Reverse engineering / fixing a PowerOne PSU

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Reverse engineering / fixing a PowerOne PSU

Unread postby jan-jaap » Thu Dec 17, 2009 3:36 am

I've got a faulty PowerOne PSU here, the model used in deskside IRIS 4D PowerSeries and Crimsons.

The fault is that, as soon as the breaker on the back is engaged and 220VAC is fed to the PSU, but before the power switch on the front is engaged, the PSU oscillates, and enables / disables the outputs a couple of times a second. This manifests itself as 'flickering leds' on the cards in the cardcage. Can't be good for the hardware :(

As soon as the ON/OFF switch is switched 'ON', the PSU starts up like it should, and the system boots up. The PSU is capable of feeding a fully loaded 4D/440VGX. As soon as I switch OFF the system, the PSU powers down like it should, and no oscillation occurs. If I kill the breaker and re-engage it, the phenomena comes back.

You could say this is an initialization problem :?

Fortunately, the PowerOne is a nice, modular design, and it is documented to some degree.

Consider this diagram from a PowerOne application note:
functional_diagram.med.jpg


The actual PSU, somewhat disassembled:
DSC_4990.med.JPG


To the left is the mains input section which (always) supplies 300VDC to the backplane. In the photo two output modules are installed, the big one supplies 5V, the smaller one supplies 12V, -12V and -5V

I have swapped the output modules and the main input rectifier board with another PowerOne PSU, and the symptoms remain. The problem of the oscillation is caused by a fault on the backplane.

Here it is:
backplane_functional.med.jpg


It is not too hard to identify the functional sections from the diagram, namely 'System Control', 'Inverter' and 'Rectifiers / Sync' (per module).

I believe the problem must come from either the 'System Control' or 'Inverter'.

Other observations:

1) Despite the oscillation, the PSU mostly functions: when 220VAC is applied the fan starts to spin, and when powered up it spins faster. The output modules work. So the inverter cannot be completely broken.

2) There's a connection between 'System Control' and 'Inverter' which is not shown in the diagram through the (white) opto coupler between them. This must be some sort of inhibit signal.

My plan is to take an oscilloscope to it and probe around, especially near that opto coupler to see if the problem starts within either System Control or Inverter.

But I have to borrow a scope, and I'm not that good with electronics ;) Anybody have any insights?
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Re: Reverse engineering / fixing a PowerOne PSU

Unread postby skywriter » Thu Dec 17, 2009 5:20 am

a couple of thoughts:
1) you have several output modules; is the fault on all the output modules?
2) does the output have a fixed periodic oscillation, or is it slow, and erratic?
3) do you have another one you can swap parts with, or compare measurements?
you may have answered this for yourself but not included them in the description.
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Re: Reverse engineering / fixing a PowerOne PSU

Unread postby jan-jaap » Thu Dec 17, 2009 6:11 am

skywriter wrote:a couple of thoughts:
1) you have several output modules; is the fault on all the output modules?
3) do you have another one you can swap parts with, or compare measurements?

I have two other, identical PowerOne PSUs. One in my 4D/440 and one in the Crimson. The oscillating PSU is my spare.

The one in the Crimson is known good, the one in the 4D/440 developed another fault: a glitch in the output would occasionally reset the system. Sometimes before it even finished booting, sometimes not for an entire evening. I have swapped out the input module and both output modules between this PSU and the oscillating spare PSU.

Two observations:
1) The system hasn't rebooted spontaneously since, but I've run the system maybe 5 hours since. Maybe this particular PSU actually had a problem with one of those modules, maybe I simply have been lucky.
2) The oscillating PSU still oscillates. Hence my conclusion that the problem is in the backplane.

So: I can probe around both PSUs, to compare a known good one with the oscillating one. I could take the known-good PSU from the Crimson and swap just the backplane with the oscillating PSU to confirm that the problem lies there once and for all.

skywriter wrote:2) does the output have a fixed periodic oscillation, or is it slow, and erratic?
you may have answered this for yourself but not included them in the description.

