Corrected the stuck bit problem in the IBM 1130, but have a core memory issue to resolve

DIAGNOSED THE HOT BIT 10 ISSUE ON THE IBM 1130

The problem was that even after a CPU reset, bit 10 in the accumulator register (ACC) and the accumulator extension (EXT) was always on. I prepared a list of signals to watch but I first took a look in the card cage, finding that card J4 which implements bit 10 looked a bit high and loose. 

RESEATED CARD, PROBLEM WENT AWAY

I reseated the SLT card and checked again. The problem was gone. ACC and EXT clear on a push of the Reset button and arithmetic and logic, which use the ACC, now perform correctly. I ran most way through the CPU Diagnostics, successfully, before I ran into an issue with parity check stops. 

PARITY ERROR DUE TO BIT 13 ALWAYS READING AS ON

This appears to an issue localized to the first half of memory, addresses 0 to 4095, where bit 13 is read back as a 1 every time. If it was written as a 0, then parity was calculated based on that. The additional 1 bit causes the error checking to detect the problem and issue a parity stop. 

I will have to look closely at the signals and which address ranges encounter the problem in order to zero in on the areas where the defect must be. The problem may be a failure to write the 0 value into bit 13, or a failure to write it back as 0 after reading it once, or an error that always reads it as 1. 

I am hoping this is not another sense line or addressing line failure within the core stack, as I had to fix a number of them to get the 1130 in shape over the past year. The core stack is a sandwich of core planes plus PCBs on the top and bottom. The bottom PCB just carries traces from edge connectors to central pins that stick through the SLT backplane to connect the signal to the rest of the memory electronics. The top PCB has a matrix of diodes that support driving current through the X and Y axis lines in two directions, one to write a bit value of 1 and the other direction to flip any bit to test for its previous value. 

The problems I had were all on the bottom PCB. Fortunately, no breaks in wires running through the cores or in the core planes, as those would be an order of magnitude harder to repair if it were even feasible. Several of the traces had failed open, all on the PCB itself. I found ways to route wires from the edge connectors out to the electronics on the backplane through alternate unused socket pins.