The MSR-904A has a few outputs (and inputs), most of them, easy to identify, but two, are pretty much a mystery to me, with no schematic – their function is clear, but with all these wires TTL logic boards -difficult to guess the pinout.
The more easy thing first, the monitor output. Arguably, this was intended to be connected to storage displays, digitizer, chart recorders, or the like – to more permanently record the activity over the bands.
So, what do we have. Using a scope, and a multimeter, and activating more or less all the functions of the apparatus, that’s what I found out.
Sorry for the rough draft, but any questions, please ask – for the given purpose, good enough.
The only pin that doesn’t do much, is pin 9 – always stays on logic 5 V – maybe a +5 V supply line? Doesn’t seem to be an essential function, anyway.