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Silver mica caps not testing as labeled
Posted: Tue 02/06/24 2:30 pm
by marty1776
Hi, I used this site almost 20 years ago to build an 18W tmb. I decided to look at it again because I wanted to return it to original tremolo build. I noticed my new silver mica caps do not test as labeled. For instance, a cap labeled “10” tests about 430pF on my dmm. These have never had electricity running through them. They all test higher than labeled. My question is, can you not test silver mica caps as you do film or electrolytics? Thank you
Re: Silver mica caps not testing as labeled
Posted: Wed 02/07/24 11:44 am
by JMPGuitars
Silver mica caps test as normal. The real question is what the 10 means. It's possible the 10 means 10%, not the value of the cap. If it's a 470pF cap, then 430pF is within 10% tolerance.
Thanks,
Josh
Re: Silver mica caps not testing as labeled
Posted: Wed 02/07/24 2:06 pm
by marty1776
I’ve included a pic. My meter should read down to 10pF. If you click on pic it turns right side up. Cap says 500 then tolerance.
Re: Silver mica caps not testing as labeled
Posted: Wed 02/07/24 6:09 pm
by JMPGuitars
Oh, I don't know what you meant regarding the labeled (10) thing.
Anyway, 0.93nF is 930pF. That cap is either bad, or your meter isn't great at capacitance (it's highly likely the meter's fault). DMMs aren't awesome at capacitance. LCR meters are better, especially when you have frequency choices.
Testing a 50pF and a 250pF cap on DMM and LCR for comparison:
lcrvsdmm1.jpg
lcrvsdmm2.jpg
You can see that the DMM is not so accurate compared to the LCR.
Thanks,
Josh
Re: Silver mica caps not testing as labeled
Posted: Wed 02/07/24 8:20 pm
by marty1776
Josh, the bottom of my range is supposed to be 10pF. Maybe so close just not accurate. I’ll just use as marked. I trust them more than I do me. Thanks for the input!
Re: Silver mica caps not testing as labeled
Posted: Wed 02/07/24 10:10 pm
by JMPGuitars
marty1776 wrote: ↑Wed 02/07/24 8:20 pm
Josh, the bottom of my range is supposed to be 10pF. Maybe so close just not accurate. I’ll just use as marked. I trust them more than I do me. Thanks for the input!
You're welcome! If you want a decent LCR that's not crazy expensive, check out the DER EE DE-5000 (or the knockoffs of it). That's a pretty standard LCR for people that don't want a crazy bench meter. I have one in drawers with lots of other meters.
Actually, if you like tweezers, the Shannon ST42 LCR tweezers are great (those actually stay on my bench). Just contact them for pricing. Their pricing is weird on aliexpress sometimes.
