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Connected AT power supply backwards...

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kishy

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Aug 22, 2009
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Location
Windsor, ON Canada
In a nutshell, I'm screwed, right?

Just wondering if there are some components that could be replaced that might undo the damage. Doubtful, but hey, gotta ask.
 
Connected it backwards to... what? What happened when you did that? Smoke, burning? What does it do now?

And by backwards, do you mean, you connected both plugs rotated 180 degrees, or you connected the plugs swapped with each other? Both situations would have taken some doing, since the plugs are keyed, but I think the latter is more likely.

-Ian
 
Nope, not keyed. That's the problem. They're supposed to be, but I think I've only seen one power supply that actually had them keyed.

Connected them swapped with each other to my backplane board and fried my single board computer, evidently. It won't power on with another known-good PSU now. One of the installed PCI cards went up in smoke, but there was no evident smoke from the SBC itself.

The 'going up in smoke' happened after I realized the mistake, corrected the connectors and tried to power it on with them correctly connected. The machine refused to power on while they were connected wrong.
 
Those LSI chipset ICs can be pretty delicate. My guess is that you fried one of them at a minimum.

Rule of thumb is that on AT power supplies with the generic 2-connector output is that the black (ground) wires on both connectors are on the inside and adjacent.
 
Oh yes, I know. It boggles my mind that I missed that because it's something I'm very aware of and normally very careful about. I suspect it has something to do with it being 2AM last night...

I figured it was probably toast. With any luck the internal LCD (this is a luggable box) didn't get roasted too, but I've heard nasty things happen when the polarity into an LCD is reversed.
 
It used to be that some motherboard manufacturers put a few silicon power diodes across supply leads to guard against this sort of thing. Probably not a bad addition to valuable equipment that doesn't already have them.
 
Well, like I said, it didn't power up with the PSU connectors reversed...it just went poof when I corrected it. You know a good amount about PSUs and their interaction with their systems, I've noticed, so do you have any insight into what exactly might have happened here? Is it possible that there were any 'sacrificial' components designed into it that should fail first and protect the rest of the system?
 
Maybe--you didn't give any specifics on the motherboard, other than it had a PCI card on it.

First off, when you say "reversed", exactly what do you mean? AT power supplies have two connectors, so there are 7 wrong ways to plug them in. Let A and B be the two connectors in their "normal" orientation and let a and b be the same connectors, but "flipped"--inserted upside-down. So, if AB is the correct way, then the wrong ways are BA aB Ab bA Ba ab and ba.

When we know that, then we can extrapolate some likely failures.
 
It's a CI6BM single board computer. P2/P3 Slot 1, i440BX, integrated video and LAN, PICMG 1.x form factor (PCI and ISA), plugs into a backplane.

Connectors were installed "correctly", but with their positions reversed. BA in your example.
 
Okay so, if the normal signal ordering is:

+5 +5 +5 - 5 G G | G G -12 +12 +5 PG

the reverse would be:

G G -12 +12 +5 PG | +5 +5 +5 -5 G G

The first thing I'd do is check for shorts--it could be something as simple as a shorted capacitor. Since +5 and Gnd would share the same traces on the motherboard, I don't think it's any major IC.
 
Well, took the board out and did a visual inspection. There is only one visually damaged component. It is labeled on the PCB as TC55, and melted beyond recognition, but all the other TC-prefixed components are small yellow boxes with the marking "C 10u" where the u is the micro symbol. It is surface mounted with two contacts.

Ceramic capacitor?

It's right above the ISA connector which I presume is where the power feeds in.

How much resistance should I expect to see across a capacitor to know it isn't shorted?
 
Bad pic, scanner isn't the best for this sort of thing, but here it is:

burnt_component.jpg


Lower left is the destroyed one, the two other yellow boxes appear to be the same thing.
 
That particular part might not be *too* bad to have to change, at least out of all surface mount parts it's a little bigger one. I've had positive experiences in the past with using a regular soldering iron to change out that style of capacitor.

It helps to find a fine tip for the iron, but basically you would need to melt the solder on one side and slightly pry that side up just a little bit. Then work the other side the same, usually it'll take a couple times back and forth to get it to lift without prying a trace off the board, but it's entirely do-able.
 
Had a positive ID on the component as a capacitor, and the good news is I probably have some in my junk pile already.

Yeah, for surface mount, it's about as good as it gets. Two spread-out contacts, nothing nearby, good size. My iron tip is in bad shape though so I may have to get creative. I've desoldered and resoldered tiny surface mount things enough to make contact, but the finish is never as pretty as original.
 
Chuck - did as you said, set it up inside a plexiglass containment chamber with a new PSU and guess what - SINGLE POST BEEP! IT'S ALIVE!!!

What consequence might there be to running it without that cap present? It directly connects to the -5V line of the ISA connector (which, keeping in mind this is a single board computer, is one of the lines from the PSU into the computer)
 
Probably none, if it's just a lone PCI board. However, the more heavily you load the power supply with expansion cards, the more important this part becomes. I'm sure that you can manage to solder in a new one--SMT isn't that difficult. Just don't create any solder bridges and solder the new cap in one side at a time, holding it in place with something like a toothpick (these things get hot!). Use a light touch and you should be fine.
 
Thanks for the guidance, system is back together and running fine :) (with original PSU even)
I have two PCI cards for it, one is a miniPCI wifi card in an adapter and the other is wired ethernet.

Lesson learned - glue those stupid connectors together from now on!
 
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