So far I have only used a multimeter to test it, but the flickering of LEDs on the IP7 and VGX looked like a fixed, low frequency, maybe 5 ~ 10HZ. As far as I know, inverters in DC-DC converters operate in the 100KHz range.

As I see it, the problem could be in the feedback circuit of the inverter, or in the control circuit toggling an inhibit signal to the inverter. In the latter case I should probably see a square wave on that opto coupler. In the former case I'm in deep sh*t :( Maybe I should attempt some of that Indigo2 PSU repair voodoo, and simply replace all the small elcos. They must be at least 17 years old now ...

Thanks!
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Re: Reverse engineering / fixing a PowerOne PSU

Unread postby skywriter » Thu Dec 17, 2009 6:24 am

jan-jaap wrote:
skywriter wrote:2) does the output have a fixed periodic oscillation, or is it slow, and erratic?
you may have answered this for yourself but not included them in the description.

So far I have only used a multimeter to test it, but the flickering of LEDs on the IP7 and VGX looked like a fixed, low frequency, maybe 5 ~ 10HZ. As far as I know, inverters in DC-DC converters operate in the 100KHz range.


the LED's you're looking at are probably going to be part of a POR cycle. so maybe that the periodicity you're observing is filtered through that sequence, but the 'trigger' may still be erratic. the oscilloscope will tell you better.

jan-jaap wrote:As I see it, the problem could be in the feedback circuit of the inverter, or in the control circuit toggling an inhibit signal to the inverter. In the latter case I should probably see a square wave on that opto coupler. In the former case I'm in deep sh*t :( Maybe I should attempt some of that Indigo2 PSU repair voodoo, and simply replace all the small elcos. They must be at least 17 years old now ...


my experience has been opt-isolators cause a lot of problems :) generally it's because the resistance goes to hell due to dendrite growth or some contamination inside the part, and bingo you have an intermittent current path. this is very consistent with the description of the problem. of course debugging someone elses problem with someone elses circuit at a distance is problematic, nevertheless you're on a highly probable track, if not the right one.
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Re: Reverse engineering / fixing a PowerOne PSU

Unread postby jan-jaap » Thu Dec 17, 2009 7:07 am

A more detailed image of the System Control section:
system_control.med.jpg

This circuit has some I/O (the system control signals in the functional diagram):
* Inhibit (input)
* Over Temperature (output)
* Power Fail (output)

I have identified all of these pins on a connector not visible in this photo. The PowerOne application notes suggest that the switches of SW2 can be used to make some/all of these active high or low, but this particular PSU is much older than the application note and the positions on SW2 have changed.

The 7805 is obviously used to power this circuit.

The Airpax 66L080 is a thermal switch which opens at 80deg: an over temperature protection.

Partially visible is the opto coupler I was talking about, but this article sugests it might be something other than an inhibit signal:
avago_fig1.jpg
avago_fig1.jpg (35.75 KiB) Viewed 1005 times


skywriter wrote:my experience has been opt-isolators cause a lot of problems :) generally it's because the resistance goes to hell due to dendrite growth or some contamination inside the part, and bingo you have an intermittent current path. this is very consistent with the description of the problem.

If the opto coupler goes out of spec and it's in the feedback path of the main inverter, that would explain a few things. In that case the PWM must be there somewhere too, probably built around that LM 393 just above it.

OTOH, I can't explain how the PSU can successfully power on (and off!) if the inverter doesn't function properly. This was, and still is, an initialization problem.

skywriter wrote:of course debugging someone elses problem with someone elses circuit at a distance is problematic, nevertheless you're on a highly probable track, if not the right one.

