Further dive into the 6V regulator circuit breaker spurious tripping

SET SCOPE TO TRIGGER IF REGULATOR OUTPUT HITS 6.7 OR HIGHER

I hooked the oscilloscope up to the 6V output of the regulator and set the trigger to a level of 6.7V so that I would see whether a high level is the cause of the spurious tripping of the circuit breaker. The overvoltage protection card will trip in a few tens of microseconds if the voltage is above the 6.8V trigger level I had set. 

The system tripped immediately, so I isolated the logic from the regulator and then watched the voltage on the output without the overvoltage protection card installed. It zoomed up to 15V and stayed there. I pulled the regulator card out and found a solder bridge defect where I had removed and replaced a couple of parts. After removing the solder bridge, the regulator worked properly and generated 6V. I hooked the logic back up.

The observed voltage completely dashes the theory that the breaker is tripping due to an overvoltage condition detected by the overvoltage protection card which clamps the output to zero. What I am seeing instead is that when I power down for a short interval and then power back up, the voltage comes up to about 3V and oscillates around that level, leading to excessive current draw which trips the breaker.

POTENTIAL CAUSES

It is possible that some parts out in the 1130 logic are producing a short circuit under the repower conditions I am seeing, causing the 24A capacity of the regulator to be unable to deliver 6V. For there to be 3V at the regulator output, the resistance of the load must be about the same as the equivalent series resistance of the regulator. 

The regulator has a 0.072 ohm 90W resistor that all power must flow through. If that were the ESR, just to make a simplifying assumption, then the resistance out to the short would have to be roughly the same. to create a voltage divider. That resistance is equivalent to more than 50 feet of 12 gauge solid wire, while the size of the 1130 and its use of stranded wire make this very unlikely to be the cause.

The other area of suspicion is that this is a defect in the regulator. That is either caused by out of spec parts values or a defective semiconductor device. 

I have ordered a complete set of replacement resistors to bring all the values in line with the design. I should be replacing them midweek and testing again. If the problem persists, I have seven semiconductor parts that may be at fault. Intermittent contacts in the germanium transistors or oxidation defects on the transistor could be occurring, although why that will happen only in the repower scenario is a mystery. The same is true for a zener diode and a regular diode that are part of the comparator on the card. 

The failure scenario involves the regulator driving insufficient current to produce 6V across the load. That may be a clue to the part of the regulator circuit that is failing in the repower situation. However, it could be due to a number of different parts malfunctioning or being off value. 

The regulator has six germanium power transistors (IBM 108 transistor type) in parallel to deliver the current needed for a 6V output. The six power transistors have their base junctions driven by another 108 germanium power transistor mounted on the regulator's SMS card. If we call this the drive transistor, then its delivered current is controlled by a cascade of two smaller transistors, a type 086 controlling a type 123 that feeds the base of the driver 108 transistor. 

The regulator heart is a pair of type 026 transistors set up as a differential amplifier with a shared emitter resistor. One transistor is driven by the output voltage, using an adjustable resistor to fine tune the target voltage. The other transistor is set with a Zener diode to a fixed reference voltage. As the actual voltage rises above or falls below the target, current through one of the 026 transistors moves to decrease or increase the current into the 086 transistor. That lowers or raises the current through the 123, the driver 108 and then the six power 108 transistors, restoring the output voltage to the target.