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IBM 5150 motherboard Capacitor Question

M

Mochatea396

Experienced Member
Joined
Feb 9, 2021
Messages
106
Location
Hudson Valley, New York, USA
Hello members,
I have a 5150 that I am in the process of replacing the tantilum capacitors on the motherboard. These are three legged caps. 13 in total. 11 prove the outer two holes in the board are ground leaving the center positive. Except 2. There are 2 caps up by the power input next to the LS30 that the center post shows in the ground circuit. Why is this on only these 2? This is a working board and all the caps removed have the same markings and all test good.
Could someone explain to me why this is? Crossing polarity doesn’t make sense to me.
Thanks in advance
Randy
 
Thanks for the reply. The thought of the negative voltages did cross my mind. Here’s a pic of the area. You see 4 C7 locations. The two in the center are the ones in question. If you notice the one at the bottom of the LS30 the two outer holes which should go to the ground circuit go to a trace that goes to the third terminal on the harness. I checked it for continuity. A white wire plugs into that terminal from the PSU. That’s the only trace I can see connected to any of the holes. They must be hidden. Not on top or bottom.
Thanks
 

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Yes I did. And there is continuity. Is the white wire a negative voltage? I don’t see how anything could be malfunctioning since the board is working. I’m just trying to figure out why this is. I’ll be replacing the three legged caps with 2 legged and keeping the positive leg in the center as the other 11 read as I expected. Then just put the neg into either neg hole. I’ll just follow the same on the one in question.
Thanks
 
Ah yes,
Memory needed refreshing. The 2 legged caps went on just as I mentioned and the ones in question being opposite polarity, your diagram answered that for me. Chuck was right on....negative voltage is present. I stayed consistent mounting them like all the others and the board boots perfectly.
 
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