Anomaly tracked down to failed triac

 ANOMALY NOTED DURING RESISTANCE MEASUREMENTS

When measuring the resistance from the Lamp Test control line on the terminal block to the common ground, I found a surprise. It was 6.8K, the value of one of my resistors whose other end was somehow shorted to ground. All of the resistors should run from Lamp Test to the gate of its triac and there is no ground trace near these that could account for a solder bridge causing a short.

I checked the resistance from each triac gate to ground, which should have been infinite but was 13.6K for almost every triac - the resistance if its local resistor plus the mystery circuit whose resistor was sunk to ground. Eventually I found that the gate of the triac for Interrupt Level 1 lamp was at ground. No place where a solder bridge could have joined it to ground, leading me to conclude that the triac itself was internally shorted, gate to ground. 

I had reclaimed most of the triacs from my earlier boards to use here, they were not new other than 30 I still had in the box unused. Somehow this one was damaged and needed to be replaced. 

I whipped out the hot air rework tool and the soldering supplies, removed the triac and cleaned up the pads for installation of another of my reclaimed triacs (after testing it for a similar short of course). I set up a number of air dams to avoid loosening any adjacent parts when I unsoldered the bad triac. 

TESTING THE FAILED TRIAC

 Very puzzling indeed - with the triac removed, the short to ground was gone on the board but the triac itself didn't show a short. Nothing on or around the pad could have bridged to ground, so it must have been some failure inside the triac, perhaps some package stress that was relieved when I removed it. 

RECHECKING THE CIRCUIT WITH THE REPLACED TRIAC

I used a different triac, installed it into the position and checked over everything again.  All appears well with the board so it is on to the powered testing phase.