Your help is appreciated, thanks!
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Re: Reverse engineering / fixing a PowerOne PSU

Unread postby skywriter » Thu Dec 17, 2009 7:14 am

seems like we're on the right track. can you at least check the output of the opto? there should be a pull up, or pull down to VCC or VSS respectively. if the output is not at a solid high or low, then that's your culprit, or very nearly it assuming it's not another passive component fault.
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Re: Reverse engineering / fixing a PowerOne PSU

Unread postby hamei » Sun Dec 20, 2009 7:34 pm

skywriter wrote: the oscilloscope will tell you better.

Definitely. If you want to do anything beyond the very basics, pick up an oscilloscope. Digital voltmeters are especially unreliable for anything related to time. You can see a lot of stuff on a scope that is otherwise invisible. And they are simple to use.

my experience has been opt-isolators cause a lot of problems :)

You ain't just whistlin' Dixie ... I hate opto-isolators. They are definitely trouble-makers. I had an unpredictable intermittent that used to cause the carriage on a lathe to run away. When you have a 2" diameter boring bar 8" deep in a 300 lb piece of steel and the carriage decides to take off for parts unknown at 400 inches per minute, it's really ugly. After fighting that for almost a year the flaky opto-isolator finally died totally. Oh ! So that was the problem ! They can work just enough to seem okay.

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Re: Reverse engineering / fixing a PowerOne PSU

Unread postby skywriter » Mon Dec 21, 2009 8:04 pm

hamei wrote:I see that Sky is more than just a manager :P


yeah, i'm working my way up to janitor!
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Re: Reverse engineering / fixing a PowerOne PSU

Unread postby Dr. Dave » Wed Dec 23, 2009 11:36 am

In most small-ish switching supplies, the optoisolator is usually used as part of the analog voltage feedback loop, tied to one of the output rails (for a group-regulated design) so it is not usually just used to isolate a static signal. Fortunately opto's are not terribly expensive new.

If the 'oscillation' is fairly quick, thinking about what sorts of things might change at that rate is useful. If I had to guess - and has been noted above - that the supply starts but doesn't like one of the rails/sensors, it may initiate a new restart cycle. Rinse and repeat...

And for any PSU of this age, check your caps. If an electrolytic has dried out somewhere, it may not be holding enough voltage somewhere to latch the PSU into the 'fully-ON' state. Cap issues should be statistically more prevalent than ones involving lead migration issues.
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Re: Reverse engineering / fixing a PowerOne PSU

Unread postby GanjaTron » Tue Aug 02, 2011 8:24 am

Hi JJ & co,

I'm reviving this thread to contribute my experience in troubleshooting a PowerOne with very similar symptoms to those described by JJ.

My Crimson's PowerOne acted up and began to power cycle rapidly and erratically a few minutes after flipping the circuit breaker (so obviously a thermal problem). On top of that, the output voltages apparently caved in if the system was fully loaded (as mine was), so I had to remove the RE and/or the 2nd HDD to even get it powered up.

Since JJ suspected the backplane as the primary culprit, I focused on that when I dismantled and inspected the PSU. Sure enough, almost all the 47uF/35V electrolytic caps on that board needed replacement; some had already leaked, while others had ridiculously high ESR.

I also noticed that the Y2 class EMI suppression caps (four on the primary input board, one each on the backplane and SGI custom board) were seriously cracked; while these were probably not responsible for any misbehaviour, it was only a matter of time before they blew.

Having replaced the above and reassembled the PSU, I gave it a test run on the bench with no load attached. While the PSU powered up, the output modules didn't, nor were they supposed to, since inhibit is configured to be asserted if pins 10/11 aren't connected. A 2nd test with attached load yielded the same result, even after 10 minutes of warmup time.

Satisfied with that, I reinstalled the PSU and double checked (!) wiring, then fired up. No probs, and the voltages were all ripple free and within spec. The PSU no longer power cycles on its own, so that looks pretty good... so far.

A few caveats to those embarking on such an endeavour: it's highly advisable to test the PSU with a combination variac / isolation xformer... assuming you've found a way to prevent it from knocking out your fuses! ;) If you have one, use it.

