Fonas Tri-1 has an unlabeled 40-pin DIP that I haven't been able to figure out. The PCB has a date code in 1979, and one trace is marked VSS 0 and another trace is marked VDD -15, so it's NMOS. I desoldered it and there are no markings on the bottom or on the PCB under it. I haven't had any luck with patent searches. Any ideas? Here's the pinout:
1 N/C - looks like clock output - I measure 86.5 KHz at 12.5% duty cycle 2 VSS 3 VSS 4 resistor to VDD 5 VDD 6 VSS 7 VSS 8 switch input 9 button input 10 switch input 11 button input 12 button input 13 VSS 14 button input 15 VDD 16 VSS 17 reset button - not sure if this is an actual MCU reset 18 out to LED 19 out to LED 20 out to LED 21 out to LED 22 out to LED 23 out to LED 24 out to LED 25 out to LED 26 VDD 27 N/C 28 VSS 29 VSS 30 VSS 31 out to 75492 32 out to 75492 33 out to 75492 34 out to 75492 35 out to 75492 36 out to 75492 37 out to transistor 38 out to transistor 39 out to transistor 40 out to piezo