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486 System Won't Post

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First post, by Almoststew1990

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I bought a motherboard and CPU from eBay and it won't post. I've tried a few things to see if I can coax it into life but I am sure you guys know what I am doing wrong (or if it is a faulty board). The ATX PSU spins up but nothing else happens.

It's a Tomatoboard 4DPS V2.1
AMD DX2-80 CPU
Topless 2MB*32 RAM 2 sticks.

a40W7m6.jpg
This photo was before I changed any jumper settings.

- I've triple checked that the jumpers are set up properly for the voltage (3.3) CPU (AMD DX2) system speed (40MHz) and L2 cache (256k). There are two I am less sure about: JP22 is called "Default Jumper: JP22 Open" so I've left it open. The other is the BIOS, JP4 lets me choose between EPROM and FLASH ROM; I've tried both but leave it on FLASH ROM
- I've re-built the PC out of the case with just the CPU and speaker installed, no beeps; added RAM, still no beeps
- Tried a new CR2032, cleared the BIOS, tried no CR2032.
- Tried ISA and PCI video cards

The RAM is 2MB * 32 70ns but the manual only talks about *36 RAM. Is this a big difference?
My CPU is a 3 volt CPU. I thought it would at least post at 3.3v ? (checked this - it is 3.3v so it should be fine.)

Any ideas???

Ryzen 3700X | 16GB 3600MHz RAM | Nvidia GeForce 1070ti | 1Tb NVME SSD | Windows 10
Athlon 3200+ @ 3800+ (Venice) | Some Ram | Nvidia GeForce GTX645 / 7950GT
Slot 1 896MHz | 384Mb 112MHz RAM | Nvidia GeForce 3 ti200 | AWE32

Reply 2 of 21, by Almoststew1990

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Nope I don't have any other CPUs, RAM or board to swap around with 🙁

Ryzen 3700X | 16GB 3600MHz RAM | Nvidia GeForce 1070ti | 1Tb NVME SSD | Windows 10
Athlon 3200+ @ 3800+ (Venice) | Some Ram | Nvidia GeForce GTX645 / 7950GT
Slot 1 896MHz | 384Mb 112MHz RAM | Nvidia GeForce 3 ti200 | AWE32

Reply 3 of 21, by kalohimal

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Perhaps you could try pulling out the RAMs and see if it beeps. Memory test is done very early during POST, so if it doesn't then POST probably didn't start. If it beeps then it is likely to be RAM issues (FPM/EDO/parity/etc). x36 RAMs are with parity, and x32 are non-parity. If you have a multimeter, can also measure the output of the voltage regulator to see if it has 3.3V. The UV EPROM/FLASH jumper probably won't matter as it usually just enable writing to the BIOS chip (your BIOS chip is a UV EPROM).

Reply 4 of 21, by Almoststew1990

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I don't have a multimeter - I really should get one. I might just open a return with the seller (they're probably on here anyway tbh) as the board should be doing more than its doing - I don't get any beeps with RAM installed or uninstalled in the system. The chipset (northbridge? If they were a thing on 486s?) gets slightly warm but the CPU's heatsink doesn't and the southbrdige doesn't.

If the VRM had "blown" would it look any different? It looks fine on the board....

Ryzen 3700X | 16GB 3600MHz RAM | Nvidia GeForce 1070ti | 1Tb NVME SSD | Windows 10
Athlon 3200+ @ 3800+ (Venice) | Some Ram | Nvidia GeForce GTX645 / 7950GT
Slot 1 896MHz | 384Mb 112MHz RAM | Nvidia GeForce 3 ti200 | AWE32

Reply 5 of 21, by kalohimal

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You could take off the heat sink and touch the cpu directly while powering on momentarily. If it doesn't get hot then it's not getting the Vcore. If it does then just turn it off immediately. If the VRM is a buck converter, then it has a driver IC and a MOSFET (the one beside the cpu that has a heat sink). You could touch them to see if they are very hot, if so then most probably is shorted (but then the PSU would shut down in this case).

