Today I had some time to look into the Wang 360SE that was recently donated to the museum.
Visual inspection was the first order of business. First, the cover over the backplane was removed so that a complete visual inspection of the backplane and power supply circuit board could be performed. The backplane in the 360SE is a hard-wired affair. Wang used a unique method for wiring the backplanes on these machines that used special edge connector sockets with long rectangular tail pins on the backside that special clips attach to. The clips are soldered to the individual wires in the backplane wiring harness. The clips visciosly grab the tails on the edge connector sockets to provide connectivity between the various edge connector sockets. Intuitively, this method doesn’t seem like it’d be very good in terms of long-term reliability, but amazingly, it seems to work very well. Even though the edge connector tails and the clips are made from what appears to be a tin alloy, the connectivity this system provides seems to be very relaible, even after 40 years.
Looking over the backplane carefully showed no signs of any loose connections or broken wires. The power supply circuit board looked as if some of the components (mainly rectifier diodes) had been replaced at some time during the machine’s life. The machine’s service tag did not indicate anything of these repairs, so it is likely that these repairs were not done by a Wang service technician.
The backplane cover was replaced, and the cover over the Logiblocs was then removed. All of the Logiblocs were then carefully pulled from the sockets so that all of the edge connector socket fingers could be inspected, as well as checking the edge connector fingers on the Logiblocs for corrosion. The edge connectors were all in good shape, and there was only light corrosion on the Logibloc fingers, which were all cleaned with a contact cleaning solution. Each Logiblock was also inspected to make sure there were no sign of obvious problems, and all looked good. The core memory board was inspected under a magnifying glass to check for any broken wires in the core array, and no problems were found.
The power supply transformer and large computer-grade capacitors were visually inspected, and looked good. The line voltage fuse was checked, and it was found to be good. Without power connected, an ohmmeter was placed across the power supply hot and neutral prongs on the plug, and the power switch turned on to check for too little resistance, which could indicate a shorted line filter device, or the mains winding in the transformer. The ommeter showed about the same resistance reading as the known good Wang 360SE already in the museum.
The power supply of the 360SE generates +11V and -11V, along with 230V for drive of the Nixie tubes on the keyboard/display units. With all of the Logiblocs removed, it was possible to power up the machine and check the power supply voltages. The 360SE electronics package was plugged into a Variac, and the voltage very slowly raised up to full line voltage, while monitoring the +11 and -11V supplies using two digital voltmeters. It became apparent as the line voltage neared 50% that the power supply wasn’t doing anything. The +11V and -11V DVM’s showed +0.000 on their displays. As I ramped the voltage up to full line voltage, there still was no output at all on either of the logic power supplies. Clearly, there’s something amiss with the power supply, that will require more digging to sort out.
There are two thermally activated "pop out" circuit breakers that provide protection for the +11V and -11V supplies. The first thing to do was check them. Visually, they weren’t popped. They were both checked wtih an ohmmeter, and had full continuity. Unfortuntately, the problem was not going to have a simple solution.
I ran out of time to work on the machine any further during this session. The next step will be to check the transformer secondary voltages with power applied to see if the transformer is working properly. If a problem is found with the transformer, finding a replacement is going to be difficult. Any replacement would have to have the proper primary and secondary windings, as well as be very close to the size of the original, as the transformer is pretty tightly packed in the chassis. If the transformer proves to be OK, then the problem is likely somewhere in the circuitry that takes the AC voltages out of the transformer, rectifies it to DC, filters it, and then regulates it to the proper supply voltages.
I’ll write about further digging into this in a future blog entry.