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Decstation 5000/200 Power Supply Failure

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legodude

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Oct 3, 2023
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I got a Decstation 5000/200 and started installing Ultrix. The install crashed, and then the power supply died (maybe the power supply dying caused the hang) with 0v on the 5v rail and the LED is off.
I can replace the power supply with a new 5v unit and 5v to +/- 12v module, but I'd also love to get the original power supply running. There is nothing obviously wrong with it in terms of caps or blown resistors. Any common failure modes for these power supply?

thanks
mike
 
I have a DECSystem 5000/240 that I thought had a faulty PSU. It was actually a faulty tantalum capacitor on the main board near to the PSU that was faulty.


20220616_094308.jpg
 
Rob and Rob,
Thanks for your hints, these are very helpful starting points. One other question - my power supply has yellow and white wires going to a molex connector, along with two black, green and blue. For completeness, the black wires are common, blue is -12v, and green is 12v.
It looks like the yellow label is "WARNING" and the white label is "POK." I assume POK lets the system board know when power has stabilized and it can come up, I don't know what warning does though?

thanks
mike
 
My photograph was taken when I was troubleshooting some time ago, it would also have the Molex plug connected but I had it unplugged for troubleshooting.
 
I don't think I was clear - I wanted to make sure that I wasn't missing a need for an "enable" signal to activate the 5v output and that is why the power supply seems dead.

mike
 
Same problem here!
Should a healthy power supply start with no load / not connected to the board?
 
legodude said:
Rob and Rob,
Thanks for your hints, these are very helpful starting points. One other question - my power supply has yellow and white wires going to a molex connector, along with two black, green and blue. For completeness, the black wires are common, blue is -12v, and green is 12v.
It looks like the yellow label is "WARNING" and the white label is "POK." I assume POK lets the system board know when power has stabilized and it can come up, I don't know what warning does though?

thanks
mike
Click to expand...
I have to say I don't know or don't remember as it was quite a long time ago.
 
Its been a while and may notes are vague, but I think that the 5V was present with the Molex plug disconnected. When I plugged in the Molex plug and switched the machine on the PSU crow-barred and i saw a flash near to the Molex connector that is when I noticed the black burn mark around the tantalum capacitor. The machine would no longer start. I removed the capacitor and my machine started up ok. My post around this https://forum.vcfed.org/index.php?threads/decsystem-5000-240.1239044/


See my post
 
Thanks for all the helpful replies
I've gathered a 5v power supply and 5v to 12v/-12v buck/boost converter to use as a stopgap solution as I am debugging the real power supply. Before I hooked these up, I checked the motherboard and unfortunately, I have continuity between 5v and ground, ~26ohms of resistance. For completeness, there is no continuity between -12v or 12v rails and ground.
I inspected the board carefully for any signs of blown components as above, without luck. Any hints on where to go next? There are a handful of tantalums and I did desolder a leg of each one and lifted while monitoring resistance without improvement in the short.

mike
 
Last edited:
Give it a try. you can use glass fuses with a reasonable breaking current (like 2.5A) on each rail to be sure you don't have a hard short somewhere
 
Okay, I gave it a try. The 5v rail stays at 5v so I don't think there is a short and the other power rails are okay. I looked at the board with a thermal camera and didn't see any crazy (all <80C) temps. No hot discrete components.

However, there is no activity out of the system. I tried various combinations of ground and 5v for POK and WARNING, without luck. Does anyone know the expected behavior of these pins for this system? I can't find any direct references for the Decstation 5000 machines, but I did look at some of the Vax power supply replacement threads here.

thanks
mike
 
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