Reply 6 of 21, by Roman555

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kalohimal wrote on 2020-07-06, 17:22:

If the VRM is a buck converter, then it has a driver IC and a MOSFET (the one beside the cpu that has a heat sink). You could touch them to see if they are very hot, if so then most probably is shorted (but then the PSU would shut down in this case).

No, it's rather a linear regulator (there is no coil). Something like this (just e.g. , based on LP2951 and TIP127):

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Reply 7 of 21, by Almoststew1990

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I touched the CPU and it was getting slightly warm in one spot.

The thing I think is the VRM is a LP 2951 if that helps?

Ryzen 3700X | 16GB 3600MHz RAM | Nvidia GeForce 1070ti | 1Tb NVME SSD | Windows 10
Athlon 3200+ @ 3800+ (Venice) | Some Ram | Nvidia GeForce GTX645 / 7950GT
Slot 1 896MHz | 384Mb 112MHz RAM | Nvidia GeForce 3 ti200 | AWE32

Reply 8 of 21, by Roman555

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Almoststew1990 wrote on 2020-07-06, 18:18:

I touched the CPU and it was getting slightly warm in one spot.

The thing I think is the VRM is a LP 2951 if that helps?

You've just supported my guess. A multimeter would be handy.

Reply 9 of 21, by Horun

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Almoststew1990 wrote on 2020-07-06, 14:24:

The RAM is 2MB * 32 70ns but the manual only talks about *36 RAM. Is this a big difference?
My CPU is a 3 volt CPU. I thought it would at least post at 3.3v ? (checked this - it is 3.3v so it should be fine.)

Are you saying the ram sticks are 2Mb each ? Only Macs can generally use those. They need to be 1MB, 4MB, 8MB, etc. As for *32 that would be non-parity where *36 is parity.

kalohimal wrote on 2020-07-06, 15:54:

Perhaps you could try pulling out the RAMs and see if it beeps.

Yes do that ^^

Hate posting a reply and have to edit it because it made no sense 😁 First computer was an IBM 3270 workstation with CGA monitor. 🤣 Second computer a 286 12Mhz with real IDE drive ! After that came 386, 486, Pentium, P.Pro and everything after....

Reply 10 of 21, by mkarcher

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Horun wrote on 2020-07-06, 23:36:
Almoststew1990 wrote on 2020-07-06, 14:24:

The RAM is 2MB * 32 70ns but the manual only talks about *36 RAM. Is this a big difference?
My CPU is a 3 volt CPU. I thought it would at least post at 3.3v ? (checked this - it is 3.3v so it should be fine.)

Are you saying the ram sticks are 2Mb each ? Only Macs can generally use those. They need to be 1MB, 4MB, 8MB, etc. As for *32 that would be non-parity where *36 is parity.

2MB * 32 means there are 2 Mega-Units (or Mebi-Units, depending on how you feel to name the units today) of 32 bits each. As one unit of 32 bits is 4 Bytes, 2 Mega-Units are 2,097,152 * 4 = 8,388,608 Bytes. So 2MB * 32 is an 8MB SIMM without parity.

Even if the imprint on the CPU is just "3 volt", it means 3.3 - 3.6 volts, often 3.45 volts nominal.

Reply 11 of 21, by Horun

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mkarcher wrote on 2020-07-07, 00:00:
Horun wrote on 2020-07-06, 23:36:
Almoststew1990 wrote on 2020-07-06, 14:24:

The RAM is 2MB * 32 70ns but the manual only talks about *36 RAM. Is this a big difference?
My CPU is a 3 volt CPU. I thought it would at least post at 3.3v ? (checked this - it is 3.3v so it should be fine.)

Are you saying the ram sticks are 2Mb each ? Only Macs can generally use those. They need to be 1MB, 4MB, 8MB, etc. As for *32 that would be non-parity where *36 is parity.

2MB * 32 means there are 2 Mega-Units (or Mebi-Units, depending on how you feel to name the units today) of 32 bits each. As one unit of 32 bits is 4 Bytes, 2 Mega-Units are 2,097,152 * 4 = 8,388,608 Bytes. So 2MB * 32 is an 8MB SIMM without parity.

