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*

[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...

[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?

[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.

[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.

[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?

[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!!!

[Blatboy]

Oooh.  Thanks!  That gives me hope.  Replacing caps I can do.  (famous last words)  And I'll remember to put the cover on when testing...

[Blatboy]

Resistors and Caps ordered.

I'll tell ye how it goes...

[Blatboy]

Yeah, the screen was fine when I smelled smoke.  The fuse is ok. I think it was a short though the chassis to the motherboard that may have caused the fault. (I did a half assed job of reassembling the computer to test.)   Hopefully nothing else got fried.  Thanks for that extra info about the main reservoir caps!

[Blatboy]

The floppy's power was connected... though I believe there was another molex there that wasn't... and, yeah...who knows where that thing was.

[Blatboy]

Well, I was finally able to open up my PSU last weekend and test it.  Everything more or less seemed ok, believe it or not.  Boy those load resistors got hot.  I expected that though.  

Regardless, since I had already purchased the caps, I replaced the caps on the PSU with some high quality new electrolytics.  

I reassembled the PSU and installed it in the A3000.  It worked... to an extent.  However, I do feel 90% sure the power supply is ok.  I'm going to do one more little bit of testing with it to turn that 90% sure to 100% sure before I move further.

I do have some other issues (story of my life) but I'll spare you those until I've confirmed that PSU can't be an issue, and only then will I start another thread of questions.

I wanted to make a special point to thank for all your helpful suggestions (especially you Tenacious for your detailed explanation of using load resistors.)  I really appreciate it!! You all are a huge help.

[Blatboy]

Interesting you say that.  Is that just a matter of cleaning the contacts w/isopropyl?

Here's the update:

I did a double check of all the voltages coming from the PSU and every output seems to check out ok.

However, I only get the screen asking to enter a floppy. (Which is further than I got after I thought I fried it, and before I replaced the caps.)

The power light and the drive lights don't come on.  The light on the hard drive comes on, but it's obviously not booting from it.  This is still consistent w/what I've read (thanks Castellen!) as symptoms of a power supply issue.  However, the power supply fan does work...

When I originally got the computer, this was also a problem.  However, when I hooked up a bunch of SCSI equipment to it (that came w/the computer) the computer magically booted from the HD, and the power/drive leds worked.  Now, when I hook up that SCSI equipment, in ANY configuration, it still won't boot up from the HD.  Furthermore, one piece of equipment, an Iomega Bernoulli, now, when it's connected and powered up, the A3000 power supply won't even turn on.  (something shorted?)  Ugh.  I worked with SCSI a lot in the early 2000s on Macs.  I remember feeling like SCSI was some voodoo technology hahah.  

That said, this makes me think that there still may be an issue with the power supply.  If the output voltages read ok, and I have new caps, does that rule out a problem with the power transistors or the rectifier?  Or could that still be an issue?

What I have done:

• Replaced the caps in the PSU with quality Nichicon caps
• Double checked output voltages on all pins - all good here
• Checked pin 24 on the external SCSI port for a reversed diode - I get 5V when power is on - no continuity to ground when power is off - seems ok
• Checked the jumpers on the HD - it is terminated
• Made sure all ICs are seated well (as well as the 3640)
• Tried powering up with no Zorro cards attached
• Tried powering up with just one card attached (I only have two)
• Tried powering up with no floppy attached
• Tried powering up with every possible combination and order of SCSI devices to see if I could recreate the earlier configuration that booted.

There is no problem with the battery.  It seems the previous owner took care of that (replaced it) before it became an issue.

It has a 3.1 ROM.  I think it's running WB 3.1 on the HD as well, if I remember correctly.  From the looks of it, it's pretty maxed out on RAM.  It's a tricked out A3000.  I want it to work!  :) :)

[Blatboy]

That's a good idea.  I've yet to get the floppy to boot from a disk, but I'm using 20 year old WB disks.  I'll make a new on on my 1200 and give that a go.  Not 100% sure the floppy drive is good.  I don't remember it reading anything correctly in the short window I had the computer working.  That said, I had 2 other floppy drives hooked up at the time and I didn't make good notes of what worked and what didn't.

Thanks

[Blatboy]

One problem leads to more...

Seems the floppy drive doesn't want to play nice.  I created new 3.1 Install and WB disks using my 1200.  DF0: on the A3000 keeps saying "Not a DOS disk in device DF0".  I took an old floppy drive from an A500 that I know isn't working (gee I wonder where this is going) and it almost worked...at least it got to the WB screen... but then I got errors.

I guess if I'm going to go this route to see if the HD shows up with the 3.1 install disk, I'll need to pilfer a working floppy drive from my 500 or 1200...

My gut still thinks it's a PSU issue, especially since the power and drive lights aren't lighting up on the front panel.  When the computer worked magically before, those lights came on. I think perhaps the HD isn't getting enough juice or something. I'll take the PSU apart again and test the rectifier, which I neglected to do before.  That said, if the rectifier or the power transistors were failing, I wouldn't be getting correct voltages on the outputs, right?  

I don't really have a way to test the drive, as I don't have another machine that takes SCSI drives (ok I have a Mac Classic II but I don't think that will do me any good at all.)