Wednesday, December 20, 2017

Both IBM 1401 systems fully operational after work today


A couple of the team members attacked the door latches for the 1402 card reader/punch, which use cables to transfer a pull on a pivoting handle, inset into the door, to rotate a keeper plate out of the way of a fixed tongue on the machine frame. With the keeper pulled up, the door can be swung open by the handle. 

The cable became too loose, thus unable to pull the keeper out of the way. The door remained latched closed - actually both the left and right doors suffered the same malfunction. The team was able to reach through the top openings of the machine and get down to snag the cable, pulling it enough to open the doors. With them open, adjusting the cable tension properly was an easy task.

Meanwhile, we focused on the 1401 system which refused to power up. This is the one which originally suffered some failure that knocked out the bottom 4000 characters of memory, but during the repair of that fault, a bad card inserted into a socket caused something to fail in the power supplies.

We had found last week that the -6V power supply was producing no power. Some component testing identified one shorted power transistor and a couple that seemed suspect.

I replaced the six power transistors in the supply and did a quick bench test with 110V supplied. The power supply looked good so we reinstalled it into the machine at the end of our session last week. Alas the system still didn't power up. 

Today we saw that -6V supply was still missing, but a quick check of the input 110V supply showed that it wasn't recieving input power. The input to this supply is routed through one of the power sequencing relays which hadn't activated.

Starting at the beginning, with relay 1, we should see the +6V and +30V supplies verified. The relay coil is hooked to +30V on one side and to the power sequence card on the other. The sequence card will ground the other side of the coil if a transistor switches on. That transistor has a resistor network acting as a voltage divider between +6V and -20V. Thus, if both +6 and -20 are active, the transistor is biased to conduct and will switch on relay 1.

We tested the voltage coming through relay coil 1 and found it was not present. The +30V supply was working fine, but 30V didn't come through the coil.  Tracing voltages, we realized that the 30V was interrupted somewhere. Our schematics for the machine showed a direct wire to 30V, but clearly that wire wasn't working properly.

Looking over all our schematics, we found that the German 1401 system has a circuit between +30V supply and the relay 1 coil, not simply a wire. Digging further, we found other schematic pages showing the same circuit for the Connecticut system. The circuit was labeled as the 18V differential memory supply. Since our failure came with a bad card in the memory circuitry, this seemed promising.

Looking closely at the fairly simple power supply circuit, it highlighted the bizarre way that IBM sometimes labels things on their machines. The circuit is fed +30V from the power supply, returns the +30V to the relay 1 coil, and drops the voltage down to +12V to feed the memory driver cards.

So, a circuit that drops 30V down to 12V is called an 18V differential supply - as 30 minus 18 is 12. It has a potentiometer to make adjustments, but the label next to it says 18 when in fact you adjust it for 12V. Words fail me..

We verified that +30V went into the supply, but the 30V back to the relay coil was missing. One schematic showed a fuse, directly between the 30V input and rest of the circuitry. We pulled the supply out, looked at a seemingly intact glass fuse but the VOM quickly confirmed that it was blown.

With the fuse replaced, the machine came right up. Memory works, power works and all is well. In hindsight, the -6V supply that we repaired was probably limping along adequately on the 5 of 6 working transistors, but it is now fully refurbed and at factory strength. If we found the 18V differential supply fuse, hidden deep inside with no signs, we might never have looked at the -6 supply at all.

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