Investigating recreation of DSKY power supply circuit

Since NASA published the schematic and most of the component values for the power supply module that fits in the DSKY (Display/Keyboard) that is the human interface to the Apollo Guidance Computer in the Apollo spacecraft, it seemed possible to build a replacement.

I was loaned a power supply module from the block I DSKY, which is close enough to the version from the block II DSKY used with all the missions flown with people aboard. I did some testing of the pins, but the parts are encapsulated inside a metal rectangular solid which makes it infeasible to probe inside or repair anything that might be malfunctioning.

My first tries to power the EL panel used that original power supply module, driven by an equivalent circuit to the one inside the AGC that generates an 800 Hz square wave which drives the DSKY power supply. The module should produce at least 250V AC at 800 Hz, but at best I was only getting 85% of the target.

That lower level didn't seem to be enough to light the biggest single element in the electroluminescent (EL) panel - the COMP ACTY rectangle - but did give me glowing output on all other elements,. Building a replica power supply circuit would let me produce the full voltage, or debug and correct any issues I might find.

Block I DSKY Power Supply Module

The circuit seemed to be fairly straightforward. The 800 Hz square wave drove one half of a center tapped transformer primary, with the secondary creating a 13V AC signal. The AC signal was fed into a pair of transistors to drive the primary of a second transformer (T2), whose secondary windings boosted the AC signal to 24V but with more current flowing.. The 24V AC signal is fed into a third transformer (T3), whose output would be the full 250+ V that drives the LEDs.

The EL panel works by energizing a capacitor, the top plate being a transparent conductive material and the bottom plate having the shape of the segment to be illuminated. The insulator material luminesces under the high voltage, high frequency field. There are more than 150 elements or segments that can be energized, with the number varying as the panel displays different output values or blanks.

Because each illuminated element is another capacitor added in parallel across the power supply, it increases the load on the output. With more segments lit, the output voltage might droop, causing the EL panel to vary its brightness based on what is displayed. To adjust for this, the third transfer (T3) is a reactor whose core can be saturated or controlled by a DC current in a control winding. That change in core saturation varies the inductance thus changing the step up ratio of the AC windings of the transformer.

The clever circuit puts a load resistor in series with the high voltage, high frequency going to the EL panel. As load goes up, the current changes and the voltage drop across that load resistor will increase. A full wave bridge rectifier converts the voltage drop of the load resistor to a DC value which is fed into the control winding. The load resistor value was tweaked with each instance of the power supply to cause the output voltage to change the least as the number of energized segments varied.

Finding (or winding) a transformer with a core which can be saturated with the current produced from the load resistor is going to be essentially impossible. We don't have winding counts, stats on the core, manufacturer part number or even manufacturer name. One document hints that the part was bought from "Bush", but google is not my friend when I search for any sign of such a company or product.

I suspect that I would have to design a different circuit to produce an invariant high voltage across a varying capacitive load. It would have been much more straightforward and period authentic to build an exact replacement power supply.