08-30-2009, 08:50 PM
g8wayg8r Wrote:I haven't paid much attention to the low capacitance mica caps since they read the same as the replacements. Both read high using the cheap multimeter I've got. I've notice that the the old paper caps almost always read higher than spec. Not sure why the new and old micas would do that unless it's a systematic fault with the meter.
Coupla different things here.
On small value caps the leads from your tester can account for at least several pf of difference. Thats substantial when you get down to values like 25pf. Keep that in mind and go accordingly. Mica caps don't typically change in value unless they go way out of whack.
On the inexpensive meters you'll always get a high reading with leaky caps. Thats because they measure using the "time-constant" of the cap under test. T(time) = C(capacitance) times R(resistance). Once you add R into the equation (good caps don't have R) it bolloxes the readings. You can proof this on your meter by measuring a fresh yellow cap and then adding 470k or 1M of resistance across the capacitor under test. Thats a typical amount of leakage (DC resistance) found in old paper caps.
Not all meters behave this way. Just by accident I bought a new no-name Chinese do-it-all meter and it seems rather impervious to leakage. I don't know why? It doesn't have a brand name so I can't hardly go look up the specs My old meter did as you describe and I developed the mindset that if the cap doesn't read correctly then its bad for whatever reason.
The place in Canada for caps is http://www.justradios.com
Pay the extra buck or two for Expedited Shipping.
>>>quote: I also use just a small screwdriver to whack off melted globs of molten solder,
I always keep several x-acto knives around the bench. Whichever is in the worst condition at the time gets used for desoldering. Don't let the blade heat up. Makes a fine pick and is good for drawing off those solder globs.
-Bill