VERAULT
Veteran Member
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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Apple III just lost the -5V rail completely, all other voltages fine
- Thread starter VERAULT
- Start date
GiGaBiTe
Veteran Member
I'd say start with the basics in checking for shorts to ground on the logic board with the PSU disconnected.
daver2
10k Member
Assuming this https://www.apple3.org/Documents/Sch...r%20Supply.jpg is your power supply, proceed as follows:
Do you have -12V available (you state you do in the title)? If so, this feeds the 7905 regulator U1.
The regulator U1 could be faulty - or the output of U1 could be shorted out.
I would check (in order of likelihood): C22, CR15 and C23 for a short circuit.
After that, it could be any of the decoupling capacitors on the -5V rail - or any of the components that utilise the -5V supply (mainly 4116 DRAM in my experience).
Dave
Do you have -12V available (you state you do in the title)? If so, this feeds the 7905 regulator U1.
The regulator U1 could be faulty - or the output of U1 could be shorted out.
I would check (in order of likelihood): C22, CR15 and C23 for a short circuit.
After that, it could be any of the decoupling capacitors on the -5V rail - or any of the components that utilise the -5V supply (mainly 4116 DRAM in my experience).
Dave
VERAULT
Veteran Member
I didnt know the -5v rail was made from the -12v rail. So I will check the input and output of the 7905 and see if its shot. Yes Dave the -12 is present and good. I will also check the other components you mentioned.
VERAULT
Veteran 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?
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?
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GiGaBiTe
Veteran Member
Cable that old is likely unobtanium, unless Apple used a standard board to board connector that is still available 38 years later. I would say as a temp fix, just chop the bad part of the wire out and solder on a section to make up the length needed back to the PSU.
As for it going bad overnight, you said yourself that isn't the case. If it was "badly pinched" for who knows how long, the copper strands were on borrowed time. There could have been a microns thick connection between two pieces of kinked copper that decided to suddenly let go.
As for it going bad overnight, you said yourself that isn't the case. If it was "badly pinched" for who knows how long, the copper strands were on borrowed time. There could have been a microns thick connection between two pieces of kinked copper that decided to suddenly let go.
VERAULT
Veteran Member
Yeah it seems that is what happened. I reached out to VintageMicros on eBay. He seems to be the only guy with Apple III parts and LISA parts. He has a cable for $23.00 so I bought it. Ill keep the broken one for a down the road attempt at repair.