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Ceramic Disc Capacitor Testing
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Question
Kaizen
I've been going over the 4 Player Pong PCB and there was a 100nF (0.1/104) ceramic cap that was physically damaged so I've replaced it, it tested 'Unkown or Damaged' on my tester. I was doing some probing around the board and noticed a low on both sides of another and removed it and it tested 'Unknown or Damaged' so I replaced that one as well. Out of interest I pulled another random one and it read 68nF. After pulling a few more they all read between 65-72nF on both my cap tester and capacitance range on one of my multimeters. I also found another that also tested faulty on both the tester and multimeter. I've never really had issues with them on most of the '80's boards I've worked on so I don't have much experience on knowing when to determine if one is faulty and should be replaced.
I put the 2 brand new caps I got from Jaycar in the tester and they both read 100nF so the tester seems accurate enough.
Though a cap of lower value may still function in the circuit could it possibly cause issues?
The caps are 43 years old and I'm guessing they are drying up inside.
What I'd like to know specifically is at what value should I determine that the cap is faulty and replace it?
My understanding is that if it's a 100nF with a 5% tolerance then anything under 95nF should be determined as faulty.
I'm not a fan of blanket re-capping but if there's a valid reason to do so then I'm happy to replace them all but I don't wan't to go causing further problems with the board by doing so.
Most of them are the decoupling caps on the +5v rail but one of the faulty ones was feeding an input to a 74LS00 which affected the output.
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