SGI Challenge L 12VDC Undervoltage - Printable Version

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SGI Challenge L 12VDC Undervoltage - Xav101 - 09-13-2021

I acquired an SGI Challenge L yesterday and a good amount of spare goodies with it. Unfortunately it wasn't totally working, the system LCD throws a "12VDC undervoltage" error and kills the main 48V power to the regulators. The two amber LEDs on both the 505 regulator and the 512 regulator are on but the red fault light isn't on for either of them. Unfortunately the 512 is one of the only spares I don't have, if that is indeed the issue. Any ideas what could be wrong? If you guys need pictures I'll post them when I get a chance.

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RE: SGI Challenge L 12VDC Undervoltage - Xav101 - 09-13-2021

Update: I tried swapping the ebus to see what would happen because I thought maybe it wasn't monitoring the backplane voltages properly. It now throws a -12V overvoltage error. The voltage menu shows -10.7V and -23.7V at the IO4 for the -5V and -12V lines respectively. The voltages for the 512 and the 505 are both zero still.

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RE: SGI Challenge L 12VDC Undervoltage - jan-jaap - 09-13-2021

Replace the VCAM on the IO4. It's the only place where -12V exists in these systems.

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RE: SGI Challenge L 12VDC Undervoltage - Xav101 - 09-13-2021

Hmm, it's definitely possible that those have gone. One thing I don't understand is why would swapping the ebus cause the negative rails to come up but wouldn't cause the 5V and the 12V to come up?

Here's the voltages menu with the old ebus installed


And here's the voltages menu with the spare ebus installed


I'm starting to wonder if there's a short somewhere on the 12V rail that's dragging it low. I found a point on the 512 that I could probe for +12VDC while it's in the machine and it seems to power up temporarily and even hit 12V one time before shutting down.

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RE: SGI Challenge L 12VDC Undervoltage - jan-jaap - 09-13-2021

You should read the CHALLENGE™/Onyx™Diagnostic Road Map. There's an entire chapter devoted to the power subsystem of these systems. You'll see that the VCAM makes -5 and -12V for the VME subsystem, and these assert the POKA (power-OK-A) condition. Multiple voltage rails assert the same error line, LEDs on the boards will tell you which one is out of spec.

The negative voltages are created with inverting regulators. It seems they are more likely to fail than e.g. a +12V regulator fed from 48VDC. I've seen my fair share of these fail.

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