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[-1-]  Precautions and Warnings before you start !

[-2-]   Introduction to Type-4 Pentium 90 Platform

[-3-]   The importance of cooling certain components

[-4-]   Modification A) Using a Pentium Overdrive 180 / 200

[-5-]   Modification B) Hardwired BF0 / BF1 Pins for different Bus / Core ratio

[-6-]   Modification C) Altering the base-clock from 60 to 66MHz

[-7-]   Modification D) Using a MMX Interposer and P-233MMX CPU

[-8-]   Problems, Workarounds and other stuff

Visiting the MCA Enthusiasts Page

Modification C) Altering the base-clock from 60 to 66MHz

The 60 MHz Oscillator
Position of the 60 MHz crystal oscillator

Against common believe this is no simple crystal - it is an oscillator chip with an integrated crystal and some electronic circuit in one part. It runs on +5V and it generates the processor base clock signal with a very high accuracy. The printing atop says '60.0000 MHz', which is quite good and sufficient for e.g. a radio transmitter, which safely keeps the frequency.
This chip is soldered to the printboard with 4 pins. They are +5V, GND and output. The fourth pin is usually internally not connected, sometimes (on those metal can types) it may be connected with the shielding case and should be tied to GND as well.
The one on the Type-4 platform is a plastic style chip and theoretically it should be easy to swap it out for a 66 MHz type.

For this purpose you need a solder iron and a wooden toothpick. Yes - a toothpick. You need it as a lever to get it off. Start at one side and warm up both pins alternating while gently trying to lift up that side until both pins come up from the board. If you break the toothpick during the operation you'd levered way too hard. Therefore: don't use a screwdriver. If you apply too much power and manage to get the chip off there is a good chance that the copper contacts come with it and you'd successfully destroyed the board.

Once the one side has come free continue with the opposite side until the chip is removed.

Now clean the contacts from the remaining solder tin. They need to be evenly flat. "Mountains of Thing" make it hard to properly align the new chip on the solder pads.
Align the new 66 MHz oscillator on these pads in proper direction with the pin 1 bottom / left (or the case mark towards the CPU).
Then warm up the solder pad - not the pin until it starts to stick a bit. Don't heat up the individual pins longer than a few seconds at one time. Work around the other pins and make sure you have a good solder spot on each.

That was it. Now you may want to replace the P90 CPU against e.g. a P133 while you're at it. The P90 would be overclocked with a 66MHz base clock and you better use a 133 type from now on.
The platform should come up like normal when you put it back into your system. QCONFIG from PC-DOS 7 will nontheless claim the machine runs at 60MHz (it does not know better), but the "estimated CPU speed" will be given as "99 MHz", which is the mathematically correct result from a 66 MHz base clock and a 1:1.5 (or 2:3) multiplier ratio. Other programs like WinTune will confirm that it runs at the correct speed.

There are oscillators available with 66.6666 MHz output. You may use them too and will get 100 MHz straight, but that is fairly unimportant. You are going to overclock the platform anyway if yours got the 60 MHz rated cache RAMs and controller only.
I'd tried both with no major effect at all. Those platforms that worked with the 66.0000 Mhz type worked with the 66.6666 MHz too and the final result (in the test result pages) are of cosmetical value only - in practice you see no difference.

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