The Original Fault

The cincinnati milling machine, with milacron sx 850 controller, dating from 1993, was occasionally complaining of a SIBA failure. This is the IO rack that connects to the main controller by a fast serial link. Not much information is available and second-hand replacement parts cost a fortune out of the US.

The power supply was old and pretty dirty. It has been serviced before with new capacitors and some minor burning cleaned up.  Testing on the bench it seemed to be OK, although some voltage droop on the output under load. Considering the age of the supply it was working. It’s design, condition and low operating frequency (by current norms) produced some odd chirping and buzzing sounds as the load varied. The voltage adjust pot was dirty making the supply noisy during adjustment.

The original supply is a Power-One SPL50-3200, rated at 50 watts, +5V @ 6A, +12V @ 2-4A Pk and -12V @ 0.5A

Re-purposed AT Computer PSU

As the problem got worse over time and the obvious was eliminated, it was decided to replace the power supply. We needed a 5V/12V/-12V similar supply quickly and in the weekend.

A surplus 200W AT computer power supply was reconditioned (new caps) and fitted. This was chosen because it was available and did not have the 3.3V high current output that is not needed, and more than enough capacity on the 5V and 12V rails.

The main heat-sink was cut down to reduce height and painted black to improve dissipation without a fan. We will see how warm it runs in the machine, but I suspect it will only be supplying 20-30 watts maximum.

Apparently Solved

We installed the power supply board and replaced the serial data link cable. 30 minutes of testing and all seems well. After 30 minutes of operation the power supply was not more than just detectably warmer than ambient, suggesting the load was maybe 5-10 watts.