Overvoltage Protection Card failure - temp repair waiting on parts - machine working fine again

BENCH TESTING OF THE -3V REGULATOR SHOWED THE CROWBAR CARD WAS TRIPPING

In spite of a proper low 3V output, the overvoltage protection SMS card energizes the 150A capacity SCR to short the regulator output, inducing an immediate trip of the circuit breaker. I pulled the card and began examining components on it. The list of suspects were two IBM 033 Germanium PNP transistors, a Zener diode, a small 1.6A SCR and the big 150A SCR. 

Further individual component testing today showed the zener and the two SCRs were in good shape. I could trigger either SCR to lock on, using current limiting bench power supplies to not overstress the parts. The Zener diode appeared to be working properly as well. 

The two parts which didn't look good were the IBM 33 transistors that form a comparator circuit to match the Zener driven reference voltage against the sampled actual output of the regulator. I don't have any spare boards with type 33 transistors on them, so I had to find an outside source. The sometimes dubious cross reference chart between IBM part numbers and industry standard numbers claimed that a 2N1303 would be a suitable replacement. 

Bad "033" transistors

Schematic of the failed SMS card

TEMPORARY SUBSTITUTE OF TWO 2N4403 SILICON PNP TRANSISTORS

I put in two silicon transistors while I wait for my order of the germanium 2N1303 to arrive. Silicon has a larger diode voltage which changes the circuit operating point a bit, but I can verify that it will provide the crowbar protection at some reasonable voltage point.

Using a current limited bench supply. I was able to observe the card triggering at 3.9V. Since it works consistently at that level, it would be safe to run the machine with the Silicon transistors until the more correct parts arrive that will lower the trigger point a bit more. 

TESTING REGULATOR WITH THE REPAIRED CARD SHOWS IT WAS GOOD

The bench setup supplied the regulator with about 8.2VDC as well as a -8.2VDC bias voltage from a second bench supply. An electronic load was used to change the current draw as a test of the regulation accuracy. The voltage stayed exactly on its set point over a wide range of current draws, so the regulator is good to return to the machine.

REINSTALLED AND POWERED UP THE MACHINE FOR A QUICK CHECKOUT

The regulator went back into the machine and I brought up the system for a quick checkout. It ran the keyboard test loop I had entered previously and seemed quite healthy. Now I can get back to working on the known issues and testing more of the machine.