Wednesday, September 2, 2015

Light day, some work on SAC expansion cable


On top of a heavy work day, I went over to the CHM in the late afternoon to help out with a few problems. There is a problem related to a broken wire inside the cable from the German 1401 to its first 729 tape drive, but a few other members were working on that. I sat down with another team member and looked at a power supply which was tripping out on the Connecticut machine, keeping it from coming up.

Ron had first checked the electrolytic filter capacitors, five 11000 uf units in parallel across the raw supply, and found that they had essentially no capacitance at all. Temporarily, he hooked up a 33000 uf electrolytic, but this is temporary until we can get the proper replacements for the five units that were in the power supply.

However, with the filter capacitors, it still wouldn't adjust to the proper -12V output level. It was sitting at a lower level, which was causing the crowbar protective circuit to trip the breaker as a protective measure. After a bit of hunting, we found that one of the ten parallel power transistors was shorted, which effectively bypassed all voltage regulation.

We replaced the transistor with a spare and then tested the unit. It needs a reference voltage of -6V for the voltage regulator card, which we supplied from a workbench supply, then verified that we could adjust the output voltage to exactly -12V.

Next, we pulled out a load fixture we have, which allowed up to place huge high power resistors across the supply and set up the unit to 0.75 ohm resistance and drive the supply up to 16 or so amps. The supply stayed at -12V, with a variance of only about 30 millivolts from zero to full load. This is ready to be put back in the system on Monday.


I picked up the heat shrink tubing and some connectors to use with my new SAC Interface expansion cable that adds interrupt levels 0 and 1 to the box, plus an emulated Prog Load button. I went out to snake my twisted pairs through the 17 feet of heat shrink, which was much harder than I expected.

I chose an Amphenol connector that can support up to 14 pins, twist-lock, which I will put on the 1130 end of my expansion cable. The cable will be hard wired into the SAC Interface Box on its other end. I have to designate the signals for each position, solder on the wires, and insert the pins in order to complete the cable. 

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