NeXT Cube powers up, 3 secs later powers down

[daniel.dlds]

Hello community,

I have a NeXT cube that was running solid for a few days ago. I was not near the computer, and when I came back it had powered down. I tried turning it back on but nothing. I then replaced the BIOS battery and now it comes up. But it only stays up for about three seconds, and then power goes off. During the time it is up, I get a picture on the main screen, a grey background. (I am using an LCD monitor)
The power supply model is a 983.

Anyone know, what may be the cause ? Or where I should start looking ?
Do these power supplies have short protection ?

Thanks,
Regards

[barcher174]

You will want to disconnect any drives and try. Sounds like a delayed start for a scsi device shorting and taking the system out.

[daniel.dlds]

The cube has a SCSI2SD board attached to the SCSI bus. I disconnected it, but the problem persists.

[barcher174]

Does the LED on the logic board flash?

[daniel.dlds]

I kept the power plug in the machine overnight. The next day, I powered it up and it turned on, and started testing the system, displaying the image of the cube. It then went into the ROM monitor window. After a few seconds it went back to the testing system screen started the process again, and then it powered down.
Right now I have removed the power supply from the case, and opened the top lid, to see if there are any bulged or leaking caps. I am not shure of what is the problem, but I suspect it may be capacitor related. I have not noticed the motherboard led flashing. I will assemble it back and check that.

Thanks,
Regards

[barcher174]

New caps seems like a reasonable place to start.

[daniel.dlds]

Yes, I also think so. But replace ALL the caps ? Are there any specific caps that I may try replacing first ?

[barcher174]

I would start with all caps on the logicboard. If the power supply is going bad you can also check for noise/dips on the power rails. I would suspect the logicboard first in this case since it boots consistently. Another possibility is that the realtime clock chip is going bad, but thats less likely than caps.

[daniel.dlds]

I got the circuit board out of the case. Now I have to discharge the capacitors.
There is no way of identifying any faulty capacitor while its still on the board, right ?
Seems like I will have to replace ALL of the capacitors, am I right ?

[MrPix]

The capacitors on the main board will all self discharge within 20-30 seconds. The high voltage ones in the PSU may take a couple of hours or 30 seconds depending on PSU type.

[barcher174]

Honestly if you dont see damage its faster just to replace them all than to try to debug it. They are past their useable life now anyway and will certainly fail in the future.

[daniel.dlds]

I would really like to be able to repair it. It's one of the most common points of failure.

[daniel.dlds]

I would start with all caps on the logicboard. If the power supply is going bad you can also check for noise/dips on the power rails. I would suspect the logicboard first in this case since it boots consistently. Another possibility is that the realtime clock chip is going bad, but thats less likely than caps.

When you refer to the 'logic-board' are you talking about the mother-board of the computer or the power supply PCB ? The mother board was recapped when I bought it.

[barcher174]

I meant the motherboard. It is possible the power supply is at fault but I would suspect the motherboard fist since you are booting to a grey screen.  If the caps have been replaced I would then suspect the real time clock chip.

[daniel.dlds]

can you help me locate the clock chip on the board ? maybe through a picture ?
As for the power supply, can I test it with no logic board ?