VOGONS


First post, by christal87

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Hi,

I've been playing around with my Jetway J-403 VLB GREEN mobo I bought with an oddball Cyrix DX2 CPU a year ago. Got some overkill size 2x32Meg EDO modules from china. DOS won't really use it much. I got hooked on exchanging it to a DX4. Went ahead and bought an AMD DX4. Unfortunately the mobo only supported 5V core processors, while every jumper manual I could find had a 3.3V option. Mine had fixed bridges instead and no 3.3V regulator. I couldn't find any 5V DX4s, most of them are 3.3V Vcore only.
I'v read a few application notes from datasheet archives from that time and started some reverse engineering. Interposers were not an option, because of their availibility and pricing. Finding just a Socket3 connector is hard nowadays. The AMD DX4 had the same 5V I/O voltage as the DX2, so I only needed to find a regulator and two resistors to set up the output voltage that can fit in the unpopulated pads. Some jumpers and headers too. After fiddling a bit I'd found the correct one, a general LM350 and two 1/4W resistors. My major concern was overheating it when the CPU starts to sip up more power. Even dropping only 1.7V while there is 3A current flow can lead to like 5W. That's real heat right there! Of course I had to install a heatsink too to keep it cool.
The BIOS isn't recognizing it correctly even after setting the correct CPU type jumpers. This board doesn't even have the ID pin connected in the CPU socket. But hey it works and it's much faster and stable then the Cyrix one was!

It's still working flawlessly after a few months of usage!
I'm thinking of overdoing it by making a switch mode regulator PCB in place of that regular LDO.

I'll try to slow it down with the turbo button or cache tricks if I want to play some Space Quest. Still have to find a Sound Canvas and build an MPU-401 card for it first.

Good hacking everyone!
Chris

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Reply 1 of 8, by Tiido

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I have added 3.3V support on all the mobos I have which are supposed to have support but regulator isn't present. I have one board which is not stable with 3.3V, I will have to try to see if boosting voltage makes it stable.

T-04YBSC, a new YMF71x based sound card & Official VOGONS thread about it
Newly made 4MB 60ns 30pin SIMMs ~
mida sa loed ? nagunii aru ei saa 😜

Reply 2 of 8, by Anonymous Coward

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There never were any 5V DX4s, at least not production versions. the closest you can get is the Intel DX4 overdrive, which is really a 3.3V core with a VRM just like all the other 3rd party upgrades.

"Will the highways on the internets become more few?" -Gee Dubya
V'Ger XT|Upgraded AT|Ultimate 386|Super VL/EISA 486|SMP VL/EISA Pentium

Reply 3 of 8, by christal87

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Tiido wrote on 2020-03-14, 21:11:

I have added 3.3V support on all the mobos I have which are supposed to have support but regulator isn't present. I have one board which is not stable with 3.3V, I will have to try to see if boosting voltage makes it stable.

I would try watching the voltage drop on a datalogging multimeter while heavily loading the CPU.
The other thing comes to my mind is checking all you filter caps, especially if they all are old tantalums.
Maybe try to search for unpopulated caps near the regulator and the CPU socket Vcore pins.

Reply 4 of 8, by christal87

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Anonymous Coward wrote on 2020-03-15, 05:39:

There never were any 5V DX4s, at least not production versions. the closest you can get is the Intel DX4 overdrive, which is really a 3.3V core with a VRM just like all the other 3rd party upgrades.

That's what I suspected, thanks! Yeah I came across the Overdrive, but it was like 5-10 times the price of the Am486. I had one of these back in the day too. Nice to see one again!

Reply 5 of 8, by Tiido

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christal87 wrote on 2020-03-15, 08:36:

I would try watching the voltage drop on a datalogging multimeter while heavily loading the CPU.
The other thing comes to my mind is checking all you filter caps, especially if they all are old tantalums.
Maybe try to search for unpopulated caps near the regulator and the CPU socket Vcore pins.

I have scoped the voltage rail and there's no problem there, it does seem chipset just isn't playing along with the lower voltage levels coming out the CPU at the speed it is needed to run at. Lower bus speed (25MHz) made things work, sometime later I'll up tho voltage rail and that should make things work at 33MHz and if I'm lucky 40MHz.

T-04YBSC, a new YMF71x based sound card & Official VOGONS thread about it
Newly made 4MB 60ns 30pin SIMMs ~
mida sa loed ? nagunii aru ei saa 😜

Reply 6 of 8, by christal87

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Tiido wrote on 2020-03-15, 12:08:
christal87 wrote on 2020-03-15, 08:36:

I would try watching the voltage drop on a datalogging multimeter while heavily loading the CPU.
The other thing comes to my mind is checking all you filter caps, especially if they all are old tantalums.
Maybe try to search for unpopulated caps near the regulator and the CPU socket Vcore pins.

I have scoped the voltage rail and there's no problem there, it does seem chipset just isn't playing along with the lower voltage levels coming out the CPU at the speed it is needed to run at. Lower bus speed (25MHz) made things work, sometime later I'll up tho voltage rail and that should make things work at 33MHz and if I'm lucky 40MHz.

Interesting symptoms! I'll try to preserve mine, so 33MHz is the default, no O.C. This one had recurring boot issues with the DX2 green cyrix. Now it only has it like 2 times out of 30.
Playing around with the FP I got it with and EDO modules from china causes different results. I guess it's because of timings. This mobo has an OPTI 82C895 chipset. What type of chipset do you have ?

Reply 8 of 8, by christal87

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maxtherabbit wrote on 2020-03-15, 14:13:

stick with the LDO and enjoy your clean power rail and stable system, switching supplies are gross

Yeah I have to get very bored to go into the patience-game of filtering out ripple noise. 😀