This motherboard already supports 3.3v~4.0v cpu's!!

https://win.elhvb.com/mboards/chicony/ch498b/CH-498B.html

I had to do a similar mod to my Jetway board, which you can find here: http://www.win3x.org/uh19/motherboard/show/5505

In the unofficial manual, I explained my way of modding the board with a voltage regulator, by replacing one of the resistors with a potentiometer, so I could adjust the voltage to my desire.

In your case, the IC's datasheet would help you understand where to install those resistors.

My board is a REV 1 and only supports 5V, REV 2 boards support 3.3v.

I tried to make the mod but it is not that I have deep knowledge of electronics, and I did not succeed. From what I understood the voltage is taken from the difference between R20 and the other resistors that are selected with the jumpers. It seems simple but I tried two trimmer potentiometers and I was not able to get values higher than 2.5 volts.
So either I don't understand correctly how it works (very likely) or the two MICs I bought were faulty.
Due to frustration, I left the project and continued with other pending things.
In the future when I feel like it, since there is always danger of damaging the motherboard, I will try again, this time with an LT.

Just found that my CH 498B is also a Rev 1.0, and yes you're right, I tried one 3.3v and it didn't boot, so it doesn't support any 3.3v~4v cpu. I guess i'll stick with 5v processors, just bought an AMD 486DX2-80 (25220 E6) and it should be powerful enough for most of the games of that era.

Well, I did something crazy, an Intel 486dx4 100 with a big heat sink and fan at 5v, and since it was already overvolted, why not setting an 40mhz fsb, and well its wonking, stable and cold.
This madness was due to the fact that I read that intel cpus tolerated 5v and that some member of the forum was using a 3.3v cpu at 5v for years without realizing it.

Did you already found your solution for this?

I do have a Rev 2.0 with these resistors installed. I can make a picture tomorrow if you want.. But will post the picture to help others out as well.

There seems to also be yellow small capacitors inside the cpu socket.

Too bad I didn't see this post before, I have this exact same board (here http://www.win3x.org/uh19/motherboards/7821)

Though I got mine this year, it wouldn't have been possible back in 2020.

In the meantime, could you please dump your bios ? There seem to be many revisions and mine isn't the latest one apparently.

I have to look for that computer in the heap, I'm sure it's there. I left it after testing the stability at 40Mhz and 5v. I did not want to risk damaging anything more since without realizing it had been running at 5v for some time. As I already said, my level of electronics does not go beyond replicating a circuit following a diagram and its instructions, but in case of having a clear photo of the resistors and the voltage regulator, the only problem would be to find the components.
As for the BIOS, what utility should I use to read it?

Use this utility to dump your bios : http://cd.textfiles.com/microhaus/mhblackbox3 … MORY/GETROM.ZIP

As for the picture, check the link I put previously, there's a clean photo 😀 (if it's not good enough I can make a closer one)

Here is the BIOS dump.
Bios string 40-E301-001116-00111111-121593-UMC498-H
In the photo I can't read the model of the voltage regulator, maybe the resistors color but no the model of the MIC, thanks.

Deksor wrote on 2021-10-18, 11:19:

Use this utility to dump your bios : http://cd.textfiles.com/microhaus/mhblackbox3 … MORY/GETROM.ZIP

As for the picture, check the link I put previously, there's a clean photo 😀 (if it's not good enough I can make a closer one)

I'm gonna suggest taking a closer one, since I can't distinguish the exact colors on the blue resistors that are required 😀

Here is the bios from my motherboard.

Seems to be version 3.03

40-P303-001116-00101111-072594-UMC498-H

Deksor wrote on 2021-10-18, 07:59:

Too bad I didn't see this post before, I have this exact same board (here http://www.win3x.org/uh19/motherboards/7821)

Though I got mine this year, it wouldn't have been possible back in 2020.

In the meantime, could you please dump your bios ? There seem to be many revisions and mine isn't the latest one apparently.

Do you also know whats different the new version from the older one. Did they added support for AMD 586 133 mhz later maybe?

Hoping wrote on 2021-10-18, 17:45:

Here is the BIOS dump.
Bios string 40-E301-001116-00111111-121593-UMC498-H
In the photo I can't read the model of the voltage regulator, maybe the resistors color but no the model of the MIC, thanks.

Its a MIC 29152BT I guess that one is also used on their CH-471B motherboard.
I think that the resistors are hard to come by..

The blue resistors are called precision resistors with a tolerance of 2% or less, they aren't common but not hard to find, expensive if you compare it with general purpose resistors. Back then I tried with a trimmer because in the board manual of the ver 2 there are ten pins on the voltage select jumpers but in the ver 1 I have there are only eight so maybe the lower I can set using the resistors of the ver 2 is 3.45v but I don't know the value of the the first resistor used as a reference for the others. So the first steps are, finding the voltage regulator and the value of the resistors,a clear photo of the area would help a lot, back then, since I couldn't find much info about this I used a MIC 29302 that is rated for 3A and I think is similar to the Mic 29152BT rated for 1.5A but not identical.

Here's a closer photo

R20 ? brown red blue black violet ? 126
R21 brown black red orange brown ? 102K
R.. brown red brown silver silver that don's make sense other way round and silver is gray sigh... 881

Wait a minute... there's only place for 5 on OP's board, but deksor's board has six.

silver red green red brown ... gray again 82.5k
brown red green ? white ... can't tell what color that missing one is... looks like lime but ... does not compute 😉 I'm guessing it's gray. and reversed 98.5K
brown green silver orange brown 158k also gray not silver :-p

edit :
violet black blue red brown 70.6K
brown black red orange brown 102k
gray gray brown red brown 88.1K
gray red green red brown 82.5K
white gray yellow green red brown 94.5K
brown green gray orange brown 158K
edit2:
reversed
126
132
12100M <- does not make sense
12K5
12500M <- also does not make sense
13M8 <- also does not make sense

edit : lime = yellow not gray
deksor's board has 5 voltage jumper settings, Op's board only has 4 :-p

weedeewee wrote on 2021-10-19, 16:57:

looks like lime

Yellow perhaps?

[edit to add...]
dunno if this makes sense, but using the Vout = 1.24*(1+R1/R2) from the datasheet, and using the 158k as R1, that gives (I think)

94.5k = 3.31V
82.5k = 3.61V
88.1k = 3.46V
102k = 3.16V
70.6k = 4.02V

snufkin wrote on 2021-10-19, 17:20:

Yellow perhaps? […]
Show full quote

weedeewee wrote on 2021-10-19, 16:57:

looks like lime

Yellow perhaps?

[edit to add...]
dunno if this makes sense, but using the Vout = 1.24*(1+R1/R2) from the datasheet, and using the 158k as R1, that gives (I think)

94.5k = 3.31V
82.5k = 3.61V
88.1k = 3.46V
102k = 3.16V
70.6k = 4.02V

Well, yep, then it's yellow not gray.
thanks I was getting hung up on the math.