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Generating ~4v on 486 motherboard for overclock?

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First post, by squelch41

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Hi,
I'm trying to get a AMD 5x86 133 to run at 160MHz
I can post and get to dos etc but once start stressing the CPU eg playing duke3d for more than a few seconds, it crashes.

My motherboard (model number in signature) supports 3.3v and 5v via a jumper.

I wanted to try and get a slight over-volt to stabalise the CPU - I was aiming for something like 3.8-4v.

I'm trying to work out how the voltage generation works.
There is an LM431 next to a couple of electrolyic capacitors, some resistors and then a TIP31C
(image attached)

Using a multimeter, it seems that the VREF pin of the LM431 attaches to the R15 resistor and then the other side of that resistor goes to ground
With the voltage jumper open (so 3.3v supply) this seems to connect R3 to the VREF pin and it's other side goes to 5v
If the jumper is closed (so 5v supply) it seems R5 is used instead of R3.

The output of the LM431 goes to the TIP31c

However, I don't understand how this works.
Measuring VREF with the jumper open gives 2.4v relative to ground and 2.6v with the jumper closed.

The data sheet for the LM431 says that the output voltage = Vref * (1+ (R1/R2))

R15 is marked 302 on the resistor but measures 1k when place probes on either side of the package
R5 is 122 but measures 870 ohms
R3 is 200 and does measure 20 ohms

What I was hoping I could do is to add additional resistance across the jumper to reduce the 5v supply (so essentially increasing R3s value), or if the resistance had to be decreased, piggy back a resistor in parralel.

However, I am obivously missing something or am just totally off on the wrong track!
I was hoping for some advice.

Picture of (what I think!) is the relevant part of the board attached.

Think this is the rough wiring of the LM431:

Shut jumper (5v):
GND--> R15 --VREF----R5-->5v

Open jumper (3.3v)
GND --> R15 ---VREF---R3--->5v

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V4P895P3 VLB Motherboard AMD 486 133MHz
64mb RAM, CF 4Gb HDD,
Realtek 8019 ethernet + XT-IDE bios ROM, ES1869 soundcard, VLB Cirrus Logic GD5428 1mb VGA

440bx MSI 6119, modified slocket , Tualitin Celeron 1.2Ghz 256mb SD-RAM, CF 4GB HDD, FX5200 gfx

Reply 1 of 8, by rmay635703

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4v is a little high for a mild OC,
I would start at 3.6-3.8 (even 3.5 if it’s there)

My experience is that if it’s unstable at 160mhz with a small amount of extra voltage it’s either
1. Motherboard/Cards don’t like 40mhz
2. Memory or Cache doesn’t like 40mhz
3. Inadequate Cooling
4. Motherboard/PSU VRM issue
5. BIOs issues/Settings

If it’s 1 or 2 you can occasionally overclock “non-cpu” components by turning the pot dial for your 5v rail inside the PSU up to a max of about 5.5v
One of my Micronics boards despite the cache being 15ns and the board having settings up to 50mhz seemed unstable at 40mhz

A tweek to the PSU voltage and all was well

Reply 2 of 8, by Disruptor

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An AMD 5x86 133 (4x33) should be overclockable to 150 (3x50) or 160 (4x40) MHz without overvolting.
You'd better try to continue your experiments by under-over-clocking to 120 MHz (3x40) to find out whether your motherboard runs flawless with a 40 MHz FSB or requires some waitstates. Do not forget to run a few passes with memtest86+ 4.10

Reply 3 of 8, by squelch41

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Disruptor wrote on 2021-05-16, 05:19:

An AMD 5x86 133 (4x33) should be overclockable to 150 (3x50) or 160 (4x40) MHz without overvolting.
You'd better try to continue your experiments by under-over-clocking to 120 MHz (3x40) to find out whether your motherboard runs flawless with a 40 MHz FSB or requires some waitstates. Do not forget to run a few passes with memtest86+ 4.10

I've done all that before - solid at 120MHz with 40MHz bus speed but my system can't handle 50MHz bus even with waitstates (POST card suggests the video card on VLB is the issue which makes sense, vlb being vlb)

V4P895P3 VLB Motherboard AMD 486 133MHz
64mb RAM, CF 4Gb HDD,
Realtek 8019 ethernet + XT-IDE bios ROM, ES1869 soundcard, VLB Cirrus Logic GD5428 1mb VGA

