68kMLA Classic Interface

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LCIII Repair
Posted by: 3lectr1cPPC on 2023-11-11 17:35:55 Got myself in an LC III today. Needs a recap but hopefully that’s all. Board looks great. Caps leaked a bit but it’s all good after a clean. I test booted and it chimed (surprisingly), and began to boot off the HDD. The RAM wasn’t in properly though so I was booting with just the soldered RAM and it didn’t have enough to boot. Strangely, ADB appears dead. No mouse or keyboard function. After cleaning the board again, it no longer starts. No chime, no video. PSU works great, tested 5 and 12v rails and they’re spot on. PSU is Astec. I threw a working PRAM batt in too and reseated the ROM I know of course that it needs new caps, and that’s step one. But I do find it strange that it would die suddenly like this, when it was just working and even still chiming. Does anyone know any other potential cause of that besides the caps? Will update again once I’ve recapped the board.
Posted by: Nixontheknight on 2023-11-11 17:36:45 Got myself in an LC III today. Needs a recap but hopefully that’s all. Board looks great. Caps leaked a bit but it’s all good after a clean. I test booted and it chimed (surprisingly), and began to boot off the HDD. The RAM wasn’t in properly though so I was booting with just the soldered RAM and it didn’t have enough to boot. Strangely, ADB appears dead. No mouse or keyboard function. After cleaning the board again, it no longer starts. No chime, no video. PSU works great, tested 5 and 12v rails and they’re spot on. PSU is Astec. I threw a working PRAM batt in too and reseated the ROM I know of course that it needs new caps, and that’s step one. But I do find it strange that it would die suddenly like this, when it was just working and even still chiming. Does anyone know any other potential cause of that besides the caps? Will update again once I’ve recapped the board. View attachment 65033 Did you recap the power supply?
Posted by: 3lectr1cPPC on 2023-11-11 17:39:04 PSU is fine, rails tested at 11.99 and 5.02. Haven’t tested -5V though. I believe the Astec is the better of the two PSU brands for these, the other one is full of ELNA caps. I think I’m gonna leave the PSU alone unless it dies.
Posted by: 3lectr1cPPC on 2023-11-11 18:05:11 I just look my first L with the twist method. Not sure exactly what happened, but now I have to bodge to what’s left of the right pad, the resistor on the left, and best of all, that tiny via. Fun days!
Posted by: bigmessowires on 2023-11-11 18:13:30 Check the schematics - that via probably goes to GND, so you can just run a wire to another GND pin and ignore the via.
Posted by: LaPorta on 2023-11-11 18:15:45 Very nice find....hope things go smoothly after the little hiccup.
Posted by: 3lectr1cPPC on 2023-11-11 18:16:39 Via does not appear to go to ground. And yes, nice find indeed. Case is in great shape. Will post pics when it’s done… just gotta get a wire to that via.
Posted by: 3lectr1cPPC on 2023-11-11 18:27:09 Behold! My ingenious bodge.
Posted by: LaPorta on 2023-11-11 18:38:40 Sometimes.....whatever you need to make it work.
Posted by: 3lectr1cPPC on 2023-11-11 18:47:42 Dang these were some fragile pads, or maybe I went too quickly. Turns out this one broke too. Luckily some of it is still intact so I’ll just solder to that… so dang. Might have to reconsider my methods if this keeps happening. But it hasn’t before, so I don’t know… Neither of these are fatal fails, but still makes this more of a pain.
Posted by: 3lectr1cPPC on 2023-11-11 19:05:07 Board is done. Time to test!
Posted by: 3lectr1cPPC on 2023-11-11 19:12:13 Alright great. Recap worked and we have a reliably booting system again! Not bad for a night of work. The bad news: ADB is dead. What should I check here? Edit: mouse works when plugged direct to the back. Gonna do a sanity check to rule out that the pass through ADB port on my AEK went bad and report back. Edit 2: just a bad connection, we’re fine now after reseating everything.
Posted by: bigmessowires on 2023-11-11 19:28:59 Nice job! FYI the LCIII schematics are here if you need them: https://www.macintoshrepository.org...i-iici-iicx-iifx-iisi-lc-ii-iii-quadra-840av-
Posted by: LaPorta on 2023-11-11 19:37:11 Always so satisfying seeing that question mark disk after a night's worth of work.
Posted by: 3lectr1cPPC on 2023-11-11 19:40:14 It is satisfying indeed. I do however suspect that my issue lied elsewhere than the caps - to be clear, they were bad. They’ve leaked, and I needed to replace them of course, but it still doesn’t POST sometimes. Once I get it going it seems to run perfectly, but there may be a startup issue somewhere that’s intermittent. First suspect would be the EGRET, but it’s pretty clean. Not sure, but it doesn’t bother me too much unless it becomes all the time. Everything checks out well though. Floppy worked first try, but I’ll still go in and do a tuneup at some point. Boots fine off the hard drive too. Gonna clean install then post a final photo - not bad for a night’s work!
Posted by: 3lectr1cPPC on 2023-11-11 20:05:09 Alright so it’s really a struggle to get this thing to boot. I can get it to, but it takes a while. The issues feel exactly like weak caps but the PSU seems completely stable so I’m not sure… It’s also seemingly VERY CLOSE to working at the times when it doesn’t. The CPU and other chips get warm.
Posted by: lobust on 2023-11-12 05:08:53 Alright so it’s really a struggle to get this thing to boot. I can get it to, but it takes a while. The issues feel exactly like weak caps but the PSU seems completely stable so I’m not sure… It’s also seemingly VERY CLOSE to working at the times when it doesn’t. The CPU and other chips get warm. I assume you don't have another PSU to test, or you'd have done that already... I'd do the PSU. Stable voltages when it's powered up doesn't guarantee that it's turning on cleanly. Most digital systems* are somewhat sensitive to their various voltage busses going high in a somewhat synchronised and timely manner. Dead caps in the PSU can prevent this from happening and you need a scope to see it. * I lack the real world experience with macs to know if this is something that they are particularly sensitive to, or not...
Posted by: 3lectr1cPPC on 2023-11-12 08:35:52 I’m probably going to recap the PSU next just to rule that out. In any case, here it is on my stack of macs! It does seem to boot every time if it’s been off for a bit. Besides That one issue, everything works on it. I did almost screw it all up though. In troubleshooting, I reseated one of the two socketed DIP chips and didn’t realize there was an extra set of pins in the sockets. Reinstalled it wrong, and found out when I gave myself a nasty burn touching the top of the chip. It got HOT! I thought for sure I’d just killed it, but nope. Reseated it correctly and it started right up first try… not gonna do that again. It’s in great shape, I think I’m going to enjoy this one quite a lot. Nice little Mac.
Posted by: cgp on 2023-11-12 08:57:49 Board is done. Time to test! Be aware that you've installed the infamous C22 reversed. Since you've used an aluminum cap (as per original) you shouldn't see a problem -- but if you'd used a tantalum, you'd would have. A reversed-polarized tantalum here will sink a high current and cause the -5V line to droop (or worse).
Posted by: 3lectr1cPPC on 2023-11-12 08:59:32 How is it infamous? It’s installed the same way around in my reference photo I took pre-recap.
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