SGI Iris Indigo CPU overclocking and Battery replacement (R4K) by yourself

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SGI Iris Indigo Battery replacement:
Remove power cords from your Iris Indigo. Remove CPU board. Remove the CPU. Now you can find that damn battery. In R4K computers battery is solded to CPU board. You need good (and powerfull) soldering iron to solder it out. Main problem is that Iris Indigo's boards are very thick and with lots of layers. If you use ~25W soldering iron you can't desolder battery. Use at least 40W or 60W iron. Heat board from soldering side for a while and gently lift battery. When next pin of battery, and after few steps you'll remove the battery. Now use special sucking instrument or just blow to remove solder from holes. You can place same type of battery, or if you don't have one, use battery from PC world. I used old NiCd 3.6V battery from old 486? board and it works fine. WARNING! You soldering, you and board must be grounded to prevent damage. Just check your soldering iron if it didn't have any short connections to mains. Connect with wire you, iron and board.
cpu board (29K)

And now few words about CPU.
In normal R4K Iris Indigo, CPU is running at 50MHz (Irix hinv reports internal speed x2 = 100MHz, prom hinv real bus speed). The clock for CPU is made by small oscilator located on CPU daughterboard. I desoldered oscilator and replace with socked. Now I can place any oscilator to test. The problems was that I have only: 40, 50 and 66.6MHz oscilators. When I place 40MHz hinv from prom reported 40MHz CPU and after boot Irix hinv reported 80MHz computer. Now I put 66.6MHz oscilator. This must give us a 133MHz computer according Irix hinv. But sadly, computer crashes. It can't pass prom test. I received message that CPU cache produced parity errors. The problem is that cache memory can't work at such speed. CPU can! It could be possible to adjust cache memory timing, but for this I need to reprogram small serial eprom chip located near oscilator. Now I am searching for 55 and 60MHz oscilators. Or maybe I'll order reprogramable oscilator.
R4K CPU, Toshiba, 50MHz