I am afraid that it is either the Synthe IC or the Chorus IC, both are custom ICs
I have basic understanding of digital circuits, I am no EE by any means, so please feel free to correct me, if my assumptions are wrong.
Trying to figure out the boot sequence , this helps a lot :
The D50 EPROM throws a start up routine where the CPU checks if the Gate array, Synth & dyna board INTs are present. Then the Gate array also checks if reverb and chorus are present. If they are all present the CPU boots up.
The problem I am experiencing is probably CPU HALT due to SYNTHE IC not responding to INT0.
What could be causing this to Synthe IC?
By checking the signal flow the reverb IC sends Ready signals (pin 6-10) to a dual 2 to 4 line decoder (IC17) which activates OE on synthe IC and chorus IC.
By comparing readings on the working and non working board, I am getting pulse on Synthe IC’s OE pin but chorus ICs OE pin is held High.
This could be a problem with the decoder IC17 or the reverb ic not sending proper ready signal.
My hopes are on the IC 17 , it is a 74HC139, the problem is I don’t have any and due to the isolation i can’t get any. I have a 74LS139, will it work ?