Tracing down the annoying dimly lit Parity Check lamp anomaly

PARITY LAMP DOES NOT BEHAVE AS THE OTHER STATUS LIGHTS DO

The status lamps on the 1130 should be completely off or brightly illuminated, depending on the condition. For example, the File Ready lamp will light when a disk cartridge is spinning and the heads are loaded, if the disk is not ready, it is off. The Forms Check lamp lights when the paper is not inserted in the Selectric based console typewriter, but goes out totally when the printer is ready for operation.

With the Parity lamp, however, there is a dim glow at all times, except when an actual Parity Error has occurred and caused the lamp to achieve full brightness. This is incorrect behavior and annoying to me. It began when the 3819 SLT card that drives the lamps took a catastrophic failure as a tantalum capacitor shorted the +48V to ground. 

The card is repaired and all traces damaged by the power event have been repaired or jumpered around. Somehow this abnormal behavior remains and I am on the hunt to find and correct the cause. 

When a Parity Check occurs, a flipflop in the machine records that status. When the flipflop is in its idle condition, the notQ output should be at 3V. Only when the flipflop sets will that output drop to nearly 0V and cause the 3819 card to light the Parity lamp. 

When I looked at the input to the 3819, I saw 0.89V, which is not a valid logic level. The card is driving the lamp weakly because of this, causing the dim glow. Since the flipflop notQ output is nearly 3V, the input to the 3819 should be close to that. 

This is not a very involved set of connections. The notQ output is in gate B, compartment A1, at card slot J3 pin D04. A trace from pin D04 routes the signal to card slot A7, pin D02. Instead of an SLT card, that slot has a cable plugged in. 

The cable carries the signal over to gate A, compartment C1, where it is plugged into card slot N6. Pin D02 of slot N6 carries the signal to card slot D5, pin D09, which is the input on the 3819 card that drives the Parity lamp. The only other connection in that link is a 0.22uf capacitor from slot N6 pin D02 (our signal) with the other end plugged into ground at another card slot of the SLT board. 

I isolated the cables and compartments to look for the cause. There are no shorts in the cable and the signal has continuity all the way from B-B1 J3 D04 to A-C1 D5 D09. There is no high resistance that might lower the 3V output of the flipflop to the observed 0.89V. 

Cables unplugged

There are not many candidates for the cause of this behavior. The flipflop output could be falsely low. The signal path could have a hidden short to some other signal that is often at ground level, but does not show up as a static short to ground when the machine is powered off. The 3819 card could have a remaining fault. 

I have already replaced the capacitor. The 3819 card has been swapped with no effect on the behavior. The flipflop notQ output appears to be validly high. That doesn't leave many remaining candidates for the failure root cause. 

 I have checked for static shorts to ground, but not dynamic ones that might be triggered by an erroneously connected trace pulling to ground when the machine is operational. There may be a completely unsuspected issue X that is behind this. In any case, I will keep after this until I find and eradicate the problem.