Even if the imprint on the CPU is just "3 volt", it means 3.3 - 3.6 volts, often 3.45 volts nominal.

Ok And yes I do know about Mbit versus MByte. Generally most do not explain ram as 2Mbit*32, they say 8MB non parity 😀
I do notice they are 4 chip simms, do they work in any other 486 board ? Most 8MB 72pin simms are 16 chip for non parity, at least most of mine are. added: maybe the motherboard does not like quad density mem chips on a simm.

Hate posting a reply and have to edit it because it made no sense 😁 First computer was an IBM 3270 workstation with CGA monitor. 🤣 Second computer a 286 12Mhz with real IDE drive ! After that came 386, 486, Pentium, P.Pro and everything after....

Reply 12 of 21, by kalohimal

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Roman555 wrote on 2020-07-06, 18:04:

No, it's rather a linear regulator (there is no coil). Something like this (just e.g. , based on LP2951 and TIP127):

Good point.

Almoststew1990 wrote on 2020-07-06, 18:18:

I touched the CPU and it was getting slightly warm in one spot.

The thing I think is the VRM is a LP 2951 if that helps?

That's not a good sign, it should be hot to the touch. So it's very likely to be VRM/PSU/CPU issue. Do you have another PSU and CPU that you could try?

Reply 13 of 21, by Almoststew1990

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I've tried it without any RAM in and it doesn't beep, and sadly I don't have another board to try the RAM in. I think will get some FPM RAM to try too. I'll also double check the CPU getting (slightly) warm again as I was quite warm, sweaty and fed up when I checked last time! Will it be be "obviously hot" to touch?

Ryzen 3700X | 16GB 3600MHz RAM | Nvidia GeForce 1070ti | 1Tb NVME SSD | Windows 10
Athlon 3200+ @ 3800+ (Venice) | Some Ram | Nvidia GeForce GTX645 / 7950GT
Slot 1 896MHz | 384Mb 112MHz RAM | Nvidia GeForce 3 ti200 | AWE32

Reply 14 of 21, by kalohimal

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Would rule out RAM at the moment since you didn't get any beeps without them (hinting that the CPU didn't start). The CPU should be rather hot until barely bearable to the touch (otherwise it won't need a heat sink). If you could return it and get a refund, I guess that would be the best option. Unless the seller is willing to refund and let you keep it, then maybe we could try fixing it.

Reply 15 of 21, by appiah4

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My money is on the RAM. PC SIMMs are 30 pin, Mac RAM is 32pin. PCs can't POST with 2x2MB SIMMs, at least none that I know of can.

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Reply 16 of 21, by squelch41

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I'd get a cheap POST card and then you can start to have a bit of a window on what is happening.
eg https://www.ebay.co.uk/itm/New-4-Digit-LCD-Di … -6/184209106608

Will at least be able to see if it getting to the point where it is checking RAM etc or if totally blank, it suggests the CPU is the culprit.

Found mine very useful for knowing what to fix on my loft-find motherboard

V4P895P3 VLB Motherboard and AMD 486 133MHz CPU
64mb RAM, CF 4Gb Hard disk,
Realtek 8019 ethernet, ES1869 soundcard, Unknown brand VLB multi-io card with Cirrus Logic GD5428 1mb VGA on it.

Reply 19 of 21, by Almoststew1990

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The RAM is by "Topless" and as far as I can tell all their RAM looked like that! I'll probably pick up some FPM RAM as it isn't expensive and also a diagnostic PCI/ISA card.

Ryzen 3700X | 16GB 3600MHz RAM | Nvidia GeForce 1070ti | 1Tb NVME SSD | Windows 10
Athlon 3200+ @ 3800+ (Venice) | Some Ram | Nvidia GeForce GTX645 / 7950GT
Slot 1 896MHz | 384Mb 112MHz RAM | Nvidia GeForce 3 ti200 | AWE32