68kMLA Classic Interface
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| Click here to select a new forum. | | Mac II refurbishment.. | Posted by: buserror on 2020-07-16 01:09:53
So well, not so good as it turns out. As soon as I slotted the batteries in I heard a little "click" noise from the part of the board near the power button or thereabout. Removed the batteries of course...
I got investigating, and there is a short on the main rail! Almost everywhere... that annoyed me a lot as I was very careful but then again... small mistake?
So I started to remove all the caps I replaced (well the SMT ones, I can't see the bigger ones being a problem!) and test until none was left and... no luck, still shorting.
So I think I'm going to also remove the 4 SOIC I installed, and then ----- grrrrr remove all the patch wires. And I still don't know if it will fix it, as I'm not entirely sure the short wasn't there in the first place! How *annoying*.
Next is to try to find the short, I got some freeze spray, and a bench supply of course, so I'll try to put 3V with increasing Amps into it and hope I can see something heat up.. unless someone has a better suggestion?
| Posted by: max1zzz on 2020-07-16 01:21:05
If your meter will read really low resistances you can measure the shorted rail in different area's or the board and the reading should get lower the closer to the shorted component you get. I haven't ever done this personally but have seen it done online a few times and it seems to work
Other than that what you have described should work in most circumstances (It's what I would usually do on modern boards) however it will not work on really really low resistance shorts (e.g solder bridges)
| Posted by: buserror on 2020-07-16 04:12:09
Thanks for the tip. I had a poke around; probe on the ground pins at the connector, and measured resistance at the + of all the SMT caps, radialy.
And yes /most/ of the board reads about 16 ohms -- give or take -- , but around the 74hcs I have replaced, it reads 3ohms on the VCC... I removed the SOICs, and my patches, and it still the same, so I suspect a short in the existing traces somehow. But it seems /pretty clear/ the short is around these parts... I wonder what it could be. tomorrow I'll look under the board and see if I can find another 3 ohm reading, if not, at least it localizes it at the top!
| Posted by: joshc on 2020-11-02 11:20:26
@buserror Did you get your II working in the end?
| Posted by: 360alaska on 2020-11-02 12:36:36
You can probably fix that board but if you can afford it I'd suggest finding an original board like this: https://www.ebay.com/itm/Very-Rare-Original-Macintosh-II-Motherboard-820-0163-03-Ships-Worldwide/193715932365?hash=item2d1a5e8ccd:g:WDIAAOSwi~9fT38u
These original boards use all Axial caps which almost never leak, even now. Furthermore, if you just want to test the board before replacing the batteries you can jump start it:
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