Morning all. My Apple III just lost the -5V rail completely, all other voltages are present and fine. Anyone familiar with this type of fault?
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Looking at the power supply schematics, it looks like -5 is derived off the -12v rail using a 7905 regulator. If your -12 is fine, maybe the 7905 died?
I'd start by checking that the 7905 is getting -12V at its input, and then checking its output. That should be pretty easy to do with a meter. Meters are cheap, you can get an adequate one for under $20. Sometimes Harbor Freight has one on sale for around $10.
If the 7905 is outputting something near -5V, it almost has to be something between there and the output jack. I'd guess there might be some caps or something else in that vicinity...
https://www.apple3.org/Documents/Schematics/Power%20Supply.jpg
Maybe check the IN4003 diode ar CR14 and the IN5400 at CR15, as well as the caps at C21, C22 & C23.
Thanks guys I will check the inputs and outputs and report back. I have a good meter. As far as the caps Its certainly possible but I would be pretty surprised as I recapped it 2 years ago.. Unless another failed component blew one out. Ill check tomorrow when I have time. Thank you for the insight!
Unfortunately, a lot of components these days, especially caps are not always of the quality we'd like. So while you are probably right, I wouldn't make too many assumptions about a component not failing just because it is relatively new.
Right, thats why I said its certainly possible. "they dont makem like they used to"
So I think my system has a power supply from a III plus instead of a standard III. The standard model III Psu is model AA11190 but mine is AA1191B (its beige not green and has a date code of Feb 1983) The layout is a bit different. I was expecting a 7905 but there is a 2n6395
So I pulled out the PSU and tested it. The -5 voltage is there. So the problem is the main board. Something must be shorting out the voltage from the psu input to the board to the rest of the board. Its worse than I thought. Seems the psu is fine.
Ok so after tearing everything apart and taking some readings. Well the culprit turned out to be the cable from the PSU to the mainboard. Apparently it has at least 3 breaks in it. I confirmed by flexing while taking readings. There is evidence that this thing was badly pinched for years against the RAM board. Well I guess thats a relief but at the same time since its not being opened or moved how did it just go bad overnight? Anyone know where I can buy another one of these cables?
Could be heat related or impact. If the machine was moved or dropped (common for ///) it may have been just what it took to let loose.
You may need to fabricate a new cable. Depending on the wires and connectors you may just be able to replace the wires that are bad if you can uncrimp and recrimp connectors. I haven't had a /// open in a while, can you post a pic of the bad part?
IMG_20210907_135127.jpg
IMG_20210907_135142.jpg
The psu board where the connect is is fine. no scorching and the solder joints are fine. The problem only exists on the cable.
Looks to be an IDC (insulation displacement connector) such as the ubiquitous MTA156
You can crimp them with a pair of channelock pliers.
Unfortunately it is going to be nearly impossible to find a new cable like that. And you don't see Apple /// units being parted out all that often, so finding a used one may be difficult as well.
Your only options are to try to rebuild that one, if the connectors aren't so badly damaged they can't be taken apart, cleaned and re-used. Or you'll have to make a new one from scratch.
I'm not sure how hard those connectors will be to find. They might have been made by Molex. No idea if those are still made. Probably not likely.
I skimmed through recent Molex and Amphenol catalogs and didn't see anything that looked like a match. But it was far from an exhaustive search, so you may have better luck.
Heat has done a number to thiese connectors and they are brittle, as seen by the scorch marks. To be honest I tried using various pliers in the past with these type of connects and I couldnt make them mate up so whatever tool is need is the crucial to making a good connection.