Thursday, March 16, 2017

Working on 1401 computer, restoring tube tester


One of the 1401 systems is down due to a cascade of problems with the "Start Reset" switch on the console. The switch appeared to be bad, someone soldered on a replacement after which the system appeared to work. 

Later, when the switch was tightened into place, it created a short across power supply lines that popped circuit breakers on 20A power supplies. Another set of soldering including some guesswork as to which wires went where, and we now have a machine that won't reset.

Back to basics, tracing the wires and comparing to the logic diagrams until we can get it working right. It is slightly complicated because we have eight wires but only six total switch terminals. We have to figure out where the two 'extra' wires go, but the ALDs are not close enough to physical wiring diagrams to help us with this. 

The switch is actually a pair of single pole, double throw momentary switches. Since the 1401 has two complementary sets of voltage levels, U and T, the switch must generate reset levels at both U and T levels for the various cards that need that polarity.

U level is -12 and 0 volts, with the lower potential equating to logical 0. The T level is -6 and +6 volts, again the lower potential is logically 0. Thus, we have two switch halves, one for U and one for T potential. 

A minor complication is that the actual signals driven are +U Start Reset and -T Not Start Reset - so that the U side of the switch will be at 0 to reset but the T side of the switch will be at -6 to cause a reset.

The U side has -12 on the normally closed contact and ground on the normally open contact. The common pole of the switch is the +U Start Reset signal. Thus, the line sits at -12V (logical 0) until the switch is pushed, when it pops up to 0V. 

The T side has -6V on the normally closed side and +6V on the normally open side. The common pole is the -T Not Start Reset signal. This is an example of the maddeningly bizarre nomenclature IBM used, as the -T means inverted logic levels so a "Not Reset" with inverted levels is 0 when "Not Reset" and 1 to Reset. That means it is really a +T Start Reset line. Thus, the line sits at -6V (logical 0) until it is pushed, when it pops up to +6V.

A second complication is that the machine must reset itself on powerup, so the normally closed contacts (-12V on U side, -6V on T side) are not directly wired to the power supply. Instead, they go through a relay that is part of the power-up sequencing logic.

The initial state of the relay before power comes all the way up will set the normally closed U side to 0V and the normally closed T side to +6V. It reverses after a short time delay to give -12V to the U side and -6V to the T side. The effect of this is to set +U Start Reset to on and -T Not Start Reset (+T Start Reset) to on initially, resetting the machine before the short delay is over and those signals revert to their inactive off state.
1401 Start Reset button wiring
We can trace the center poles of the switches, by finding SMS cards that receive these signals somewhere in the machine and tracing continuity back to the the wires, all of which are unsoldered to trace things properly. We did find and validate the +U Start Reset and the -T Not Start Reset lines.

Finding the +6V, -6V, 0V and -12V seemed simple as well, but we could not find any wire that had +6V on it. That was the condition when we ran out of time Wednesday when the museum visitors arrived for the scheduled demo at 3PM. If there is no +6V available, the reset switch can't activate the +T side of all the reset circuits.


I have to wait patiently for the next session with access to the machine in order to archive nine more cartridges we borrowed and to collect enough debugging information to clearly identify and fix the defect that keeps me from booting cartridges I have written.


I soldered together the solid state replacement for the 83 tube and stuck it into circuit. I powered up to check that no magic smoke or signs of distress arose. All seemed fine, but without a meter I will need to improvise to check things out.

I think I can hook up my modern DVM in current mode, using a 2365 ohm resistor in series with the VOM, and read off the current. Full scale should be 200 uA through the resistor, which is the equivalent of the original meter resistance.

My meter reads 400ma on the scale, which is huge compared to the actual current expected, so I might need a different method for handling this. I decided to upgrade my DMM to a model that has a 200 uA range - will come tomorrow.


  1. Can someone with another 1491 send you a sketch or photo?

  2. Hi Pete

    We have a second 1401 system in the same room, but the wiring on the switch is really buried. We chose not to possibly harm the one working machine.

    Further, you will see eight wires, all marked as white with a black stripe, so all it will tell me is which two pins have double wires, but nothing about what wire goes where.