Beware also that the backplane carries 300VDC to the modules, which can give quite a nasty shock (possibly regardless of whether you're using an isolation xformer of not). Note also that this voltage lingers for considerable time after the PSU has powered down and the fan has stopped, due to the large time constant of the primary board caps no doubt. Check the backplane with a voltmeter before poking around inside!

DON'T accidentally connect live and neutral to the terminals labelled 1&2 (sorry JJ). That will blow the PSU primary. Double check your wiring!

Don't forget the obvious: keep screwdrivers, loose screws (that includes your mental state) etc. well away when testing this baby, unless you wanna take up sculpture welding as a hobby... :D

Tighten the 5V busbar nuts, since a loose connection could (theoretically) generate some serious heat at 170A. Personally I find it particularly worrying that SGI chose to route the cable harness around them just underneath the PSU. What happens if the bars run hot and melt the adjacent insulation? :?

Finally, as JJ already pointed out, this PSU with its 170A capability should be taken seriously. Know what you're doing, and again: double check everything to avoid a catastrophe.

Hope this comes in handy for those with similar trouble. Good luck!

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Re: Reverse engineering / fixing a PowerOne PSU

Unread postby jan-jaap » Tue Aug 02, 2011 12:59 pm

GanjaTron wrote:Since JJ suspected the backplane as the primary culprit

Explanation: I have one with similar issues, and an identical, known-good one. The modular nature of these PSUs makes it easy to pinpoint the problem to a module. In this case: the backplane.

GanjaTron wrote:Sure enough, almost all the 47uF/35V electrolytic caps on that board needed replacement; some had already leaked, while others had ridiculously high ESR.

I also noticed that the Y2 class EMI suppression caps (four on the primary input board, one each on the backplane and SGI custom board) were seriously cracked; while these were probably not responsible for any misbehaviour, it was only a matter of time before they blew.

Having replaced the above and reassembled the PSU, I gave it a test run on the bench with no load attached. While the PSU powered up, the output modules didn't, nor were they supposed to, since inhibit is configured to be asserted if pins 10/11 aren't connected. A 2nd test with attached load yielded the same result, even after 10 minutes of warmup time.

I ended up replacing the electrolytic caps in mine and it didn't change a thing. Then we had a baby and I put it aside for the time being ;) I'll order some of the Y2 EMI caps, but mine didn't look bad and I have a feeling it won't make any difference.

GanjaTron wrote:DON'T accidentally connect live and neutral to the terminals labelled 1&2 (sorry JJ). That will blow the PSU primary. Double check your wiring!

LOL, yeah. Shit happens. It blew a 30A fuse, the primary works fine (after I replaced the fuse, of course). FWIW, terminals 1,2 switch the PSU between 110VAC and 220VAC operation (these are not auto-ranging)

GanjaTron wrote:Tighten the 5V busbar nuts, since a loose connection could (theoretically) generate some serious heat at 170A. Personally I find it particularly worrying that SGI chose to route the cable harness around them just underneath the PSU. What happens if the bars run hot and melt the adjacent insulation? :?

Well, it is capable of supplying 170A, but unless you apply a very low-Ohm load, it won't. With a 5 Ohm resistor loosely attached to the terminals, it still won't exceed 1A @ 5V. Unless a stray wire shorts the terminals, of course. Then you'll have drops of molten copper all over your workbench :oops:

I'm glad yours works again. Enjoy your Crimson :)
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Re: Reverse engineering / fixing a PowerOne PSU

Unread postby GanjaTron » Wed Aug 03, 2011 10:41 am

I ended up replacing the electrolytic caps in mine and it didn't change a thing. Then we had a baby and I put it aside for the time being ;) I'll order some of the Y2 EMI caps, but mine didn't look bad and I have a feeling it won't make any difference.

Well, the electrolytics are always a good starting point, so it doesn't hurt to replace them. Maybe your optocoupler is dodgy, or there's something wrong with the inhibit circuit itself.

