• Hello, Guest! Welcome back, and be sure to check out this post for more info about the recent service interruption and migration.

Apple III just lost the -5V rail completely, all other voltages fine

ScutBoy

Well-known member
I know this is kind of admitting defeat, but get the ReactiveMicro replacement supply. So nice not to have to worry about power problems!
 

Verault

Well-known member
No way thats the easy way out and there is no troubleshooting involved which is most of the fun with old computers.

I have one because I got it in a trade but I use it for bench testing. I would much rather have an actual Apple III psu.

I have fixed 3 dead apple III psus in the past (this one included); I have never seen just the -5V rail go like this.

And besides, its got 3 of 4 voltages correct so its almost fully operational. We cant give up on these old things :)
 
Last edited:

Gorgonops

Moderator
Staff member
If you can lay hands on a schematic that'd be a good start, it's no doubt floating around out there...

Okay, it looks like progress on the Applefritter thread, I second chasing that regulator is a place to start. You're lucky the Apple III uses proper regulators; I've been digging into restoring a TRS-80 Model I lately and the description of the power supply in the manual is enough to make a grown man cry.

(TL;DR: Radio Shack built it out of discrete components instead of using those 78/79xx series regulators; the rumor is they went that way because it was actually marginally cheaper to do it that way despite needing a larger PCB and at Tandy cheaper was *always* better. The way that machine gets its -5 volts is almost criminally shortcut-y but I guess it's barely good enough for 4116 DRAM chips to suffer through.)
 

Verault

Well-known member
If you can lay hands on a schematic that'd be a good start, it's no doubt floating around out there...

Okay, it looks like progress on the Applefritter thread, I second chasing that regulator is a place to start. You're lucky the Apple III uses proper regulators; I've been digging into restoring a TRS-80 Model I lately and the description of the power supply in the manual is enough to make a grown man cry.

(TL;DR: Radio Shack built it out of discrete components instead of using those 78/79xx series regulators; the rumor is they went that way because it was actually marginally cheaper to do it that way despite needing a larger PCB and at Tandy cheaper was *always* better. The way that machine gets its -5 volts is almost criminally shortcut-y but I guess it's barely good enough for 4116 DRAM chips to suffer through.)
I really need to do something with both my Model 1's. I have never really even touched them in years. I know the model 1 monitor I ordered arrived completely damaged from eBay as noone knows how to properly ship a monitor. So I should probably rebuild the monitor soon as well. Good winter project.


Anyway it looks like the 7905 uses the -12v rail to make the -5V rail so hopefully thats it.
 

Verault

Well-known member
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.





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?
 
Last edited:

Verault

Well-known member
IMG_20210907_135127.jpg
IMG_20210907_135142.jpg

You can see heat has been an issue as there is scorching on some of the connectors. I am guessing as the strands were breaking and the resistance went way up they got pretty hot. The wires that connect to the ends are brittle and the shielding is cracked in a few places.
 

Verault

Well-known member
I have found them as well, but you need some kind of crimper to push the wires into the blades to make contact. I have tried screw driver tips, pliers. ALl the cables I have tried making with those ends came out terrible. So you NEED whatever crimper/tool is required.
 

Verault

Well-known member
I think the black thing at the top is just a cover that snaps on. Most IDC connectors dont come with those. Anyway I never had any luck making my own cables with IDC ends. The wires just dont stay put. That is why I assume the right tool for the job is needed.
 
Top