I fried my A3000 PSU!

[Tenacious]

I opened up the PSU and took out the fuse... I don't think it's blown... but I'm not 100% sure.  Since I smelled bad things coming from there, perhaps I shouldn't bother with even trying a new fuse and I should just get a new PSU.

When you said this, I wondered if you have a meter to use.  Any meter should tell you about that fuse, and, which power rail (+5 VDC, +/-12 VDC, etc) failed.  It would also be handy for finding a shorted capacitor.  If yours has capacitive ranges, you can confirm the spec of each one.

Are there any burned or obviously damaged components in the PS?

You could use some 30 ohm, 5 Watt resistors to load the PS for testing the outputs.

[Tenacious]

Wow.  You are so right. I have meters coming out my ears.  (ok.  I have three.  There's no probes sticking out of lobes just yet)  I can hear my old instructor giving me hell now.  Just measure the fuse.  Duh.

 

Best to get it out of the circuit and test its resistance alone.

Regarding the possible failed power rails... is that just a simple resistance check vs ground?

I'd use the voltage scales , perhaps, with the loading resistors mentioned above.  The meter and the resistors would go from the 0 VDC (common or ground) connection to  each of the power rails.  You are correct, a switching PS should be loaded somewhat to produce its rated voltages.  The resistors are less risky than powering the A3000 for testing (Tony mentions a good precaution.).

I believe my meters measure capacitance... is that what you mean?

Again, you would have to get at least one leg of a suspect capacitor out of the circuit to test it with these ranges of your meter.  I hope you're handy with a soldering iron.  

No obvious damage from what I could see... the caps bulge a little, which is a sign of failure to come if it wasn't a cause now.

Those would be the first to check.

I think its worth your time to save the original PS if possible.  A PC supply, while usable, is not an exact replacement.

[Tenacious]

Since the PSU doesn't seem to power on (even with the fuse in - the fan doesn't come on either) I should test the rails with the power applied, if I'm looking for voltage, right?  And, I'll use the resistor to emulate a load.

 

Yes, you will have to test for voltage (the DC rails) with the power ON, BE CAREFUL.  Nicola Tesla always kept one hand in his pocket while testing to kepp dangerous voltages away from his heart.  Many of the components, heat sinks, traces, etc will have dangerous line voltage on them!  You may have decided that going inside the PS box is not worth the risk.  No harm done.

When testing the supply, I would load all rails simultaneously with resistors.  Lets revisit those loading resistors.  Using Ohm's Law, a 30 ohm - 5W resistor will load the 12 V rails, both the + and the -, with 0.4 amps.  MAKE SURE all rails are rated (read the tag on the PS box) for more current than what the loading resistors will draw!   If not, adjust the resistance values UP accordingly.  The +5 VDC rail could typically take a 15 Ohm - 5W resistor.  

If you are getting outside of your comfort zone, google Ohm's Law. You could also run these tests first on a known-to-be-good PS from a disposable PC and monitor how it behaves.  

PS fans are often powered by the +12 VDC rail rather than the 120 VAC line.

I'm taking the PSU out of the computer and will, from this point on, only be working on it on my bench until everything checks out (which may take a while.  This started as such a simple fix.  Waaaaaaaaahh!)

This has come up many times in the recent past.  There are many links to modifying PC supplies to Amiga 3000s use on this site.  If your supply is confirmed dead, maybe this route would have more valor.

Re: those capacitors... all that hot snot holding them in place!!  How do you safely remove that gunk to get to the capacitors?

Carefully and with the power OFF!  

[Tenacious]

It's very good to see people tackling the problems of vintage computers without giving up.  I think many are interested in how you solve the problems at hand.

Just to mention it, Ive had my A3000 appear completely unresponsive in the past (especially after being opened and disturbed) and it turned out to be dirty oxidized connectors where the MB mates to the Zorro daughter board.  Food for thought.