Tube🌿Timei got a new Apple! 🧵
anyway, let's try and get this machine running. i have a funny feeling the video isn't going to work right...
sure enough, the machine chimes, but the video is blank. but i think i know what the problem is.
sure enough, there's no PRAM battery. these machines need that battery or you won't get video!
but where am i going to get a PRAM battery at this hour?
and we have working video! now for the most important question of the day: what version of Mac OS should i put on it?
i'm going to create a SCSI disk image for my SCSI2SD using BasiliskII and the instructions at https://www.savagetaylor.com/2019/12/19/how-ive-setup-the-scsi2sd-that-i-use-in-my-lc-475/
I was having trouble because I was putting the *volume* image into the SCSI2SD rather than the *disk* image. turns out partition tables are important lol
trying something else. unfortunately this didn't seem to work, I suspect the HFS partition needs to come before the ProDOS partitions.
well this explains why it was hanging. there's probably some setting on the SCSI2SD that's not exactly right.
turns out this hang was caused by the SCSI2SD firmware, which was uhh shamefully out of date. with the latest version it works and i can partition the drive!
it also turns out that MacOS wants to be the first data partition (i had the two ProDOS partitions first.) but it works now!
only had these poscaps hanging around. yes they're very fancy but they'll work.
wow, it totally worked! is it an Apple IIe or is it a Performa 475? why not both!
i figured out how to change the startup sound to "Quack" which makes me think of the Quapple (my Apple II ISA card that i cloned a while back)
time to check the capacitors on the motherboard. there really aren't that many chips on it! I was also able to remove it without any tools. just bend some tabs and it slides right out.
i found a tech manual. this is a very clean design. most things are tidied up into the two big custom chips, MEMCjr and Prime Time.
heh, they even put the bus sizing features in this chip. motorola removed that in the '040 (the '030 had it) and it caused innumerable problems for 68k computer designers.
the Commodore folks solved this on their '040 Amiga CPU card by using a raft of ten (10) GALs, which burned a ton of power.
based on the pinout i think it's a 24C01 serial EEPROM, probably for a unique serial number or something.
yeah i'm wrong, this is a battery switchover circuit much like the MAX703. it provides voltage to the Cuda chip from the battery, and automatically switches to the 5V rail when power comes up.
I've 3D printed a drive bracket for the Performa. it's designed to hold the SCSI2SD.
it'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
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.
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.
Karl Gutwin@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
William D. Jones@tubetime Ahhh, so IBM _wasn't_ the only company that used an RC circuit as an integrating ADC :'D!
Brian Danger Hicks@tubetime Sounds like an ADC to me.