Once the fastest and most expensive Apple product was Macintosh IIfx. It has 68030 CPU with 68882 FPU running at 40MHz and it contains all kind of specialities like 64 pin RAM. But soon lost its place when Quadra series was introduced with faster 68040 CPU.
I spotted an advert of a second hand IIfx with reasonable price. It was sold as defective as it didn't start but it looked clean inside so I bought it. It came with 20MB RAM, SCSI drive and rather unusual ProNitron 80.21 V1.5 1152x864@78. Luckily IIfx has mostly Tantalum capacitors there are only two electrolyte capacitors. But clock batteries are really risky as they tend to leak and ruin the whole logic board beyond repair.
One of the speciality of IIfx is the requirement of two good 3.6V clock batteries for startup. The power supply doesn't provide any voltage unless it triggered with current from two batteries combined. The computer is totally dead if batteries are not ok and I believe this is not common knowledge. So I thought I would get a working IIfx cheaply by just replacing batteries.
After replacing batteries there was no sign of life. There are two usual ways to start IIfx, using the start button of the ADB keyboard or the start button behind the IIfx case. Neither one worked but jump starting by applying current directly to the PSU connector pins on the logic board worked. It was a relief to hear the startup chime and hard disk access sounds but I could not see anything as my LCD screen was not able to sync with the display adapter.
The startup circuitry of the IIfx is pretty complicated but fortunately enthusiastic Mac users have figured it out before me and there are schematics with trouble shooting advices available. I replaced two electrolyte capacitors with tantalum ones even though I didn't see any bulging or leakage. This operation did not fix the startup issue so I had to go through startup related traces. On the backside of the logic board there was one trace that had some corrosion looking changes on it. Somehow the goo from capacitor found its way there and ruined one trace. I washed the board with IPA and soldered a long jumper wire to bypass the damaged trace. Now the startup works as designed.
Lack of working video was the next issue. My old Sanyo PLV-Z4 projector was able to sync and I got to see the happy mac. Also the old SCSI hard drive was still working and system booted from it. By browsing the files I would say this IIfx spent its last days as a printer spooler in an architect office.
I bought a bit more typical Apple 8•24 display adapter and Asante MC3NB network card from a fellow Mac collector. The floppy disk drive was also working after normal cleaning. Now there wasn't anything urgent left to fix.
My initial idea was to try out A/UX on this computer but instead I started to think the overclocking possibilities. The normal operating frequency is 40 MHz which is generated by 80 MHz crystal oscillator. 50 MHz was easy to achieve with 100 MHz crystal and some hook probes. That wasn't enough and I needed more dynamic way to generate different clock signals.
I bought couple of Si5351s and SN74AHCT125Ns. The idea was to use Si5351 to generate clock signal and then use SN74AHCT125N to change its level more suitable to 5V circuit. An Arduino MEGA was used to setup Si5351 at the startup.
The Si5351 is a versatile I2C programmable clock generator that is ideally suited for replacing crystals, crystal oscillators, VCXOs, PLLs, and buffers.
After bunch of crazy looking experimentation I managed to achieve 58 MHz:
https://68kmla.org/bb/index.php?threads/micromac-speedy-for-iifx.42709/page-2#post-475491
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