POWER UP OF MACHINE DURING TESTING - AUTO SHUTDOWN AND LOCKOUT
The protection circuitry of the IBM 1130 is designed so that if anything is not correct with the primary power rails, it will drop power and lock itself out (until the breaker or CE switch is reset) so that the power toggle no longer works. An SMS card has three reed relays in series, each powered by one of the three major rails - +3, +6 and -3.
As the machine starts up a delay relay provides the main rails time to come up to voltage, before the SMS card tests that all three are correct. The machine stays in reset until the relay time expires. Then, either the SMS card drops power or a relay closes to provide +12 and +48V to the system.
As the time delay expired this time, the machine powered down. I looked at the regulators for the main rails and saw that the -3V circuit breaker was in the OFF position. Other 1130 systems have had weak CBs that sometimes tripped during powerup; a reset of the CB is all it takes to get the show back on the road.
However, this time, after I reset the -3V CB and reset the lockout, the power up again stopped abruptly as the delay time expired. Once again the CB was tripped. We have a real issue!
DISCONNECTED THE LOGIC GATES FROM -3V TO CHECK ITS OPERATION
I took the connector off the regulator so that it only had its internal 1ohm load resistor. If the issue were elsewhere in the machine, I would expect to see the regulator delivering precisely -3V. However, the CB tripped immediately. This indicates an issue in the regulator itself.
REMOVED REGULATOR AND PUT ON THE BENCH FOR TESTING
I disconnected the regulator completely and put it on my workbench where I can test with my bench supplies and loads. I set it up with a bit over 8V, the raw level delivered by the power supply to the regulator. There is also a -8V feed that represents the AC bias circuit from the power supply; the IBM design attempts to compensate for load effects by delivering an pseudo-independent voltage to the regulator.
I hooked up an electronic load set to 1A as a low load, which will also show me the voltage being generated. I then flipped on the breaker with power supplied.
I saw the same issue, with an attempt to trip the breaker. Since my bench supply was current limited, I just saw the input supply drop substantially and flipped off the test setup. Had I provided full current to the system,
I chose to pull the Overvoltage Protection card (crowbar circuit) so that I could see how much of the regulator was working reasonably. Without that card, the regulator produced exactly -3V and held steady as I bumped up the load.
OVERVOLTAGE PROTECTION CARD IS BAD
The crowbar is clamping even with the voltage at acceptable levels. I don't know for sure the exact sequence that occurred but the result now is several transistors and an SCR blown. I will figure out how that happened later, but first I want to repair the card so that I can verify the extent of damage first to the regulator and then to the entire 1130 system.
TWIN IBM 33 TRANSISTORS FORM A COMPARATOR
The card uses a pair of transistors, one sampling the regulators output and the other drive by a zener diode to a fixed volage. When the sampled output exceeds the reference zener voltage, it triggers the gate of the first SCR. This SCR is a relatively low power 1.6A device, which in turn switches a higher current to the trigger of the 150A capacity main SCR. That SCR will short the entire output of the regulator, resulting in a trip of the circuit breaker.
TESTING DEVICES TO IDENTIFY FAILED PARTS
The two transistors appear to have failed by shorting internally. I suspect that the lower power SCR is also toast, but the big one is probably okay. I will set up my testers to check out the two SCRs, determine the zener diode voltage, and test the two IBM 33 transistors just to be sure. Since these are germanium rather than silicon semiconductors, the diode checkers built in multimeters sometimes give misleading results.
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