Biostar "MB-8500TAC-A Ver.4" Mod for MMX CPUs \ VOGONS

Biostar "MB-8500TAC-A Ver.4" Mod for MMX CPUs

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First post, by majestyk

Posted on 2025-03-16, 14:00

majestyk Offline

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I recently found this nice socket 7 mainboard but had to find out at least Ver.4 (and below) don´t support split voltage CPUs correctly.

The attachment IMG_2748a.JPG is no longer available

So I did some measuring and made the necessary modifications.
Two traces need to be cut (red lines), Q6 - a TO-220 twin diode (non-schottky) plus heat sink have to be added and JP20 has to be replaced by a 3K3 resistor vor 2.8V Vcore.
No idea what the guys at Biostar did there, but the layout for D5 and D6 doesn´t make any sense at all.

The attachment IMG_2749a.JPG is no longer available

On the backside one trace must be cut (red line) a bridge wire must be added and a pull-up resistor for BF2 can be added to detect 200MHz and 233 MHz Pentium MMX correctly.

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Reply 1 of 3, by dominusprog

Posted on 2025-03-16, 15:55

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I would put the resistor in a heat shrink tube for a better protection. Nice board by the way.

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Reply 2 of 3, by Horun

Posted on 2025-03-16, 17:39

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Yeah odd, the diodes D5 and D6 would not do anything if there, from what I see.
Good work !

Hate posting a reply and then have to edit it because it made no sense 😁 First computer was an IBM 3270 workstation with CGA monitor. Stuff: https://archive.org/details/@horun

Reply 3 of 3, by majestyk

Posted on 2025-03-17, 09:19

majestyk Offline

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dominusprog wrote on 2025-03-16, 15:55:

I would put the resistor in a heat shrink tube for a better protection. Nice board by the way.

Good point, the upper lead of the resistor has to be prevented from touching the heatsink.

The twin diode causes a voltage drop from 5.0V to 4.05V, it´s followed by the regulating transistor that makes 4.05V drop to 2.75V (Vcore voltage).
As a result both heatsinks dont get really hot, with a regular socket 7 cooler running at 7V it´s just 40° - 45°.
The heatsink at the top belongs to the Vi/o regulator that is set to 3.3V. There are no heat issues anyway. It feeds the CPU´s Vi/o and the two pipeline burst SRAMs.

Biostar implemented this solutiun with revision 6 (not sure about revision 5) of this mainboard after adjusting the screwed layout.

I choose to upgrade CT35 from 220µF to 1000µF. This seems more appropriate for MMX-class CPUS.
I also tried some K6 and K6-2 CPUS, but that´s where the heat issues begin. I would not recommend doing this despite the fact that Vcore can be adjusted by changing the resistor at JP20.

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