I fried my A3000 PSU!

[Blatboy]

I was attempting to repair an A3000 I had recently obtained.

It worked when I first got it, then I moved it around some.  When I set it up again, I got a blank screen when booting...

I worked on the issue for a couple evenings.  After taking it apart a couple times I found the culprit!  The crystal had fallen out of it's socket in the A3640.  I put it back in.

When I put it back together and flipped the switch, finally I got an image on the screen!  Huzzah!  Hmmm, the power light on the front of the computer wasn't on...hmmm... what?  Is that magic smoke I smell escaping from my A3000?  Power down!!!

Alas, the smell was coming from the PSU.  And now, it won't power up again.  I was so close to getting it back working, and it seems now I fried it.  Aaaaaargh!!!

So, I see that I can possibly replace the PSU with an ATX supply, but I'm concerned.  What could have caused the original PSU to do that?  I'll admit I had really given it a serious go today, constantly powering up and down while troubleshooting.

I'll admit I didn't have the chassis attached when I powered it up (the one that holds the HD and the FD)... it was just sort of resting, more or less where it was supposed to be.  Could that have caused a short or something?

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.

any input from you very experienced Amigans would be so appreciated!

Thanks!

*sigh*

[giZmo350]

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.

Thanks!

*sigh*

Oh man... I gotta admit I know nothing about A3K PSU's.... ahhhh, sorry... but when I came to this sentence I thought "Substitute the word PSU with Girlfriend".... :lol:

[Blatboy]

Yuk yuk yuk  aw man.

ahem.

I do have the know how to replace the caps in the PSU, which look a little bloated.  However, perhaps the damage is already done?

Maybe I'll try a new fuse... if that works, then I'll replace the caps...

[tonyvdb]

If you go that route take the PS out of the Amiga befor you power it up again. If it turns on then go to the next step. No point in damaging anything else.

[Blatboy]

Totally understand.  However, if it's not hooked to the computer how do I tell it's working?  A meter?  Will the supply work without a load?

[tonyvdb]

Everything inside the PS will power up even without it hooked up so that will at least tell you is its ok.
Yes, you can use a meter on the pins to look for voltage.
http://www.hardwarebook.info/Amiga_3000_Power_Supply

[ElPolloDiabl]

Someone accidentally shorted there power supply earlier this year by letting their hard drive touch the motherboard. They eventually replaced the PSU and everything was working normally.
That might be what happened. A 12V short would have drawn a lot of amps.

It sounds like you should have been more thorough at the start. Most of us learned to find faults by 'process of elimination.' Check each card individually etc.

[Blatboy]

Thanks for the link!  This shows a diagram of the pinouts:

http://www.oocities.org/ovegun/Amiga-3000/A3000powersuply.html

The HD was on the chassis, but the chassis perhaps was touching something on the motherboard it shouldn't.  This is my guess.  I should have been more meticulous about that.  I was so excited about finding the problem I cut corners.  Thank goodness for society I don't do something for a living that puts lives on the line.  *sigh*

Anyhoo, I'll be ordering some fuses (250V 5A) and will start from there... on my workbench, not in the computer!  If it's not just the fuse (I did smell the smoke after all) I'm hoping switching out the caps will do the trick.  Those were on their last legs from the look of it anyway.

[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.

[Blatboy]

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.

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

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

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.

[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.

[Blatboy]

Thanks so much for all this info.

I took out the fuse.  It's still good, which I think is unfortunate.  That could have been an easy fix.

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.

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!)

I'm pretty handy with a soldering iron, and I've built a PSU (a bench lab supply) from scratch (not a kit), so I've got some chops... but (obviously) I'm no pro.  I think I can handle this with a wee bit of direction.    THANKS!

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

[QuikSanz]

If you have built power supplies you know the chain. Test output from transformer then move to full wave bridge and down the line for each voltage output. you'll find it.

Chris

[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!  

[Blatboy]

If you have built power supplies you know the chain. Test output from transformer then move to full wave bridge and down the line for each voltage output. you'll find it.

Chris

Yes!  I know the chain.  It's great to be reminded of the process though.  Thanks.  Seriously.

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.

That is probably the only thing Tesla and I have in common. Haha.  I feel more comfortable dealing with a PSU than I would sensitive CMOS components or logic components or something like that at this point... so I'm going to take a good hard look at that PSU before I go ATX.  It's always an option though, thank goodness.

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 no stranger to Ohms law.  That said, the process is everything and this is very helpful.

And re: hot snot... I guess it's just isopropyl one needs to get rid of it.  That, I have.

https://www.youtube.com/watch?v=dNNqSXgj6DE#t=96

This is great!  Thanks for the help!!!