soundtracks PC midi mixer problem

[newtronica]

Hello Everyone,
I've got an old soundtracks PC midi mixer which works fine for all the analogue bits but I'm having a strange problem with the Z80 controlled midi mute automation. I don't know anything about this side of electronics but tried some basic fault finding and doing things like changing all the electrolytic and the shitty ic holders. The weird thing is that after working on the board and say replacing an ic holder when I power it up it works fine and will continue to do so but if I turn the mixer off and on again it doesn't work. I know it uses a few inverter logic chips that start things on low so thought that this may be the cause. I'm sort of just replacing things until it works but no luck so far! It uses a z80 processor with a 4mhz crystal. I just find it weird that after I change/replace things and any residual power is reset it works but next time it doesn't. Any general ideas?
Thanks
Joe

[m k]

So input is not dropping or output rising.

Missing discharging load can be one, maybe a single small resistor.