I took out the old capacitor and didn't remember to check the polarity because I assumed the board would have more standard markings. Is there any way to figure it out with a meter or by the printing?
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1\$\begingroup\$ Did C1 look like C3? If so, polarity doesn't matter. \$\endgroup\$– JREOct 18, 2018 at 19:39
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\$\begingroup\$ Probably non-polar. Otherwise it would be marked. \$\endgroup\$– Robert EndlOct 18, 2018 at 20:57
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2 Answers
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The pin on the left is connected to the large power plane, the next task is to figure out if it's positive or negative with respect to the other pin, since the HC125 is connected to it, it would be the ground plane, but if the board has multiple voltages, you may have dig a bit more. If the right pin is connected to pin 14 of the HC125 then it would be positive wrt the ground plane connection.
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\$\begingroup\$ Pin 14 is the top right pin on the HC125 btw, directly across from dimple (pin 1) indicator. \$\endgroup\$ Oct 18, 2018 at 19:35
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1\$\begingroup\$ You're about 90% of the way to solving it with your plane comment. We can clearly see that the left pin is connected to the top plane, and so is pin 7 of the IC, which is ground... Edit it in that pin 7 is ground and I'd say you have the winning answer, as I was too busy typing to see the plane connection. \$\endgroup\$ Oct 18, 2018 at 19:35
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\$\begingroup\$ At first I was, but I can't tell what the other pin is going to 99% sure it's going to be Vcc, but... A quick continuity between that pin and pin 14 would confirm it (check continuity both ways to make sure there's no diode in between). \$\endgroup\$ Oct 18, 2018 at 19:53
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\$\begingroup\$ If the board had a bipolar power supply, then yes, ground could be positive for that capacitor if the other side goes to a negative supply. But it probably doesn't. And that should be something easy to determine by looking at the totality of what is on the board and its supply connections. If I had to guess the big cap is in parallel with the little one. \$\endgroup\$ Oct 18, 2018 at 19:55
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1\$\begingroup\$ Pin 14 does indeed show continuity with the right pin of the cap. Interestingly, I found that some other boards of the machine (Marquee 9500 CRT projector) had the similar closed bracket marking as negative, which can be seen on the right pin. Thanks for the answers! \$\endgroup\$ Oct 18, 2018 at 20:37
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You cannot identify this from the board printing.
However you could look up the pin-out of the 74HC125AN and use a continuity testor on the powered-off board to figure out if either lead of the capacitor is connected to its ground or supply. Or to a labeled ground connection elsewhere on the board.
Continuity testors must only be used on unpowered systems, and unpowered means not only having no supply connected, but also not having any high voltage capacitors which retain charge (especially in legacy TVs, camera flash units, etc). That's probably not the case here but still needs to be said. Also using a continuity testor is not 100% safe - in theory the injected current can damage sensitive circuits but that is also relatively unlikely here.
You may also be able to trace connections to known power/ground pins by eye.
answered Oct 18, 2018 at 19:33
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\$\begingroup\$ I'll leave this here as a general solution but isdi's observation of the top surface plane makes this particular case quite easy to figure out by eye. \$\endgroup\$ Oct 18, 2018 at 19:39