108 Transistors arrived, power supply back together but still not working correctly

RECEIVED MY FIRST BATCH OF 108 TRANSISTORS AND TESTED TWO UNDER 4A LOAD

I serially installed two of the new transistors in one of the heat sinks and verified that the unit would support the full 4A load with steady output voltage. With all the main transistors now working properly it was time to put things together and finish the testing.

REASSEMBLED POWER SUPPLY AND FAILED WITH TEST AT FULL 24A RATING

My bench supply can't deliver 25A to test this out so I had to take advantage of the 1130 which can feed that much. I hooked the 6V power supply into the 1130, but instead of connecting its output to the logic circuits I hooked it to my resistor complex at the initial load level of 6A.

The breaker immediately popped! I pulled the unit and put it back on my bench power supply, where it worked properly again. At this point I started to look more carefully at the supply of the raw voltage that is fed to the unit in question. 

The 1130 power system is divided into two halves. One part has transformers, rectifiers and capacitors, producing raw unregulated voltages at different levels to be fed to the other part, the regulator supply. The unit I am repairing is the regulator supply. The first part shows its output as 12V on the ALDs and other documentation, but measures 16.5 volts in real life. In fact, all the raw voltages are a bit high, the 48V raw runs at 52.5, the raw 12V is at 13.9, and the raw 3V supplies sit at around 7. 

I increased the voltage of my bench supply from the 9V I had been using up to 16.5V, where I immediately saw the power supply try to trip the breaker. It couldn't because my bench supply is current limited at 5A, therefore the supplied voltage sagged instead. 

CHECKED WHAT IS HAPPENING AT HIGHER INPUT VOLTAGES

My testing focused solely on whether the output voltage remained constant as I varied the load, up to the realistic limits of my bench setup which is around 4.7A. I hadn't looked at higher raw voltages on the input, but this appears to be the issue.

With experimentation I discovered the supply is happy up to about 12.5V of raw input, but above that the output voltage starts varying upwards, quickly triggering the overvoltage protection which would trip the breaker if we weren't current limited. With over 16V coming from the raw supply the breaker will trip as it did repeatedly. 

I grabbed the small DC-DC supply that had been wired into the 1130 by a past restorer. This unit is claimed to support up to 20A which should allow me to test at loads up to 18A with my resistor complex. I wired it into the line between my bench supply set to 16.5 and the regulator supply I am testing, with the DC-DC board set to 11.5 volts output. 

At 4.7A load, this held steady but when I tried increasing my electronic load to 6.7A, I saw everything sag. Measuring the voltage at the input of the regulator supply showed it dropping well below the 11.5V that it should be outputting. It appears to be current limited quite a bit below the faceplate capacity of 20A. 

NEXT UP - TROUBLESHOOT THE VOLTAGE REGULATOR BOARD

The likely culprit in all this is the voltage regulator board, an SMS card with five transistors and a medley of other parts, which should be modulating the drive of the six main transistors to delivery steady power at the target voltage.