Pulling one card stops the runaway line spacing behavior - mystery short discovered on backplane

DETAILED OBSERVATIONS OF THE LATCH ATTEMPTING TO RESET

The top trace is the +Line Space Latch signal, output of the latch. The green trace is the reset condition that should cause the top signal to drop down to near zero volts. The output never falls below 1.9V, way too high to be considered a logic low state and produce a reset. 

I have swapped all the cards in the controller logic. I also tested all the cards that touch the -Line Space Latch output which never gets much above zero volts even when a reset should occur. It does reset perfectly at power on but not later. 

I couldn't see any wires touching the pins in the relatively small net for -Line Space Latch. It is generated at D3 D10, runs to E3 B13, B3 D02 and B2 D07 as well as to the resistor pullup at J3 D05. With all the cards pulled, this net has infinite resistance to all power rails. 

My suspicion then turned to a short where an output of some other gate is interfering, holding this signal low in spite of the latch attempting to reset. That would be a dynamic situation, explaining why we start out with a good reset at power-on and then fail later. 

PROCESS OF ELIMINATION TO FIND CARD THAT IS INTERFERING WITH OUR NET

I left the latch card (D3) and powered up. No runaway spacing. I added in E3, which allowed for manual spacing. That worked perfectly, turning on the latch which then reset as soon as the carriage reached the next line position and output a pulse on -carriage CB. Still no runaway. 

Next I added in the card in B2. Again, no line spacing issue. Finally, when card B3 was inserted, the issue came back. This tells me that some activity of that card is impacting the signal. 

I put the card on the bench - although I have swapped them all so it isn't a defect in the card. The input pin D02 is working as it should. We just have problems when the card is active. 

TESTING OUTPUT PINS OF CARD IN B3 TO SEE WHICH IS SHORTED TO D3 D10

There are only six output pins on the card which could be driving signals to interfere with our latch on D3. I tested each and found that pin B02 was a dead short to our latch line. This is an output that should not be directly connected to our latch (D3 D10). 

The connection is on the backplane. There is no wirewrap making the connection, but I had previously found that a missing connection between this card for pin D02, which should have been connected to B2 and the reset of the D3 D10 latch net. 

LOOKING CLOSELY AT THE SLOT PINS FOR SIGNS OF THE SHORT

I zoomed in to the backplane to see if I could find a short between B02 and nearby pins on our net - the most likely culprit being the adjacent pin D02. 

I don't see anything shorting here, but previously I found D02 was not connected where it should be, suggesting this area of the backplane has suffered some failure of the printed circuit traces. 

I will need to break the connection - once I find it. The short is either on this side or on the side where the SLT cards plug in. If I can spot the bridging trace I can cut it with an Xacto knife and restore the machine to operation (or at least this problem will be fixed).