Apple CPU cards will not work in the PowerBase. The PowerBase chip set requires a couple of CPU signals which aren't present on the Apple CPU card connector.
It's hit or miss whether third party upgrades will work in the PowerBase. It depends on whether the upgrade is late enough (new enough) to have implemented the signals that the PowerBase requires.
That will not solve your immediate problem, but it will save you from wasting time on CPU cards which can't work in the PowerBase.
I would check the 3.3V power on the logic board. I had a logic board once which had a similar problem. It turned out that there was an internal short in one chip between the 5V and the 3.3V supplies and the clock buffer to the PCI bus runs off of the 3.3V supply. No PCI bus clock, no PCI devices. No PCI devices, no video. So, good chime. No video -- or other PCI dependent activity.
I found the shorted chip, once I knew what to look for, because it was very hot. But it could also be an internally shorted capacitor, or similar. Take an ohmmeter to a powered-down board between 3.3V and Ground. It should be, IIRC around 500 ohms. Check the resistance between 5V and 3.3V and 12V also. I don't remember what's typical there, but the values are kind of an average of all the leakage across the chips, so take some measurements on some known good board -- like a 7500 or similar and compare. The ballpark should be about the same.
It's hit or miss whether third party upgrades will work in the PowerBase. It depends on whether the upgrade is late enough (new enough) to have implemented the signals that the PowerBase requires.
That will not solve your immediate problem, but it will save you from wasting time on CPU cards which can't work in the PowerBase.
I would check the 3.3V power on the logic board. I had a logic board once which had a similar problem. It turned out that there was an internal short in one chip between the 5V and the 3.3V supplies and the clock buffer to the PCI bus runs off of the 3.3V supply. No PCI bus clock, no PCI devices. No PCI devices, no video. So, good chime. No video -- or other PCI dependent activity.
I found the shorted chip, once I knew what to look for, because it was very hot. But it could also be an internally shorted capacitor, or similar. Take an ohmmeter to a powered-down board between 3.3V and Ground. It should be, IIRC around 500 ohms. Check the resistance between 5V and 3.3V and 12V also. I don't remember what's typical there, but the values are kind of an average of all the leakage across the chips, so take some measurements on some known good board -- like a 7500 or similar and compare. The ballpark should be about the same.