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North Star Horizon power

daver2 said:
No problem. If it was simple, everyone would be doing it and there would be no reason for us to be here...

The short circuit between the +/-16V rails would certainly have driven the switched mode power supplies daft! It was a good job they have overload protection! You could have done some real damage there...

R32 reading 0.4 Ohms instead of 330 Ohms is (more than likely) a red herring I am afraid. All it probably indicates is a short circuit elsewhere between -16V and GND. Keep looking - you have done an excellent job so far and you are close to the finish line to getting this beast running again...

I suspect - with the short circuit between the +/- 16V rails - you may have inadvertently reverse-biased some of those tantalum bead capacitors. If (say) the -15V supply shutdown, this could put +15V across some of the capacitors (relative to GND) that are on the -16V line. My recommendation would be to recheck the pesky capacitors again. I am also not sure what this would do to the voltage regulators either.

Dave
Click to expand...
You and Dwight were correct. I pulled the 79L12 and checked it by itself; with -12V on the input, I was only getting about -5V on the output. I pulled C1 and C2, and they were both bad. I replaced all those and found no shorts on the -12V rail. I powered everything up and all three power rails are working properly. The Vcc and -Vcc on the 1488 ICs are now reading correctly, there is +5V and +12V to the disk drive power connectors, and card connector pin 52 is now reading -16V. Everything looks good at this point.

Now I'll have to troubleshoot the cards. I'm hoping the Compupro CPU-Z and RAM-17 are okay. They only use the +8V rail. The Morrow Designs DJ/DMA fdc uses all three rails, so there is the possibility of problems there
 
Great news, well done!

It’s good when a plan comes together!

So getting annoyed with the power supplies and buying another switch mode supply would not have helped.

If you had the linear supply connected, I could see some more PCB damage could have occurred as a result of the short circuit(s).

Onwards and upwards...

Dave
 
Traces don't go away unless there's an over current event. I agree that there's still a short on the minus side. Easy test to so pull up one leg of the 330 ohm resistor if you want 100% proof. Reversed voltage tantalum cap's don't always go bang and let the magic smoke out, sometimes they just short out. I also notice that the positive power must be connected to ground somewhere since you measured millivolts to ground. Make 100% sure that the general purpose supply you are using as the negative supply isn't connected to ground with its negative lead. This lead has to be used as -16 volts for the motherboard. If it is tied to ground then you will short that supply.
 
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