Single-bus keyboard

[Mark Hammer]

I'm working with the single-bus keyboard interface. I get appropriate keyboard CVs and also get an trigger indicator LED that flashes when I press a key down and again when the key is released. But the gate indicator LED remains on all the time, no matter whether a key is pressed or not. I might attribute it to something I may have done wrong, but I'm having the same issue on an entirely different build of the same interface board that somebody else built.

Was there some erratum that was found out later, OTHER than the change in value to R7?

Thanks in advance.

[Mark Hammer]

This is so bizarre and frustrating. I get a trigger and I get keyboard CVs that are what they should be, but I do NOT get a gate voltage (in addition to the gate LED being always on), and CVs do not linger longer than the trigger duration. Would I get an expected CV of too-short duration were one of the 2N5457s to be suspect?

Now, I have to add that much of this is largely true of BOTH the board I etched for myself, AND the board that the technician I'm working with purchased from MFOS and assembled himself.

I did change R7 from 1M to 10M, as recommended, but that did not address the problem I'm having.

Any hints as to what would be a telltale sign of a gate being unlikely to be generated? I'm guessing it is somewhere within the stages comprised by U5, but I'm unsure of which specific stage is the one to focus on. So far all the various bias voltages on pins 2, 6, 9, and 12 of U5 seem fine. Measured all relevant diodes and their voltage drop is all coming in between 510 and 520mv, so they're all AOK.