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Ceramic Disc Capacitor Testing


Kaizen

Question

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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I wouldn't lose any sleep over ceramic caps. They are generally very reliable and also almost never critical.

 

I choose bypass caps (the main use of the ones you are talking about) pretty much based on what is in stock at the time.

 

Usually any cap, 0.01 or 0.047 or 0.1 works perfectly well as a bypass cap in practically every application we are concerned about with game boards.

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I wouldn't lose any sleep over ceramic caps. They are generally very reliable and also almost never critical.

 

I choose bypass caps (the main use of the ones you are talking about) pretty much based on what is in stock at the time.

 

Usually any cap, 0.01 or 0.047 or 0.1 works perfectly well as a bypass cap in practically every application we are concerned about with game boards.

 

I always thought that losing one here or there wouldn't cause any major issues as I have plenty of 80's PCbs that have them missing/broken off here and there and it hasn't caused any issues.

I'm learning as I go and I've never paid much attention to them in the past like you suggest which is why I've never pulled one off a board and checked it in the past or taken the time to learn about them in detail.

Thanks for the clarification.

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To my understanding, the ceramics are only there to block AC transients, so if the DC is properly regulated & correctly working any ceramics out of tolerance really wouldn't make any difference, unless it's a dead short.

 

Sort of.

 

The bypass caps, usually a small ceramic or greencap across each TTL IC, supplies the IC with a surge of reserve power - when the IC gates switch, they can draw a lot of current for a very tiny time, the cap supplies this current to ensure the chip can switch cleanly without creating spikes on the supply rail.

 

So your comment about "blocking AC transients" is correct but not the full story. They don't "block" the transients, they prevent them.

 

In practice, there are so many caps scattered around the board (usually) that a few either missing, broken or out of tolerance here and there will make no practical difference.

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