 | RE:V1, need GOOD directions on making it support a dual volt processor. (modified 0 times) | jimmy | |
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Briefly, U16 is a dual op amp being used as the voltage regulator driver for the regulated Vcore voltage and a second regulated 2.5 volt rail. Pins 5 and 3 are tied together on the motherboard, and both are fed by a strange, high impedance version of the 2.5 volt reference voltage reference generated by Q28. Thus, you must lift pin 3 from its solder pad to set your Vcore voltage without affecting the other regulated supply. Second, you really should generate your Vcore reference voltage from the normal, low impedance 2.5 volt Q28 reference. A convenient place to get this reference voltage is the upper (toward cpu) end of capacitor C257, in the lower right corner of the motherboard. Las Vegas describes how to make a voltage divider with two resistors tied between C257 and any nearby ground (he used a pad by the large electolytic capacitor to the left and slightly above C257, near the printed word "C277"). The center point of the divider should be tied to pin 3 (lifted) of U16.
I am currently doing this mod on several V1's using AMD K6-2+ and 3+ chips that need a 1.9 to 2.1 volt Vcore. For simplicity, I let my two-resistor divider voltage derived from C257 be my Vcore reference directly. For this to work, I remove resistor R202 from the board. R202 is the ground leg of a voltage divider in the U16 feedback loop that boosts the Vcore voltage above the value appearing on pin 3 of U16. R202 sits near the large electrolytic capacitor that lies near the infamous Q16 Vcore voltage regulator dropping transistor. Once removed, Vcore will equal the pin 3 voltage, which makes life simple for making up my resistor divider network on C257. Once R202 is removed, the appropriate divider network for K6-2+ and 3+ chips is a 100K resistor connected to ground (near the "C277" word printed on the board), tied to a 27K resistor, which, in turn, is tied to C257. This generates about 1.95 volts. The center point of the divider (1.95 volts) is tied to pin 3 (lifted) of U16 using fine wire wrap wire available from Radio Shack. Use fine wire to avoid putting strain on the lifted U16 pin 3, which is quite flimsy once lifted. I glue the wire to the motherboard at some point along its length as a strain relief.
Try a 15K resistor with a 120K resistor for your 2.2 volt Vcore. Oh, of course, don't forget to move the zero ohm resistors R60 and 61 to the vacant R184 and 185 solder pads near the cpu socket to allow split Vcore and i/o rail voltages. This is important, and do this first before messing with U16!! Also, check your Vcore voltage before plugging in the cpu. All work should be done with no cpu present until this check is done. Vcore appears on the outboard solder pads of R60 and 61 (which are now vacant).
Good luck!
jimmy | |
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