How to Identify C102 Capacitor Negative Pad on 1980s PCB with Damaged Traces?

Guys, 

Its been awhile since ive delt with electronics. It's really been about 20 years. I'm getting back into it and need some help. 

What i'm doing here is just replacing some SMT capacitor's. Seems simple enough right. Well this circuit was made back in the 1980's. When taking off some of the capacitors, I really didnt have todo anything. They were just falling off with the slightest touch. Along with the capacitors, came the metal that connects it to the circuit board. So now there are a couple spots where no soldier will stick. There are a couple place where I cant trace where it would connect in the circuit because the board was messed up. I have included photos. 

Looking at the photo. I need to put a capacitor in C102 ... But where is the negative going?

A crop from the schematic is also included below.

So Looking at the crop of the schematic, I want to make sure im reading this right. So coming off the chip 380-1B at pin 20. That is the positive. the path then splits and part goes to FB5 and the other goes to the capacitor C102 (16V 10uF). The negative of the capacitor just goes to a chasis ground. So I can really tact that to any ground. 

Correct?

Once again sorry for all the newbie questions. 

Trying to attach photos

Hi,

Well it would be very helpful if you have a copy of the layout of the PCB. But from the looks of it, the larger one with the leads coming off is the negative part. There is the slightest possibility that there is a discontinuity in the traces. You better check it out.

Yes, in your photo of the board, C102's right pad is clearly the negative side. The white plus symbol just above the upper-left corner of the box drawn around C102 tells us that the positive terminal is on that side.

The ground trace above the capacitor looks damaged. I suggest you take small flat screwdriver or a grinding tool and carefully scrape away the green lacquer to see if the copper underneath is damaged. When you expose copper areas on your board like this, they can corrode more easily, so you should always tin them over with solder.

Hi Vic,

Based from the image, the right pad does look like ground. But to be 100% sure, why not try running a continuity test on the other pad and check if it connects to ground? I've seen boards with white plus symbols on them that don't necessarily connect to anything. Plus, you'd be insured that the other pad wasn't accidentally shorted to ground while trying to remove the previously mounted cap.

Also, distancing the ground from the circuit/pad can potentially increase or decrease the voltage it is regulating (specially when its a switching regulator) or shift the poles (if its an LDO), so I also suggest monitoring the voltage after mounting the capacitor if it is still the same.

Hi Vic Williams,

It is very easy to find the polarity position of the cap in your pcb, connect your
multi meter to dc range and connect the negative probe of the meter to the negative
of the power supply (usually gnd ) and check the positive voltage on both the points
of the solder points where the cap was fixed confirm which of the point is getting
more positive voltage ,connect the positive terminal of the cap to that point . It is
always better to replace old caps by new ones to improve the performance of the
circuit

Pl don t forget to review my rating. Good luck.



Regards, V.Sambath kumar