Friday, May 13, 2022

Have the Usage Meter running, repaired the rotary Mode switch, investigated the three lights that don't illuminate on display pedestal


After disassembling the switch, I discovered that the fault was caused by a plastic piece that had split, leaving debris that was jamming the rotation. I realized that the two halves would work just fine as long as I cleaned out the bits.

Broken plastic piece near center

This switch has notches for ten stops in a 360 degree rotation, although it has a metal stop ring that only allows selection of seven of the ten possible positions. The shaft of the control is a flat bar, with the commutators of the switch gangs having a slot, as well as the plastic piece which had broken.

barrel and shaft

This plastic piece has holes drilled on opposite sides, small springs sit in the holes and press a small ball bearing into the notches in the switch barrel. When you apply rotary pressure the ball bearings push in against the spring, allow rotation, then pop out at the next notch which is one of the ten detents. 

The way it broke leaves it able to work properly

I put it back together, verified the electrical connections were correct for the seven detented positions, then reinstalled it on the switch plate I had been using with my 1130 replica. Later I will solder this back into the replica and put it into my display pedestal. 

Repaired switch back on plate


Some of the IBM rental plans involved extra charges when the computer was used more than the agreed number of hours per month. To support this, IBM put usage meters on the machines and would read them, much as a power utility reads the meter to bill for electricity. The pair of meters on a machine have a CE (customer engineer) keyswitch in between, so that during repairs or regular maintenance, the customer's meter did not run. Usage was instead accumulated under the CE meter. 

IBM made it difficult for customers to circumvent the usage meter. The meter controls are in a metal box which has a plastic tamper proof button isntalled. To open the box, the button is broken and a new one must be installed if IBM was the party opening it. Otherwise it was evidence of tampering. 

The power to the meters was routed down to join the terminals that supply system power to the main power supplies. If you removed those terminals, the meter wouldn't run but without the power rails, nothing else would run either. 

Inside the meter box, the transformer could be wired for 115, 208 or 230VAC operation, same as the main power supplies of the machine. Since this machine was originally a 115V unit, it had been converted for 230V. The prior restorers who did this conversion failed to detect the meter power supply and thus ran it at 230 while it was wired for 115. That damaged the windings of the transformer and it failed even after I rewired it for 230. 

I found a substitute transformer to produce 40VAC which is the supply used by the usage meters. It was a 115 to 40 transformer, thus I needed it to be connected to the 115V produced by stepdown transformer T2. That was a problem because of the security oriented wired, as the meter would be fed by the same line voltage as the power supplies, not by the output of transformer T2. 

Meter box with slightly larger substitute transformer

I had to break apart the terminals, cut back the unneeded wire from the meter and install a single wire terminal to feed the power supplies. I cut back the wires coming from the meter box, put terminals on them, and then hooked them to the main distribution terminal strip where T2 provides 115V. 

Severed wires ready for terminal lugs to be added

After verifying that the transformer in the meter box output 40VAC, I hooked it all up and tested the meter. When I put the CPU into a spin loop so the Run light would go on, the meter began accumulating fractions of an hour. It ceased moving when I stopped the CPU, exactly as the meter should behave. 


I had noticed that bits 2 and 3 of the Storage Buffer Register display were not lighting, nor was the T2 light under the T-clock section. Hitting lamp test turned on bit 2 of the SBR but not the other two. 

At this point, it was possible that I had a component failure in the CPU that generates those bits, so I pulled the signal line off the pin of the display board and checked its output with a VOM. As I did this, I noticed that the signal line was already detached on bit 2, so I hooked that back up.

When there a 1 in bit 3, the signal line had 3V on it, exactly as it should. Similarly the T2 line had 3V when I single stepped the CPU to the T2 state. This tells me that the fault is on the display board or a bad incandescent lamp, not a defect in the 1130 itself. 

I left for the day but will investigate and repair the three lights in a future session. I am more interested in debugging the inability to Select the keyboard with an I/O instruction and will do some signal testing at the next work day.

No comments:

Post a Comment