Tested blower motors and power rail voltage regulators; the +3V regulator is bad

BLOWER MOTORS CHECKED WHEN POWERING ON

Now that T2 is wired up, when the contactor switches on, it powers T2 whose output is 115VAC wired directly to the blower motors. There are three motors in gate A, three motors in gate B and one motor under the voltage regulators and lighting power supply. 

These spun up just fine with relatively little dust emitted for a few seconds before they ran clean. Before the restoration is complete, I will need to measure the air filter openings on the bottom of the blowers, find a compatible size available today and cut it down as necessary to fit the 1130. 

DISABLING THE POWER CHECK AND LOCKOUT WHILE I TEST POWER SUPPLIES

If I pull the connector off the SMS card pin A, the signal to activate the shutdown and lockout won't activate the reed relay on the card and we will merrily continue to have power. 

The green path is the power that flows to keep the contactor (K1) energized and power applied. It does so as long as relay RR1 is not activated, because of the circled contacts. When the time delay relay TD1 fires, the yellow path flows through the coil of RR1 and activates the relay, breaking the contacts that keep K1 energized thus dropping power to the machine. Pulling pin A blocks the route to activate RR1. 

DISCONNECT REGULATORS FROM CPU LOGIC BEFORE TESTING THEM

We don't want any problems with the power rails to cause damage to the SLT cards or other circuitry, so we will disconnect the output wire from each of the three regulators. This ensures that anything we do with the regulators is not reaching any further into the machine. That is terminal 2 of the +6 and +3 regulators and terminal 5 of the -3V regulator. 

CONNECT RAW POWER SUPPLY TO REGULATORS BUT LEAVE THEIR BREAKERS OFF

The three regulators have their own circuit breakers, which we will initially flip off. Then the wires from the raw DC power supply are connected so its output will power the regulators. Raw DC flows to the inputs and the outputs are disconnected until we carefully check the power quality and correctness. 

TURN ON EACH REGULATOR AND CHECK OUTPUT VOLTAGE FOR SANITY

One by one, we flip on the circuit breaker and look at the voltage level produced by the regulator. These have a narrow range of acceptable values centering on their nominal 3, -3 or 6 volt target. If the values are slightly off, we won't adjust them here because the correct procedure is to measure the voltage as delivered to the terminal strip at the bottom of the logic gates and adjust the regulator potentiometer to get the voltage correct at that point, not at the output terminal of the regulator itself. This compensates for voltage drop in the distribution wiring. 

The results were excellent for the +6V regulator as well as the -3V regulator. The +3V looked a bit suspect, as it was at 3.26V which is outside of the maximum range of 2.88 to 3.12V. 

HOOK UP LOAD RESISTOR NETWORK TO FULLY LOAD EACH REGULATOR

I had a resistor network which I use to present full or nearly full load on the regulators. They must hold their voltage not only when no load is present, but maintain voltage regulation up to their capacities of 25A for 6V and 20A for the others. 

I wired together 10W ceramic 1 ohm resistors in series and parallel combinations to achieve the net low resistance to load down a regulator and to have each resistor at or below its 10W dissipation limit. I could demand the full 25A capacity from the 6V regulator but for the two 3V supplies, the resistors I had on hand could only get low enough to pull a bit under 17A. 

While this is not the full 20A the regulator can support, it is close enough that all power transistors must be good to handle this and therefore it would scale up to 20A with no problem. The four germanium power transistors are in parallel across the load with each able to handle about 5A. If only three were working we would have begun to sag already. 

As expected, the +6V held its voltage right on the nose at the full 25A of load. The -3V regulator held its voltage steady up to almost 17A. The +3V regulator did not do so well.

With no load, it had operated at about 3.26V. With the load applied, the voltage began ratcheting upwards from that already unacceptable level. When I saw it going north of 3.67V I flipped off the circuit breaker before the overvoltage (crowbar) card would fire and trip the breaker off. The regulator is not doing its job. 

REGULATOR REMOVED AND PUT ON THE BENCH

I disconnected the regulator and pulled it out the 1130. It was put on the bench where I could figure out the defect and repair it. The power transistors are on heat sinks that fold out from the regulator to give good access to all four. Each heat sink wing holds two transistors and an aluminum bus bar holds the two wings together electrically when folded up. 

REMOVED AND TESTED ALL 108 POWER TRANSISTORS BUT ALL ARE GOOD

Every other time I have had a bad power supply, whether on SLT systems or 1401 systems, the fault has been one or more 108 transistors that shorted or went open or are otherwise nonfunctional. I desoldered and removed the four 108 transistors from the wings, as well as the 108 on a heat sink on the SMS regulator card. 

Regulator card

Overvoltage/Crowbar card

USED MY CURVE TRACER TO IDENTIFY VOLTAGES OF ZENER DIODES

Having just used the curve tracer to verify the health of the 108 power transistors, I decided to leverage it to identify the voltages of the two zener diodes on the regulator card. It would be hard to reverse engineer the regulator circuit without specific component details like this.  They are 10V and 4.5V by the way. 

The 10V diode had a very nice almost vertical turn-on at the 10V point, but the 4.5 had a bit of slope rather than the right angle that is characteristic of a good zener diode. I guess I could substitute a new 4.5V zener and see if that fixes things, but that is a bit of a blind shot. 

STUDYING CIRCUIT SO I CAN COMPARE WHAT IT IS DOING WITH WHAT IS CORRECT

I set up the circuit with all the parts values in Circuitlab, an online tool that lets me simulate various circuits I have designed. The transistor parameters were not accurate, however as I don't know the specs for 026, 086 and 108 transistors. I tried to get the 108 set up to look like a modern PNP Germanium power transistor. 

The simulation did let me regulate by adjusting the potentiometer setting until it produced exactly 3V as an output. I put in various load resistances with zero change in the output voltage until I got down to .45 ohms. This corresponds to a design that could handle 6.5A or a bit more, not the 20A in the IBM supply. However, I attribute this to the inaccurate transistor specifications, since the drive current to keep up with higher current depends on the abilities of the transistors in the circuit. 

This allows me to record the expected voltages at different points on the regulator, which I can compare to the failing regulator on the bench. Hopefully this will point me directly at the failed component(s). The alternative is to unsolder all the transistors on the regulator card and put them on the curve tracer to look for a bad part. 

I keep seeing references to an IBM publication named IBM FE Theory of Operation : Power Supplies SLT, SLD, ASLT which would have been very helpful, rather than having to reverse engineer the supply to understand how it must operate. Back in the day, the solution to a problem like this was for the local FE to install a spare part regulator card and if that didn't fix it, replace the entire power supply. The factory would deal with diagnosis and repair.