[jan-jaap]

The twin tower machines have a special place in my heart! Beautiful to see one still running!

What went wrong with the power supply and what did it take to fix that?

Sorry, but a Personal IRIS is not a Twin Tower. The good news is I have one of those too, a Professional Series 4D/70 GT. Took me forever to get that thing working again.

The PSU of the IRIS simply didn't fire up one day. You could hear a faint humming from it so I assume the high voltage section works, but none of the outputs come up. It is a total pain in the *** to diagnose anything because it is mounted on a bracket which serves as cooling for various components so you can't get to the soldering side of the PCB while it's 'hot'.

I had an original spare, but these things are some thirty years old now and not getting better with age. I didn't want to risk loosing that one too, so I got a replacement, a new-old-stock Power-One PFC375-4000. The original PSU puts out 45A @ 5V, this one 40A @ 5V. So first of all, I measured power consumption of the system with the original spare. My 4D/35 is as loaded as it gets: all GR1.5 options, max memory, video framer, and a 5.25" FH hard disk. Without the disk, the power consumption is 172W from the wall. The disk is spec'ed as 1.1A @ 5V max, and a lot more @ 12V but that's not my problem. Assuming a high efficiency of 90% and assuming all 172W go into 5V, the system consumes 32A @ 5V. More likely, the efficiency of the PSU is ~ 70%, and some goes to +/-12V, so it consumes less than 25A @ 5V. In either case, the 40A of the Power-One unit should do.

I wired up the Power-One using the cable harness of the original PSU and attached a spare IP10 mainboard from a 4D/25 because I didn't want to risk this system, and the IP10 schematics are available. Powered it up, and something exploded :( It was a tantalum on the IP10, 12V rail. So, I removed the remains, wired up the original spare, and the IP10 still worked. Put the Power-One back in, and it *still* worked. I guess it was simply it's time to go.

I needed this system last week to test some spare parts for Ian Mapleson, so I didn't waste any more time, and with improvised cooling did that. Now I will look for a suitable fan for the Power-One PSU to provide some airflow because it does get warm, and mount it to the bracket of the original PSU, which installs nicely in the case of the Personal IRIS. I will report back with some photos when I'm done with that.