Initial checkout of the second IBM 1130 system

QUICK INSPECTION OF THE SYSTEM

Generally the system looks in good shape. There is a missing blank keycap on the keyboard, the console light panel needs a bit of work to secure the lamp pcb, and some substitutions have been made in the power system of the system. A superficial look at the 1053 console printer highlighted a missing rotate tape and the typeball rotates easily by hand. Otherwise I don't see any damage or missing bits.

New system in front of my working 1130

DEEP CHECK ON CONFIGURING THE POWER

IBM 1130 systems can be set up to run on three voltages - 115V, 230V or 208V. My shop is able to support 230V (240V) so that is how I have to power it. One of the people who dropped it off in my workshop said that it was set up for 208V.

Switching between the levels involves moving jumpers and connections at several points inside the machine. Both 208 and 230V use stepdown transformers to produce 115V as a source for fans, motors and power supplies throughout the machine. IBM made use of big ferroresonant transformers, which have the property that they produce almost flat voltage output in spite of shifts in the input voltage. They are large and require capacitors to be attached.

In this system, someone had replaced the two big transformers with smaller modern transformers. Initially I was worried that these might be 208V only transformers but upon inspection I found they had multiple windings to accept a range of input voltages including 230V. 

Substitutes for the large ferroresonant transformers

I went through all the logic diagrams and other documentation I had to determine how the machine was set up. It was a pleasant surprise to find it was already configured for 230V in all the proper locations. That means I don't have to rewire it for the sake of the restoration and won't have to reset it afterwards to match its permanent home.

I did see that one of the voltage regulators, the one which takes raw 7.8V and produces 6V output for the SLT logic, had been replaced with a modern small switching power supply. The regulator and the overvoltage protection cards were removed and the unit was unwired. Likely this was due to a bad transistor or bad regulator card component. These supplies are very similar to the linear supplies in the IBM 1401 and of course the 360 line of mainframes. I may restore the original and put it back in service, but will start off with the substitute unit just to get through other restoration issues first. 

NEXT STEP - POWER VERIFICATION

I always check the power very carefully before subjecting the logic or core circuits to power. In this case, with modifications having been made, I will check even more thoroughly to be sure that everything is good before I unleash fresh electrons on the SLT cards. 

I worked up a step by step testing plan. The first step in IBM 1130 power is a small transformer that produces 24 VAC power for sequencing. This runs through various emergency power off switches as well as the main power switch on the 1130 console. Once I know that power is good, but everything else is isolated, I can begin to apply power stage by stage.

Next up is the replacement transformers, one of which only feeds a 115V convenience outlet inside the machine and the other which feeds all the power supplies. I will first test the voltage on the convenience outlet, to be sure that the transformers are doing what they promise by stepping my 240V down to 120V. 

The regulators get switched off so that the raw DC and AC power from the supplies does not go anywhere in the machine. I can power each section to verify the +3, -3, +6, +12, +48 and 7.25VAC levels, at least the raw levels, are good. 

When that is done, I can unhook the logic circuits from the regulators, power them up and verify they produce good clean voltage at the proper level. I will focus extra hard on the substitute regulator. 

The IBM 1130 has a series of relays and a main contactor to power up everything. This logic verifies that the main logic voltages (+3, -3 and +6) are good before it brings up the higher voltages. If any of the voltage rails drop it brings down the system in the proper sequence too. A time delay relay keeps the CPU in reset for five seconds, so that power can stabilize before the system is released to begin logical operation. 

Power sequencing circuitry