Gregory's Serverit's a little loose but not a bad fit. I designed this based on photos of a real bracket, so the dimensions are probably slightly wrong
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 the design is not an ADC. it uses a quad 555 timer with RC circuits composed of the external joystick resistance and 22nF capacitors. the Apple IIe just measures the time it takes to exceed the threshold. it's just barely pegged at 250 or so. 2.8ms. the resistance is 87.8K, so this RC curve implies a capacitance of 68nF. that can't be correct. inspecting the board reveals corrosion on the pins of an 'LS125, some of whose pins are wired into the joystick inputs (for some reason). i think the capacitor (already recapped C2) spilled electrolyte all over this area. i removed the 'ls125. there was definitely a puddle of electrolyte underneath (it crackled and fizzed during desoldering). note that the potentiometer connection (red arrow) is right next to a pin tied to ground (blue arrow). electrolyte is conductive, so maybe it was creating a leakage current. removing and cleaning the area helped quite a bit! it's not perfect so i think i have more cleaning to do. i'm also thinking that the etched/oxidized solder joints in combination with the spilled electrolyte formed an electrolytic capacitor with about 45nF of capacitance. yeah that would do it. so what does the 74ls125 do in this circuit? it's just discharging the timing capacitors. the NE558 also does this, so the 74ls125 is connected in parallel. I guess it... helps? regardless i went and cleaned the board again more thoroughly, and it's working a lot better now. i'll replace the damaged 74ls125 later.
[DATA EXPUNGED]
@tubetime Did you figure out what the ‘LS125 is doing there? I looked it up and guess it’s just a buffer. https://www.ti.com/lit/ds/symlink/sn54ls125a.pdf @kgutwin it has the ability to pull the joystick lines low. no idea what this was for, none of the other apple II compatibles have it  @tubetime Ahhh, so IBM _wasn't_ the only company that used an RC circuit as an integrating ADC :'D!  |
today's status: i'm having issues with the joystick port. the machine thinks it is pegged to the lower right even when it is centered.