so I figured I'd make some sort of a log of my fix0ring adventures. fixing shit is fun! (and also, uh, keeps me busy in these troubled times, which helps keep depression at bay)

so, anyway.

the facts.

---

a while ago I was able to salvage a couple modern computers:

• Packard Bell PC from the mid-1990s
• Macintosh LC

in this post I will focus on the latter, because while the PC is working more or less smoothly, the Mac gave me one hell of a ride.

it came complete with mouse, keyboard and matching monitor. while the monitor did turn on, the Mac itself gave no sign of life.

opening it up, things looked atleast decent. it atleast hadn't been Maxell-bombed, and the old PRAM battery wasn't leaking (yet). however, it did have leaking capacitors, the audio chip even found itself covered in cap gunk somehow.

the power supply had leaking caps too. I measured the voltages out of it, the 5V, 12V and -5V rails measured respectively at 2V, 1V and -3V. no wonder why the thing wouldn't turn on...

however I wanted to see if the thing was worth attempting to fix. I borrowed the power supply from the Packard Bell, which I knew was working fine, and had the required voltages. by using staples and some trickery, I was able to extract the metal contacts from the ATX connector with no damage, and connect them directly to the power pins on the Mac's motherboard.



that ghetto setup revealed that the Mac was mostly working fine, minus having no audio (and a nearly dead HDD). after a while, the speaker started screeching at me, which is apparently caused by cap gunk. in the meantime I stuck a piece of paper between the speaker contacts.

also, the original monitor is monochrome, which is a bit of a disappointment to me.

so, with my meagre electronics skills, would I be able to fix it up? I apparently thought so, and started ordering replacement capacitors and shit...

---

I started the work with the Mac's power supply. removing the old crapoed capacitors, cleaning up the gunk where they used to be, and putting new ones in. pretty standard shit. except the power supply didn't work right after the recapping, so it was going to need more attention. in the meantime, I still had the ATX power supply set up as a replacement, so I put it aside...

I moved on to the motherboard. using the 'twist back and forth' method, I was able to remove the old capacitors with no damage. I removed the leftover cap legs from the solder pads, did some cleanup, and attempted soldering in new caps. all fine and dandy.

except it didn't work.

it was soul-crushing to watch the thing sit on a black screen and do nothing visible.

I tried cleaning the motherboard harder, checking my solder joints (some of which were bad), even redoing all my soldering with a different technique, all to no avail. well, at one point it did boot, but it was unstable.

but atleast it wasn't entirely dead I guess... I figured I needed to redo my work the right way. ordered some flux, desoldering braid, etc...

I desoldered my capacitors again. ended up with some damage to a couple pads, but nothing that wasn't quickly taken care of. I made sure all the traces were good, cleaned up the solder pads with solder wick, resoldered the caps with generous amounts of flux. all seemed good this time.

this time the motherboard was working reliably again. it still didn't have sound, but it was atleast back to being usable.

the power supply, after some cleanup, seemed to work... mostly. sometimes it wouldn't start, but when it did, it was rock solid, outputted correct voltages, etc... after a while, the issue seemed to mostly resolve itself, so I put the Mac back together, and put the ATX power supply back into its computer.



all seemed to be good again?

---

I was in for the original experience of playing Blobbo Lite and some other fun games in the good ol' way, so the lack of audio bothered me.

I wanted to see if I could fix that, so I grabbed some old Mac schematics. they don't have schematics for the LC, but they do for the LC II, and most of it is identical to the LC, including the audio circuitry.

so I started investigating it. the DFAC (audio chip) is fed 8V power by a 78L08 voltage regulator. all fine and dandy. except the voltages I measured on its input and output pins weren't looking good. the input was around 8V and the output was 7V. so something was up there.

the 78L08 input is connected to the 12V power rail through a resistor, and the output goes to the DFAC. both connections worked fine. however, something was up with the ground -- the 78L08's ground was connected to the DFAC's ground, but there was no connection to the system ground, which seemed wrong. further inspection revealed that this affected pretty much the entire audio circuitry, and also the ground plane to which the shrouds of the I/O connectors were soldered -- those were all interconnected, but had no connection to the system ground.

