NeXT Cube powers up, 3 secs later powers down

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

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

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

Does the LED on the logic board flash?

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

New caps seems like a reasonable place to start.

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

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.

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 ?

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.

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.

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

Quote from: barcher174 on July 14, 2022, 02:06:19 PMI 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.

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.

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 ?

I got the attached image from the ATX conversion page :



Which PINS of the left most column should I short ? When doing this, I think that I should put 3V on the pins of the column to the right ?

PS: if image does not appear it is at : https://ibb.co/1QD9Y3b

Thanks for any help,
Regards

The RTC is next to the battery and a small surface mount crystal. To check the power supply rails you also may need a load across the -12v rail to keep it powered. Otherwise, yes just hook the battery to the marked input.

Quote from: barcher174 on October 16, 2022, 02:45:07 PMThe RTC is next to the battery and a small surface mount crystal. To check the power supply rails you also may need a load across the -12v rail to keep it powered. Otherwise, yes just hook the battery to the marked input.

So to power on the power supply I should (in sequence) ? :
- apply a load between a pin on the 12V and GND
- supply the 3V to the input marked -3V momentarily (I suppose this is the signal sent from the battery when the power button in pressed)

Thanks for any help,
Regards

1) -12v to GND ~330ohm

2) You need to keep the 3V applied I think.

I made a small diagram of the connector of the Cube's power supply, the resistor acting as the Load and a bench power supply to turn on the NeXT power supply.
it is here : https://ibb.co/8gRjGj5

There is a question mark on the second wire coming from the power supply (GND or negative, I really don't know which). I think I need to make this connection somewhere because if not, no current will flow into the -3V of the PSU.

Am I right ?

The 330ohm should go from the blue boxed -12v to black boxed GND. The battery should attach from light blue boxed 3v input to black boxed GND.

Thanks for you reply.
What is the power rating of the 330 Ohm resistor, that I should use ?

Thanks,
Regards

A big one. Like 5w. Alternatively I use a bulb like this: CBconcept - 10 Bulbs - 12V 5 Watts, AC/DC JC T3 G4 Bi-Pin 5W Halogen Light Bulb, Accent Lighting, Chandelier, Puck Light, Microwave, Range, RV, Landscape Lighting - Designed in CA https://a.co/d/e9Fb0Vd

I tried to test the power supply in isolation, by connecting the 12V to a light bulb and the battery across the -3V pins. The power supply didn't turn on. I don't know what I did wrong.
Anyway, I re-installed the PSU in the cube, and powered it on. This time it didn't shutdown after a few seconds. I left it running for about half hour and it stayed on.
Problem is that now the computer shows a grey screen with vertical stripes, and does not boot. It even does not do the system test. It just stays on the grey screen. (link to a photo of the screen : photo (https://danielsantos.world/IMG_0215.JPG))

So I am starting to suspect that there was nothing wrong with the power supply, but with the main board.
How can I continue testing ?

Thanks,
Regards

No easy answers from here. Recap —> RTC chip —> cpu or next asic fault

Hello Daniel: You can also try removing the ram and unplugging the hard drive then powering it on to see if is the main board causing that pattern. I have had that happen with incompatible ram or a bad memory stick or like Brian is saying new caps :)

I removed all the RAM disconnected the hard drive (SCSI2SD), but the behaviour is the same.
The board was sold to me recapped. Is it possible that in the space of a year or so a cap may be bad ?

Can you post some high res pics of both sides? Also some close ups around the new caps.

Links to the pics :
https://danielsantos.world/IMG_0221.JPG
https://danielsantos.world/IMG_0222.JPG
https://danielsantos.world/IMG_0223.JPG
https://danielsantos.world/IMG_0224.JPG
https://danielsantos.world/IMG_0225.JPG

Best Regards

There is a cap missing on the left hand side of pic 1 next to the video ram. Some of the ceramic replacements look questionable. I would redo the work.

See the cap placements here:

http://www.asterontech.com/Asterontech/040_cube_restore.html

I don't know how to locate it. Could you post the picture with a drawn circle around the cap ?

Thanks

In the link I provided check the green circles at the bottom.

Back at this. It has been a while. So, do you recommend that I remove the ceramic capacitors and put electrolytic ones, or just do a better job at re-soldering the ones that are already in place ?

Anyway, I am thinking of replacing all of them with the ones listed in the link you pasted.
Any one of them electrolytic or not may have failed.

Ceramics are fine. Just clean up and add the missing one.

The computer worked before. Is it possible that one of these capacitors went bad ? If so I have to get new ones. Or it is just a case of doing a better job of soldering them ?

I would be concerned the missing cap destroyed the video chip. It definitely needs to be there.