Finding commonality in the two 1130 misbehaving lights

LOOKING AT LOGIC DIAGRAMS SHOWS THE TWO ARE DRIVEN BY A SINGLE CARD

The lamp driver for the KB Select lamp and the Parity lamp are both on a 3819 card in slot D5 of gate A, compartment C1. My first thought then was that the card was not properly inserted or bad, thus I looked into the card design to see how two failures might occur at the same time on the card.  

SCHEMATIC OF 3819 STUDIED

The schematic for the card contains eight identical drivers, each can sink 300ma of current at 48V, roughly 15W. They are used to power the lamps on the console plate on the left of the keyboard, as well as solenoids in the keyboard, typewriter and disk drive. 

One of eight circuits on the card

One of eight circuits on the card

Looking at the detailed schematic of the card, there are not many common elements that could cause two of the circuits to fail simultaneously. The lamps are illuminated, so we know that the transistor on the right is not conducting to ground, letting the +48V reach the output pin and thus the lamp. 

For the right transistor to be cut off the transistor on the left must be conducting. The base of the left transistor goes independently in each of the eight circuits. The +3 and +48V supply have to be connected on the card as well for these results to happen. If the +6V supply is missing, then the right transistor can't conduct in any case and the lamps would remain on. 

TRACING SOURCE FOR THE TWO LAMP DRIVERS

The KB Select signal comes from an inverter at A-C1 D3 which gets its signal from A-C1 E2, all of which are on the same backplane as the lamp driver card at D5. The Parity Check signal comes from gate B, compartment A1, card J3 across a cable that runs from B-A1 compartment connector at A7 to A-C1 compartment connector at N6. Thus no commonality that would suggest a cabling or trace fault. 

OUR COMMON CANDIDATES

We might have a trace that failed for the +6V pin in the 3819 card. Similarly, we might have a bad trace in the board that doesn't deliver +6V to the D5 socket. Ground failure would also produce the symptoms, either on the 3819 card or at the socket. If grounds at D04, D05 and D08 are not the same, it might explain the results.  Finally it is possible that +3 supply pin could influence the behavior. Cracked traces around the socket are also possible, although the input signals are too far apart to be likely to run next to each other. 

NEXT STEPS

I have to put the scope on the driving signals at D5, pins B03 (KB Select) and D09 (Parity) to see if they are at logic low or high. 

I will also check voltages at D5 socket and ground continuity. 

Another check is to swap the card with another 3819 from the machine.