Voice Warning Generator from F-4 fighter jet (continued)(Continued from earlier)I traced the high-level connections on the PCB and tried powering it up a while back, and poking the input pins one at a time with 5V, but got only white noise from the audio output. So, I succumbed to temptation and sawed open both hybrid modules:
As expected, the smaller one is for interfacing. It protects the external inputs with series resistors, clamping diodes, and filter capacitors, and feeds them to a Schmitt-trigger inverter (the CD40105). One of the other dies onboard is a CD4011 quad NAND gate which does a little bit of logic with the inputs (a global enable/disable); the third die is an op-amp which takes the current-mode output from the DAC and converts it to the final audio output voltage to send through the external connector. There's also a power-up reset circuit which generates a global reset for the larger hybrid module.
(I've got a full schematic for this one but it isn't worth posting until I finish tracing the connections in the large hybrid module)The large hybrid module, also as expected, contains the circuitry for playing back the audio samples from memory, implemented entirely in RCA's CD4000-series digital logic. There's a couple counters, a few shift registers, a bunch of gates, and a lot of 4013 D-type flip-flops. I haven't finished tracing this one, so no schematic yet: it has at least 3 layers of connections that can be seen faintly through the thin aluminum-oxide(?) substrate, so a lot of continuity checks with sewing needles are needed to dis-ambiguate a lot of the connections, and have a shot at eventually making it play sound.
Some microscope work at my local hackerspace was essential here in getting a close enough view of the chips to read out the ID numbers written on the metal layers and identify them from that, starting from a lucky hit on the correct digital logic family while browsing Zeptobars. (See attached images below)