Do you have a 5v cpu lying around you could test with?

I have a few 486 motherboards that blew their VRMs silently and won't post with 3v CPUs anymore.

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

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).

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....

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).

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):

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?

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.

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 ^^

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.

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.

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?

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?

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.

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.

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

I don't have anything to add but to join the chorus about the SIMMs. Those are the weirdest looking 72 pin SIMMs I've seen. Why do the chips look like chewing gum?

The rams are ok, unless they're Apple specific. I had sticks featuring the same package.
Buy a diagnose card - now.

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.