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Tube🌞Time

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.
2 Jan 2023 at 2:25 | Open on mastodon.social
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Tube🌞Time replied to Tube🌞Time

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.
2 Jan 2023 at 2:29 | Open on mastodon.social
Tube🌞Time replied to Tube🌞Time

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.
2 Jan 2023 at 2:33 | Open on mastodon.social
Tube🌞Time replied to Tube🌞Time

removing and cleaning the area helped quite a bit! it's not perfect so i think i have more cleaning to do.

2 Jan 2023 at 2:38 | Open on mastodon.social
Tube🌞Time replied to Tube🌞Time

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.
2 Jan 2023 at 2:40 | Open on mastodon.social
Tube🌞Time replied to Tube🌞Time

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?
2 Jan 2023 at 22:41 | Open on mastodon.social
Tube🌞Time replied to Tube🌞Time

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.

2 Jan 2023 at 22:42 | Open on mastodon.social
[DATA EXPUNGED]
Tube🌞Time replied to DELETED

@homelessjun the NE558 already does that though.

3 Jan 2023 at 3:23 | Open on mastodon.social
Karl Gutwin replied to Tube🌞Time

@tubetime Did you figure out what the ‘LS125 is doing there? I looked it up and guess it’s just a buffer. ti.com/lit/ds/symlink/sn54ls12

2 Jan 2023 at 3:08 | Open on mas.to
Tube🌞Time replied to Karl

@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

2 Jan 2023 at 21:12 | Open on mastodon.social
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