I've barely started with the repair process on this system--I'm mainly just documenting what I've done for future reference.
In June I bought a Mac II on eBay. It was sold as non-working, so I knew it'd be a project machine. I opened it up, and it turns out it has a IIx logic board. It seems to be some sort of prerelease version of the logic board. Instead of "Macintosh IIx", it says "Seed1" on the silkscreen. On other forums, I've seen that max1zzz also has a logic board like this one. Sounds like it was probably a developer seed system. Some of the silkscreen component numbering seems to closer to the Mac II based on various examples I've found online. It seems to have a standard IIx ROM SIMM, which I dumped and confirmed.
The electrolytic caps on it had leaked terribly. I removed them, cleaned around them, and replaced them all with tantalum caps. I also removed the two soldered batteries and replaced them with holders (stretched the legs to make the holders fit) -- right before tt came out with his awesome battery adapter board clone. Oh well
Unfortunately, the machine still doesn't boot. The power-on circuitry doesn't work (yes, I put good batteries in). If I jump-start it, it still doesn't work. It's definitely not surprising to me considering all of the corrosion I'm seeing on the ICs and traces nearby the capacitors:
I've already identified some traces with breaks that will require some wires to be run, and it's pretty clear from the pictures that the SCSI chip needs some work. Ignore the pencil eraser -- those are remnants from me cleaning off the capacitor pads, which were also terribly corroded. I'm hoping I can bring this poor logic board back to life! Luckily, everything with corrosion seems to be off-the-shelf parts that can be found easily, and the Apple ASICs all seem to be in good shape. I think I'm going to remove all the ICs near the capacitors, clean their pads and their legs, and verify their traces before repopulating them. I don't think the NCR 53C80 SCSI chip is worth trying to clean when I can find them on eBay for $5 either...
Out of interest, whats the board doing when you try and jump start it? Mine would come on for about 5 seconds (with no chime and nothing onscreen) then die. The root of this was corroded solder joints on UC7 and UC8 (assuming the silkscreen on your board is the same as mine they are the two chips by the NuBus slots next to the top battery), After cleaning and re soldering these chips it fired up just fine.
Thanks guys! Yeah, the IIx is going to be a cool machine if I can get it up and running.
techknight: yeah for sure UB8 is in terrible condition. Like I said in my post, I'm not even going to try to clean the chip itself up. It's just not worth the time it would take. I ordered a new one on eBay instead. It's the SCSI chip. There is some visible gooey stuff in the other pictures I posted too.
max1zzz: Glad you found this thread! What I found is that with the jump start, the power supply came on, but I didn't hear a chime and it didn't feel like anything was happening. I didn't have a video card in when I did that, but I'd assume it should have at least chimed. I didn't leave it on long enough to tell if it was powering off after 5 seconds. Yeah, UC7 and UC8 are right near a capacitor. It looks like it definitely leaked, considering the top text is faded on UC7. I'll definitely remove and clean those two chips, because it does appear that there could be damage in that area. Hopefully I won't need to find new ones...they appear to be a 74ALS651, which doesn't appear to be readily available. Luckily if there is any corrosion, I don't think it's very bad on them.
Thanks for the tip on that trace! I just checked and it still has continuity on mine.
No real update on bringing it back to life yet, but I did remove, clean up, and resolder UA15 and UB15, which are both near capacitors that leaked. In the "before" picture, you can see that the downward-facing half of each chip looks a little gunky. I thought it would be fun to show my technique for removing surface mount chips, especially ones that are right next to plastic parts (the RAM sockets in this case).
I covered the surrounding area and components with Kapton tape, then covered the tape with aluminum foil tape, especially ensuring all nearby plastic was protected. The Kapton tape helps with heat resistance and also makes it easier to remove the foil tape, which tends to really want to stick down. Then I used hot air to remove the chips. I checked all the traces underneath to confirm that they're still good. I soldered the chips back down by hand when I was finished--looks much better!
Now I just have to repeat this process several more times...
LOL, believe me BMOW...my setup is nothing special!
bbraun, you're right...I skipped a step in my description. To get the clean shiny pads, I used desoldering wick. Works like a charm! On really nasty corroded pads, I've had to use a pencil eraser before. That SCSI chip's pads may require something like that. We'll see...
Edit: Oh, and I used 99% isopropyl alcohol to clean the pads after using the wick. I put on a little bit of flux when I used the wick too.
I just repaired a couple Mac II boards for uniserver yesterday, they both had broken traces and corroded stuffs as well. Luckily though only a few trace patches brought them back alive, It seems that 90% of the problems occur at the soft start circuitry just above the nubus slots.
Heh, I'll bet! I can totally see how the soft start circuitry by the NuBus slots would be a common problem area. I found a broken trace under one of the NAND chips and a broken trace by the heavily corroded SCSI chip. I'll fix those traces after I replace all these chips with brand new ones. Getting closer...here's a "before and after" of how I cleaned up underneath the latest chips. I'm finding that a pencil eraser works pretty well for removing the really nasty corrosion. Of course, for the non-corroded pads, desoldering wick works fine. I only use the eraser if the desoldering wick won't do its job.
UB1, pin 6 has lost its connection to all the other pins on UB1. The pin on UB8 just under the "8" in silkscreen has also lost its connection. I can see several other traces that I'm surprised still have a good connection...
The IIx is a beast. It has so many caps and some of the through hole ones have a bit of heatsinking where they can be difficult to get the legs out of their holes. I have one that has the soft-start issue, but I can jump-start it and otherwise seems OK. I was able to use your method to shield the components, but used regular aluminum foil taped down with Kapton tape.
Yeah, I had some trouble getting those through hole caps off too. Good to see you on here tt! It's been a while.
Today the replacement NAND gates arrived. I ended up having to make several repairs to some traces that lost continuity. There's definitely some damage to the traces on this board, and I have no doubt there are still some damaged traces I haven't found yet. I went all the way through Gamba's Mac II/IIx startup circuit diagram and all of my connections in the startup circuit are solid (I found one broken trace on capacitor C5 that I had missed). A lot of the traces in the startup circuit had gone bad, as you can see by my patchwork:
Unfortunately, this thing still will not power on! Everything in the startup circuit has continuity just like the diagram shows, although I can't rule out unwanted shorts. With two batteries in it, it won't turn on. If I jump-start it, the power supply will turn on, but no chime or anything. I'd like to focus on getting the startup circuit working first before I bother with why I don't get a chime though.
Here's the last clue I found tonight before I'm going to bed: when I stick the batteries in, B2 ends up with a negative voltage across it, which makes no sense to me. Something really goofy is going on, and I don't know what it is. All the batteries are in the correct orientation. Why would B2 have a negative voltage across it? The negative side of B2 is ground, so I have no idea how a voltage below ground is appearing on its positive side. I must have a short somewhere...right?
So...the power circuit is working correctly when I have the power supply disconnected. I can see the PFW pin go high when I press the power button. I've traced the behavior of Q3 and all the NAND gates on UB2. They are all doing what they should be doing.
As soon as I hook up the power supply to the logic board, I start seeing weird results like one of the batteries having a negative voltage (-0.5V to -0.7V or so). Also, it's the same battery doing the weird negative voltage thing, regardless of whether it's in B1 or B2. It's starting to look like one of my batteries is old/bad, so I'm going to start out with two brand new fresh batteries before I do any more testing of the power circuit.
The good news is that after jump-starting the machine, pressing the power button turns off the power supply...so the power off portion of the circuit with the SR latch and inverter (made up of all the NAND gates on UB1 that I had to heavily patch) is working!