REMOVING FUSES, DISCONNECTING TRANSFORMER OUTPUT AND OPENING BREAKERS
In order to carefully step through the checkout stage by stage, I had to disconnect various parts of the power circuitry. There are seven fuses which control power to various power supplies, convenience outlets and the power sequencing transformer that is essential to turn on an 1130.
Each of the voltage regulators has a circuit breaker, which I opened to block them from energizing logic circuits throughout the machine. The +6V substitute regulator does not have a breaker; I disconnected its input instead.
There are two transformers which were substituted by a prior restorer. One of them is connected only to the convenience outlet inside the machine and its inputs are connected through a fuse. The other, however, is directly connected to input power when the main contactor energizes when the power switch is thrown. It powers various fans throughout the machine which I don't want energized initially, so I have to pull the wires off the secondary to block that transformer.
With all these disconnections and fuse removals, the machine should not provide power to anything besides the power sequence logic itself. I can then insert fuses, connect wires or switch on breakers selectively to test section by section.
VERIFY ABSENCE OF POWER THEN CHECK 24VAC POWER SEQUENCING VOLTAGE
With the plug inserted into the 240V outlet, no fans should run, the unit should not power on and I shouldn't find AC anywhere outside of the power sequencing box. I verified that this is true.
Next, I inserted fuse F5 which provides power to the 24VAC transformer. I could see the 24V appear on the sequencing lines and the relay R3 energized which among other things allows AC to flow from transformer T3 to the convenience outlet.
VERIFY SUBSTITUTE TRANSFORMERS DELIVER 120V
Inserting fuses F3 and F4, while F5 was still in place, energized transformer T3. I checked the voltage at the convenience outlet which was indeed 120V as intended. This gave me some confidence but I still will check the output of transformer T2 to ensure it too is delivering the proper voltage. To test this I had to switch on the power switch of the 1130 system, causing the contactor to energize.
This was a good time to verify that all the fans began spinning around the machine. However, they were spinning without T2 connected, thus they are wired in some nonstandard way.
ANOMALOUS WIRING AND COMPONENTS DISCOVERED
The fans should be 115V blower motors whose power is fed from the stepdown transformer T2. This allows machines to be used with 115, 208 and 230 volts, the transformer converting the higher voltages down to 115 as a standard input to motors, fans and power supplies.
However, they are spinning when the only power present is 230V. I see they are wired in odd locations on the terminal bus, which is how they get power early, but I am waiting to hear from the prior installer to learn whether these are 230V substitutions or something else was done to make them run.
When I was checking all the power sequence wiring I came across a replaced relay. Instead of the small relay used on the SMS card for power level checking, a large IC style component was soldered in place. As long as this switches on when the voltage rail is active and switches off when the rail drops, the substitute is okay to keep.
I did look at the specifications of the power regulator for the +6V rail, since a modern substitution was put in place. The regulator used by IBM can deliver up to 24A at 6V, while the modern part has a capacity of 10A on paper and probably less without serious cooling. Even if it provides noise free and stable power at 6V, it lacks an overvoltage protection which is an important feature of the IBM supplies.
I am not happy with the 6V regulator in the system and will have to address this. Long term, I will repair the IBM regulator and put it back into service, but in the short term I will look for a different high current regulator with overvoltage protection.
CHECK RAW DC AND AC POWER LEVELS
I hooked up the output of T2 to the power circuitry and once again switched on the 1130 system. The contactor engaged but with fuses F1, F2, F6 and F7 out, there should have been no power to the power supplies. I checked and this is true.
Next, I inserted the fuses one at a time, checking the output of the power supply it energized. F1 provides power for the raw voltages that produce the +3, -3 and +6V. F2 provides power for the 12V and 48V DC supplies for core and relays. F7 energizes the 7.25VAC power for lighting circuits. F6 is power to the various peripheral devices, which I verified at the power connectors for those devices.
The fuse for the lighting supply was missing from the holder. When I put one in, I did not see the 7.25V appear at the lighting PCB. I will need to track this down and figure out what is happening, then repair it.
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