[jan-jaap]

I'm busy rebuilding power supplies for my old 4D series. The one in my Crimson would kill the system before it had even fully booted. When opening up the internals I found lots of leaky capacitors, plus additional damage so I decided it was time for a comprehensive rebuild.

This PSU is rated for a total output power of 1050W. The PowerOne SPM5 series is a modular power supply. Not like modular PC power supplies where it simply means you can remove unused cables, but it could be configured with a range of output modules for various voltages and currents. The SPM5 chassis can handle at most 1500W. The SGI configuration was rated for:

• 5V @ 170A
  
• -5V @ 5A
  
• 12V @ 16A
  
• -12V @ 7A
  

The reason I'm saying this is that old USENET posts are sometimes confused between the 1050W and 1500W numbers and draw incorrect conclusions about which models are supposed to be capable of running a Reality Engine.

Here are two SPM5 chassis side by side:



The left is from an SGI, and has all output modules removed. The right is an SPM5 configured for 5V @ 150A, 3.3V @ 150A and triple 12V @ 10A. I bought it for spares. You can see why I like modular PSUs: with the output modules removed you can poke around measure. Just beware that almost everything left of the SGI PSU in the photo is not galvanically separated from the mains! I use an isolated oscilloscope probe.

This diagram is from the PowerOne datasheets:



We'll start at the mains input terminals. This is the rectifier board, removed from the PSU:



From left to right: input terminals, EMI filter, rectifier (under heat sink), filter capacitors.



You can see that the film capacitors have cracked with age (they absorb moisture). I replaced them. I also replaced the thermal paste between the rectifier and heatsink. I measured the main filter capacitors. There are two banks of 3x  1500uF and they checked out fine so I left them alone:



Replacements would have costed me around 100 bucks and availability is problematic.

Did I say modular PSUs are convenient to work on? Here's a test of a rebuilt rectifier:



Next up, the backplane board. this is an old photo but it shows where various parts of the functional diagram are located:



In my experience, functional problems with these PSUs have always been with the bias supply. So I decided to figure out how it ticks. This is the relevant part in more detail:



The implementation follows the UC3844 datasheet:



Basically, the BUK456 is Q1, and the PWM frequency is set with Rt/Ct where half of Rt is the blue potentiometer. So that can be used to adjust the PWM frequency.

This is the drain-source voltage drop over the BUK456:



This is with an x200 (isolated) probe, so you're looking at a square wave of some 250V. It is sent to the transformer which has many secondaries. One of them powers the fan, one the control logic, and one each for every module slot.

This circuit is always 'live' when the PSU is connected, even with the system off. At this point I screwed up. When you remove power from the PSU, the voltage drops and at some point the PWM cuts out. You hear the fan spin down. But some 70V remains on the main rectifier filter caps and it takes > 10 minutes to drain. I heard the fan spin own and thought it was safe to move the scope leads. I shorted drain and source of the BUK456, it sparked and it was dead. I was pretty sure the high voltage had killed the UC3844 and probably the BUK456 switcher. There's a handful of other semiconductors, all small stuff. I decided to simply replace the lot. Of course nothing is simple when you're dealing with obsolete parts and global shortages of components, so it took a bit of time to rebuild the circuit.

It didn't work.

That's when I found out the fuse had blown. I should have measured it and I didn't. It looked intact...

But I'm happy to report that after replacing the fuse, all is good again.

I think there's an attachment limit to a single post so I'll stop here. There's an interesting story to how the inhibit circuit works (standby vs operational results in different fan speed, and how the control circuit works). I also have some photos of the carnage of the filter capacitors from the output modules which were beyond disgusting.