REMOVED CARDS ON BOTH SIDES AND CHECKED FOR SHORTS
One possibility for the problem is a partial short of the signal trace between the card on B-B1 J3 and the one on A-C1 D5. These are 3817 and 3819 cards, respectively. With the cards out, the path was a complete open circuit and no adjacent pins or power rails had a resistive path.
ADDED IN THE DRIVER CARD AND OUTPUT WAS 2.9V AS EXPECTED
With the 3817 card back in the machine, I powered up and saw that the signal path was at 2.9V with no parity error and low when an error was encountered. This cleared the 3817 card of involvement.
ADDED A DIFFERENT LAMP DRIVER CARD, VOLTAGE NOW .0.9V BUT LAMP
There were a couple of extra 3819 cards in the machine because it had a paper tape punch configured into the system with its attendant logic cards placed in B gate compartment B1. I took one and inserted it in A--C1 D5, where the voltage with no parity error remained at 0.9V but the lamp was completely dark. A real parity error was forced and the lamp lit up fine.
FIRST PASS DEBUGGING BAD 3819 CARD BUT NOTHING OBVIOUS
The 3819 card is a 300ma driver, using a transistor in an SLT module as the first stage and a discrete transistor on the card to support the full 300ma current flow. It has eight identical such circuits but we are concerned with one of them which is connected to the Parity lamp.
I couldn't find any signs of difficulty on the circuit in question. All its characteristics were identical to the other, correctly working circuits. Apparently the discrete transistor is partially conducting, causing the dim lamp. I would need to set up the circuit on a testbed and probe the various voltages and currents in order to figure out why this is happening.
STICKING WITH EXTRA 3819 CARD SINCE WE DON"T NEED THE ORIGINAL
Because we have a couple of spare 3819 cards, due to peripherals which the museum does not have, I simply left the spare card in place so that the issue is resolved.
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