I couldn't find where the break occured, so I ran a bodge wire between the two grounds to reunite them back. that didn't fix the audio output, it was only outputting some barely audible garbage at best, but atleast it was getting proper power now -- the 78L08 input measured at 12V and the output at 8V, so atleast that was fixed.

digging more into it, I looked at the audio output circuitry. in particular, the DFAC's audio output line goes through a circuit with a bunch of passives, then to pin 4 of the amplifier at UC9. so I started checking continuity, which was a bit of a pain as the trace goes to the other side of the board where most of the small passives are. those also have different numbers than those in the LCII schematics, but you can still make out the circuit. anyway, all that circuitry was fine, up to the point where it went back to the top of the board to connect to UC9. the small trace connecting the via to the chip was broken. all the other traces running from UC9 were fine, but this particular one was not.

the trace goes under UC9. I don't have a hot air station, so I wasn't going to attempt desoldering it... I ran a tiny bodge wire. I gave it a try and... bong!

so, having fixed the audio output, things were looking pretty good...

---

...until the damn power supply started acting up again.

I started trying to diagnose that thing. did some more cleanup, but then it didn't turn on at all...

eventually I did something stupid and managed to blow a couple resistors. after replacing them, the fuse blew. blarg.

I looked deeper, found that two other resistors had gone bad, as well as the MOSFET, so I took these out. the resistors were easily replaced, but the replacement MOSFET I ordered never showed up.

in the meantime I did a more thorough search for cap gunk, and cleaned up all affected components I could find. any time heating a solder joint caused bubbling, I desoldered the component and cleaned it and its spot before putting it back. eventually all looked pretty clean... that bastardly cap gunk.

...

eventually I traveled to my parents' place for the holidays. being the incredibly stubborn person I am, I took the Mac's power supply with me.

there, I went through all the crap I had in my childhood bedroom, eventually finding 6 random switching power supplies. one of them had a 2SK2611 MOSFET, which is electrically an acceptable replacement for the Mac power supply's original 2SK1024. (note: I have a TDK power supply, so this might not apply to other brands)

I quickly salvaged the 2SK2611 and hodgepodged it into place. I replaced the blown fuse and tested the power supply, expecting it to blow up again... I was ready to just order a new power supply module if that one failed again.

but it didn't. it just kept clicking, like those do when they have no load. I connected the two LEDs I was using as a load for testing, but it was still clicking and the LEDs were going on and off. I thought that maybe these didn't put enough load, even though they worked fine before... I connected a random HDD. this time, it worked fine.

however, the 2SK2611 is a little too tall, meaning I couldn't put the power supply back together.

so I placed another order for a 2SK1024 MOSFET, which, this time did show up. I installed it and put the damn thing back together. it had been tested regularly throughout the vacation and worked absolutely fine, so I was excited to finally get the damn thing working and have fun with my Mac.

---

getting back home, I put the power supply back into the Mac, and...



lord, what a fucking saga.

anyway, I made an adapter cable to connect this to a modern LCD, which is working absolutely fine, and with glorious color. maybe I got lucky with this LCD, there are several reports of old Macs being finicky with LCDs.

I installed a 512K VRAM SIMM, so I could get 256-color display (which, for example, Blobbo requires for color graphics).

nicely, the RAM was already maxed out, at a whopping 10MB. there is also a FPU addon card, which I guess might be good to have?

I set up a scsi2sd to replace the dying HDD. the install was a bit of trouble, but now it's working absolutely fine.

the floppy drive is also working fine after a cleanup, and the eject mechanism is good, which is nice. I got a USB floppy drive so I can install games and shit on the Mac.

all of this has also inspired me to get other things to fix up, and maybe sell them. we'll see how that goes, I guess.

____________________
Kuribo64 - melonDS

I have some more things to post about, but, in the meantime, coming back to the LC.

y'know how I noticed, on my LC, that the audio ground wasn't connecting to the system ground...

I saw this video by Adrian Black, he's had the same problem with his Color Classic, and it turns out that the digital and analog grounds are just not connected on the motherboard on Macs.

looking at the LC, sure enough, when the motherboard is seated in the case, the back connector shrouds touch the RF shield, which the power supply case also touches, so the analog board does end up connecting to the ground... through the RF shield.

well, guess I learnt something there.

to verify this, I quickly cut the bodge wire I had done that connected the two grounds, and, sure enough, audio was still working fine. I'm going to completely remove that bodge to make things cleaner. this may also improve stability, I guess.

____________________
Kuribo64 - melonDS