440bx MSI 6119, modified slocket , Tualitin Celeron 1.2Ghz 256mb SD-RAM, CF 4GB HDD, FX5200 gfx

Reply 4 of 8, by squelch41

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rmay635703 wrote on 2021-05-15, 23:05:
4v is a little high for a mild OC, I would start at 3.6-3.8 (even 3.5 if it’s there) […]
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4v is a little high for a mild OC,
I would start at 3.6-3.8 (even 3.5 if it’s there)

My experience is that if it’s unstable at 160mhz with a small amount of extra voltage it’s either
1. Motherboard/Cards don’t like 40mhz
2. Memory or Cache doesn’t like 40mhz
3. Inadequate Cooling
4. Motherboard/PSU VRM issue
5. BIOs issues/Settings

If it’s 1 or 2 you can occasionally overclock “non-cpu” components by turning the pot dial for your 5v rail inside the PSU up to a max of about 5.5v
One of my Micronics boards despite the cache being 15ns and the board having settings up to 50mhz seemed unstable at 40mhz

A tweek to the PSU voltage and all was well

Adjusting the input voltage wont change anything as the voltage regulation circuit will regulate it to 3.3v (or 5v).

Agree would prefer to go to 3.6-3.8v rather than full on 4v but principle will be the same.

(has read some people have run their 5x86 without incident on 5v but think this is somewhat risky!!)

V4P895P3 VLB Motherboard AMD 486 133MHz
64mb RAM, CF 4Gb HDD,
Realtek 8019 ethernet + XT-IDE bios ROM, ES1869 soundcard, VLB Cirrus Logic GD5428 1mb VGA

440bx MSI 6119, modified slocket , Tualitin Celeron 1.2Ghz 256mb SD-RAM, CF 4GB HDD, FX5200 gfx

Reply 6 of 8, by squelch41

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Disruptor wrote on 2021-05-16, 08:22:

Have you also tried the 3.6 V setting (used for some Cyrix CPUs)?

No 3.6v setting - only 3.3v or 5v

V4P895P3 VLB Motherboard AMD 486 133MHz
64mb RAM, CF 4Gb HDD,
Realtek 8019 ethernet + XT-IDE bios ROM, ES1869 soundcard, VLB Cirrus Logic GD5428 1mb VGA

440bx MSI 6119, modified slocket , Tualitin Celeron 1.2Ghz 256mb SD-RAM, CF 4GB HDD, FX5200 gfx

Reply 7 of 8, by rmay635703

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squelch41 wrote on 2021-05-16, 07:28:

Adjusting the input voltage wont change anything as the voltage regulation circuit will regulate it to 3.3v (or 5v).

On 486 boards the voltage regulator does not always extend beyond the cpu

AKA ISA, Memory and other motherboard components are many times run right off the tap even if the CPU is on a regulator and unstable motherboards are more common than people give credit

Also a marginally stable 40mhz motherboard FSB can break at higher clock speeds.

Also if he checks his PSU voltage he may find it’s on the weak end and the source of his issues

Reply 8 of 8, by squelch41

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rmay635703 wrote on 2021-05-16, 15:26:
On 486 boards the voltage regulator does not always extend beyond the cpu […]
Show full quote
squelch41 wrote on 2021-05-16, 07:28:

Adjusting the input voltage wont change anything as the voltage regulation circuit will regulate it to 3.3v (or 5v).

On 486 boards the voltage regulator does not always extend beyond the cpu

AKA ISA, Memory and other motherboard components are many times run right off the tap even if the CPU is on a regulator and unstable motherboards are more common than people give credit

Also a marginally stable 40mhz motherboard FSB can break at higher clock speeds.

Also if he checks his PSU voltage he may find it’s on the weak end and the source of his issues

Psu voltage on the 5v rail is good - is a modern supply.
I accept that the motherboard might be the source of the overclock issue but would like to try increasing the voltage a little on the cpu, especially as it is stable at 120MHz cpu on 40MHz fsb

V4P895P3 VLB Motherboard AMD 486 133MHz
64mb RAM, CF 4Gb HDD,
Realtek 8019 ethernet + XT-IDE bios ROM, ES1869 soundcard, VLB Cirrus Logic GD5428 1mb VGA

440bx MSI 6119, modified slocket , Tualitin Celeron 1.2Ghz 256mb SD-RAM, CF 4GB HDD, FX5200 gfx

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