factory overC locked running 33 or 40mhz in a small late model AT motherboard very similar to yours.

ISA Bus overclocking via divider only works if your motherboard has settings to change it, I found it very effective if you have onboard IDE/serial/floppy and a strong video card, finding a multi-IO that overclock well is very hit or miss.

RAM speed and bios timing is critical as well as the date of manufacture of your chip, I’ve found that new 16mhz sx’s tend to overclock much more easily than late 80’s SX25’s
I also would recommend a little heat sync for your chips

Good Luck

I agree. If the board and CPU are from the early 90s, it should probably overclock to 33MHz. I have an AMI 386SX-XT series 20 motherboard with what I believe is a relabeled VLSI 311 chipset + proprietary cache controller. Mine has a 20MHz 386sx, and I can overclock the board to 33...but I am using a clip-on Cyrix CPU that deactivates the SX-20 because I didn't want to risk frying it). My board also runs the ISA bus in async mode so it's always running at 8MHz regardless of the bus speed. You might want to attempt to figure out how your ISA slots are wired up, or check if your BIOS has options to change the ISA and memory timings.

Did you know that intel deliberately held back their 386SX chips so they wouldn't undermine their DX versions? For a while, the SX only came in 16 and 20MHz versions. It was only when AMD started producing 386s that they got off their asses and spat out the 25 and 33MHz versions. The 33MHz version in particular came out really late. I don't have the exact date, but it may have been 1992, so very few people bought them outside of laptop users.

Ok great that motivates. Thanks.
Last question: anyone has a good instruction how-to-exchange the Quarz? Google just shows me some watches stuff 😀
Many thanks again
Marco

Well desolder the old one and solder the new one in place making sure it's int the correct orientation (pin 1 is indicated by a dot on the quartz and by a different pad on the pcb)

Great. Thus next step: find and buy the new one. To keep Vendor and naming I will try to find this one:

https://www.mymectronic.com/part-search/KTS/KDN020C66667MHZ

Hopefully I’ll find something similar in Germany as well.

Thanks

Hello again,

I found an adequate Quarz oscillator - even with 60MHz.
Now the issue: the Quarz is soldered that close to existing lanes that I doubt it’s being replaceable manually. At least when having a look on the back of the MB. Not even 1mm is between the solderpoints and neighboured lanes.

Any idea here?

Looking on the upperside of the MB I see that the Quarz has a distance of about 2mm to the board. Possible to cut these connects and solder the new Quarz on the existing cut(ted) solder-pins?

I try to attach a foto
Thanks already upfront

You have to desolder it, not cut it.

That was not my preferred answer 😀
Ok but understood. I’ll try to figure out how to keep the lanes untouched

Why neighboring traces are a problem ?

I „solder-iron“ will touch these I could create a shortcircuit? Pls see this as a question

Yeah but as on all PCBs, there's a solder mask which prevents solder from getting on the traces, as well as protecting them against moisture and other things.

Ok thanks. So I will go for it! Thx

Get turned IC socket and remove the pins using heat and push them out to create 4 pin socket for the oscillator.

Cheers,

Yeah a good needle iron and very little pry pressure from small flat blade screwdriver from the top and slowly work the two pins on either the left or the right of your top picture out first. Trick is holding the board stable vertical if you do not have some type of soft clamps (sitting holding board between the knees works in emergencies 🤣). just some thoughts... Definitely put a socket in so you can easily swap Xtals back if needed.

Thinking about doing the same thing to my sx25 + 311 chipset.

pentiumspeed wrote on 2020-06-28, 23:36:

Get turned IC socket and remove the pins using heat and push them out to create 4 pin socket for the oscillator.

Cheers,

Great idea. If I understand you correctly I should solder a socket in instead of the Quarz directly. Great idea this helps exchanging them / switch back to 60 or the original 50MHz osci.

Thanks

Also thanks to Horun

Update: activity completed.

Running at 30MHz: working. 3Dbench ran for about 20min. Doom Benchmark completed. After that System hang reset not working. CPU quite warm

Running at 33MHz: system won’t even boot. I found this strange. Would have expected some early system hang but this... strange. Normal? Quarz not working?

Anyway work completed via your socket hint. Thanks again a lot

Edit: doom behavior can be „repeated“ with some Result. System hang and after hard reset system won’t start. Strange but that’s it

Need a fan or heatsync (orboth)

My AT PSUs had a trimmable 5v, some motherboards could be made more stable by adding a little more than normal voltage

Also worth noting that you may need to change the ISA Bus divider in case that is going flakey.