+6V Regulated Power Supply in more depth

THIS SUPPLY REGULATES DC DOWN TO A VERY STEADY +6V

The IBM 1130 has a DC power supply that is fed input mains voltage, transforms it down, rectifies it and smooths it with large capacitors. The resulting voltages can shift up and down with changes in the building power as well as with the effects of varying loads as different circuits operate. This raw DC is fed to a set of regulating power supplies that produce regulated steady power for use with the logic.

The +6V supply is nominally fed with 12V raw DC from the DC Power Supply, although in our case it is actually up at 16.5V. The negative side of the input is fed into this regulator and the positive side of the raw DC is tied to the positive output terminal. Thus the supply acts by moving the negative or common output terminal to be six volts below the positive output. 

The output transistors will conduct enough current from the negative side of the input to bring the common output to zero volts. The regulator circuit is on another SMS card and will drive the output transistors based on what is needed to keep the difference across the output at the target set.

It does this starting with a differential transistor pair that uses a zener diode as a reference voltage on one side and a potentiometer adjusted fraction of the output voltage on the other side. The power transistors are then driven by other circuitry so that it keeps the differential pair essentially balanced. 

You can think of this as a kind of servo or feedback mechanism, with the regulator card producing an error signal that is amplified to a strong drive signal for the output transistors. IBM's official name for this SMS card is "6V Amplifier".

Redrawn schematic used for circuit simulation

FOLDING AND STACKING A CHAIN OF SIX OUTPUT TRANSISTORS

The supply puts each output transistor on its own heat sink, along with an emitter resistor that causes the transistors to balance the load among the various transistors. The emitter and base connections are daisy chained with appropriate thickness of wire, producing a long train of heat sinks initially. 

Six heat sinks each handling 4A of output current

These are then placed side by side and electrically connected by a heavy aluminum bar spanning the heat sinks. If all six were linearly placed the supply would take up too much real estate, so the design will place three across, then fold the chain to run back in the other direction for the next three. These are therefore stacked, two levels of heat sinks, to fix all six in a reasonable volume. 

First three in place, layer one complete

Full stack assembled

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CHECKING OUT DC POWER SUPPLY

I removed the DC Power Supply to be sure there was nothing wrong there that led to the 16.5V output. This box feeds raw power for +3, -3, +6, +48 and +12 thus is large and extremely heavy. I wanted to be sure there wasn't a problem with the windings involved in 230V operation, or with the rectifier diodes. 

DC Supply removed, all the wiring in place.

The diodes are good and even when I rewired and hooked it up to 115V, the output was the same stepdown ratio. Thus, this is the level the supply is going to be dealing with, so the focus remains solely on the regulator card.