I also doubt the Y2's will make a diff. I have seen these blow spectacularly though, so it's a good idea to replace them after they've been sufficiently heat stressed and begin to develop cracks.

Btw, I take it you put aside the PSU, not the baby? :)

LOL, yeah. Shit happens. It blew a 30A fuse, the primary works fine (after I replaced the fuse, of course). FWIW, terminals 1,2 switch the PSU between 110VAC and 220VAC operation (these are not auto-ranging)

Well, glad to hear it survived! Tracking down a 30A "Littlefuse" is bothersome enough tho. I take it the 110VAC configured units have a jumper there?

Well, it is capable of supplying 170A, but unless you apply a very low-Ohm load, it won't.

Has anyone ever stuck an ammeter in there? :D

I've always wondered what a Crimson RE draws. My AC mains meter indicated over 600W (800W peak with 2 RM4s), but that's only a vague indication of how many amps the 5V bus actually carries (particularly due to power losses in the PSU).

I'm glad yours works again. Enjoy your Crimson :)

Thanks mon. Installed the RE today, and it doesn't redscreen anymore. Changing console from d to G (for the funky RE logo) however corrupts the display and apparently hangs the machine, so it's back to the serial port for now. Maybe the RE firmware is corrupted, so I'll have to refresh that if/when I manage to boot.
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Re: Reverse engineering / fixing a PowerOne PSU

Unread postby jan-jaap » Tue Aug 09, 2011 3:43 am

GanjaTron wrote:Btw, I take it you put aside the PSU, not the baby? :)

Yeah ;) Should have buried it deeper though. It was still on the floor behind the dinner table. Yesterday my oldest son fell from his chair and landed head-first on top of it :oops: Good thing the PSU was on top of a box (which collapsed), or that stupid thing would have cracked his skull. Lots of blood everywhere (had to take him to the doctor), but it looks like he'll be alright. PSU is now in the garage. Not a scratch, of course (I know you were wondering :mrgreen: ).

Tracking down a 30A "Littlefuse" is bothersome enough tho. I take it the 110VAC configured units have a jumper there?

Actually, clamps 1 & 2 *are* the AC input selection jumper. I probably just shorted the 220VAC input. Farnell & co will happily sell you a 30A Littlefuse but they cost ~ 30bucks so I decided my "spare parts" PSU will have to do without.

I've always wondered what a Crimson RE draws. My AC mains meter indicated over 600W (800W peak with 2 RM4s), but that's only a vague indication of how many amps the 5V bus actually carries (particularly due to power losses in the PSU)

My Crimson RE (w. 2x RM4, FDDI and a modern(ish) 18GB disk draws ~ 750W idle @ 220VAC. The efficiency of the PSU is documented somewhere in the PowerOne PDFs, and it's not half bad (IIRC between 70 and 80%). So if you measure power draw @ 220VAC with and without RE you should be able to figure it out.
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Re: Reverse engineering / fixing a PowerOne PSU

Unread postby GanjaTron » Fri Sep 02, 2011 2:42 pm

jan-jaap wrote:
GanjaTron wrote:Btw, I take it you put aside the PSU, not the baby? :)

Yeah ;) Should have buried it deeper though. It was still on the floor behind the dinner table. Yesterday my oldest son fell from his chair and landed head-first on top of it :oops:

Holy crap JJ, you are getting more trouble from that PSU than you bargained for. Hope neither has longterm damage. At least you can fix a PSU...

My RE began redscreening again overnight for no apparent reason. Will have to reseat it a few times to see if that helps. I'm ruling the PSU out here. Just came back from a longhaul trip with a nagging cold so haven't felt like checking it out yet.

Have a great weekend!
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Re: Reverse engineering / fixing a PowerOne PSU

Unread postby GanjaTron » Wed Sep 21, 2011 8:54 am

Desteeng desteeng one too tree...

Oh wow, I can actually post from my Crimson!

Still have issues with redscreening (GE8?), but that belongs in another thread.

Good luck JJ!
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