Trying to check pF (silver mica) capacitors

[pullshocks]

It's Monday, and I was in the mood to head down a rabbit hole.

So I need a 120 pF capacitor for the SoLow Watt (C18 on the enclosed schematic, in the LTPI).  Don't have any on hand.  I have some marked 82, 100, 150.  I know the caps themselves have a nominal +/- 10% tolerance, so I thought 100 +10% or 150-10% would get me pretty close.

My not-a-Fluke  DMM seems to line up with the marked value of the .01 uF film caps, but as you get down into the pF range, the readings deviate substantially.  But I do notice that the 100 pF caps I have are all read pretty close to eachother.  I'll use the one that reads 154, but it is probably not meaningfully closer to the 120pF value in the schematic.

Marked value          DMM reading    Deviation
.01 uF (film)           10.01 nF             <1%
.01 uF (film)           10.08 nF             <1%
.01 uF (film)           10.11 nF             ~1%
.01 uF (film)           10.08 nF             <1%
.01 uF (film)           10.01 nF             <1%
220 pF (SM)             .275 nF             25%
150 pF  (SM)            .212 nF             41%
100 pF (SM)             .152 nF             52%
100 pF (SM)             .153 nF             53%
100 pF (SM)             .154 nF             54%
100 pF (SM)             .140 nF             40%
82  pF (SM)             .131 nF             52%
82  pF (SM)             .133 nF             62%

[dwinstonwood]

I'm pretty sure that's what is sometimes called the Marshall Fizz cap or snubber cap. I Googled this a while back, and I don't think the value is all that critical.

https://el34world.com/Forum/index.php?topic=15816.0

[shooter]

my actual fluke gets a little cranky trying to measure <100pF

I don't think the value is all that critical.

+1
I typically just used a couple short gators and swapped caps til the guitarist smiled, solder be done

[jordan86]

Yup. I’d gator clip in and try them all: 82,100,150, and 220pf. Hit the standby off before swapping.

[sluckey]

Was there a question?

You should not expect your meter to accurately read anything lower than 4nF (4000pF).

[pullshocks]

Even though I did not ask my question clearly, you guys answered it….what a great forum.
Thank you

[sluckey]

The lowest range on my Fluke 87V is 10nF.

There are some cheap capacitance meters that go really low. I just trust the value that's stamped on the cap.   

[pullshocks]

... I just trust the value that's stamped on the cap.   

This little exercise makes me inclined to do the same.  Sometime there might be a question if  a cap has drifted, but even though my meter would not read **accurately** I could compare the suspect cap to a new cap.

[shooter]

If you're a picker and a builder, "swapping in parts" beat's the calculators n meters to "get you there"
Then
break out the meters, books n notes when your fingers are to sore to play

[pullshocks]

If you're a picker and a builder, "swapping in parts" beat's the calculators n meters to "get you there"
Then
break out the meters, books n notes when your fingers are to sore to play

In principle I agree. But when your hearing is as damaged as mine, you feel less confident about tweaking by ear, and more apt to rely on what the designer and previous builders did, and instruments.


I used to be a hi-fi speaker building enthusiast.  One day I got to borrow an octave band sound level meter (this was before smart phone apps).  For the higher frequencies, the meter was moving, but I could not hear the sound.  A combination of heredity (my dad had bad hearing and was never exposed to noise), my brief music career, woodworking, etc.  My right ear was finished off by a virus.  Hearing aids help, but I do not have the golden ears anymore...

[Willabe]

Very sorry to know about this pullshocks.

[PRR]

The meter and LEADS have capacitance. Unless you have a Zero/Tare feature, it will always read high by some pFd. Note the reading with leads laying on bench. And in various positions/tangles. And on metal or wood. I suspect you will find 20pFd-80pFd "depending".

Then, like shooting cross-wind or playing with a tripping drummer, you aim a bit away from what you want. You want a 150, you look for "190".

EDIT -- ah, your errors are far more than a few extra pFd. Someone screwed-up making that meter. Which is odd, because Exetech is historically OK, better than junk.