Printer controller debugging continues

LINE SPACE LATCH SHOULD RESET WHEN CARRIAGE MOVES ONE LINE

The line space latch will unlatch when the carriage of the printer reaches the next line location. The printer produces a 5ms pulse once per rotation of a gear which moves the paper up the distance between printed lines. This pulse should reset the line space latch, which would remain in the off condition until it is activated by execution of an XIO Control command to the printer with the bit set to space one line. 

One the scope, I can see the pulse from the printer (-carriage CB) and the latch attempt to reset. The +Latch line lowers partly but does not go all the way to logic low. The -Latch line increases a bit but does not get up to a good logic high condition. Thus, there is a brief attempt to reset but the latch stays active.

It appears that something is keeping the -Latch line from pulling up to a sufficient voltage to switch off the latch. I then spent the afternoon chasing down possible culprits, but to no avail.

CHECKING NETS FOR CONTINUITY, SHORTS

I removed all the cards connected to the -latch line and confirmed it had good connectivity between its pins but no connection to any power rail or ground. The net connects pins on B2, B3, E3, D3 and J2 card slots. The pin on J2 is labeled 'resistor' which I expect means it is the pullup resistor for the line. If that failed it would certainly cause issues with the latch operation.

LIMITED SCHEMATICS, MISSING ONE CARD TYPE

The cards in those slots are 0000, 3130, and 6250 types. I don't have any schematics for the 6250, which is the most likely to be causing the issue of course. The card provides debouncers for eleven sets of contacts,  three pull up resistors for nets, an OR gate to signal if any one of the carriage control brushes made contact, and a pulse generator to emit a signal when the print disc slot passes a photocell. 

The print disc slot pulse means that a new character is coming up on the spinning print wheels - thus the CPU is interrupted so that the program can issue an XIO Read to see the character code on the disc. This code indicates which of the 48 characters is next. 

The card has six SLT modules, 13 tantalum capacitors, other capacitor and resistor modules, plus a discrete resistor and 6.8uf capacitor. I will try to figure out how I can test the pullup resistor circuit to assess its functionality. Since there are three such circuits, comparing them might turn up a significant difference. 

REASONS I DOUBT THE 0000 AND 3130 CARDS ARE KEEPING THE LATCH LOW

I have several of each type and swapped them with others. No change to the symptoms. Also, looking at the schematics, both of these card types have diodes isolating the inputs making it unlikely they could pull the line down. 

0000 card circuit of one gate

3130 card circuit of one latch