HOW THE CLOCK SIGNAL IS GENERATED IN THE APOLLO SPACECRAFT
The 800 Hz signal is produced by a counter that ticks to alternate the binary state of an output signal. That essentially produces a square wave at 800 Hz. The type C interface circuit delivering that signal out of the AGC consists of a PNP transistor with its emitter at ground and a 2K resistor in series between the collector and the output pin.
Thus, it acts to pull the external line down to ground or let it float depending on whether the clock signal is on or off. Within the DSKY power supply, +13V passes through half of a center tapped transformer and out to the circuit I just described. Thus, 800 times per second it is alternately pulling 13V down through the transformer towards ground or letting the transformer pull up to 13V.
FIRST ALTERNATIVE, GIVEN THE SHORTAGE OF APOLLO GUIDANCE COMPUTERS
I set up a function generator to produce an 800 Hz square wave, 0 to 5V, which I fed into an interface circuit I had built for use with the restoration of the Apollo Guidance Computer we undertook last year. The surface mount transistor that I used on those interface boards were designed to drive very small currents for inputs to Arduino or similar devices. It was not engineered for higher current levels such as the use of the clock to drive the DSKY power supply.
NEW APPROACH TO DRIVE THE POWER SUPPLY MODULE
I constructed a replacement to the interface circuit since my first approach may not have had enough current capacity to adequately drive the transformer in the power supply module. This was a 74F05 integrated circuit, an open collector chip which takes a TTL input signal (from my function generator) and whose output is hooked through a 2K resistor to the DSKY supply.
RESULTS OF NEW APPROACH 1
Before I put this into the circuit to drive the power supply module, I set up the open collector inverter with the 2K resistor pulling it up to +5V and observed the signal on the oscilloscope. The results show that this chip is inadequate, having too high an internal resistance.
|Yellow is input square wave and blue is output pin of the inverter|