[NJARC] Mega 328 Tester

Wed Dec 30 12:55:01 EST 2015

DaveESR stands for Equivalent Series Resistance so this phantom resistance is in series with the capacitor and is a small resistance when compared with leakage resistance.  Leakage resistance is in parallel with the capacitor and is usually quite high.  When leakage resistance is in the multi megohm range, it tends to be ignored much as ESR is ignored when it is below about 10 ohms or so.  It is tough to measure leakage current or ESR with an ordinary ohmmeter.  One usually needs quite a high voltage to measure leakage current and special techniques to measure ESR.
The exceptions to the above are when a capacitor is used in, for example, a high impedance circuit like a radio's AGC circuit and then even moderate leakage current plays havoc with the grid bias of the RF and IF tubes or when one is using a capacitor where very high currents are present so the ESR drops an appreciable voltage and reduces what is available to the rest of the circuit.
On consumer radios, I have no problem shot-gunning all capacitors since the PITA is getting the chassis out of the receiver and there are not that many caps to really worry about.  On commercial / military radios for example, I disconnect the filter capacitor and put near the rated voltage on it but that voltage is in series with a power resistor of a high enough value so as to limit the short circuit current to about 10 mA and a high enough wattage so the resistor only gets warm to the touch so my fingers don't blister!  
Once the capacitor reforms, and they always seem to reform since they are of higher quality then those in consumer radios, it is an easy matter to measure the voltage across the resistor with a very high impedance volt meter (DVM does nicely) and divide that value by the series resistor value.  This gives you the leakage current for that particular capacitor.  Sometimes the reforming goes on for several hours and once in a while, over night.  Other times, the leakage current falls rapidly and things are good to go in less then an hour.  It all depends on how long the capacitor sat uncharged and the quality of that capacitor.
Coupling and by-pass capacitors are always suspect in all radios.  I usually disconnect several and do the above leakage test.  If they pass with flying colors, I move on to the smoke test, otherwise I replace all.
All of this is done so the power transformer isn't stressed.  Power transformers are usually the most expensive single component and the hardest to find replacements for.  This is also a good reason to not put too high a value filter capacitor in as a replacement for the original.  A larger value capacitor will have higher charging current which will cause the power transformer to get much hotter then normal due to the transformer's winding resistance and the higher charging current.  Not to mention that the higher charging current will stress the vacuum tube rectifier too.Jim

      From: David Sica <dave.sica at njarc.org>
 To: Aaron Hunter <ahunter01 at comcast.net> 
Cc: NJARC <njarc at mailman.qth.net>
 Sent: Wednesday, December 30, 2015 10:40 AM
 Subject: Re: [NJARC] Mega 328 Tester

Tester purports to test ESR (although using an unspecified frequency and presumably a low voltage. I'd need to be a 'real engineer' to have an idea of how that might influence things.)  But assuming there's a correlation between ESR and leakage current (and maybe there's not?) I'd figure you'd get a bad ESR reading on a cap with damaged insulation.  (There's a lot I need to learn about ESR.) 

New Jersey Antique Radio Club
On Wed, Dec 30, 2015 at 11:31 AM, Aaron Hunter <ahunter01 at comcast.net> wrote:

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I hope everyone realizes when checking capacitors, only the value is checked, not the leakage current. 
I had some fairly new electrolytic capacitors that had dents in the cans.  I checked them with the checker and a good capacitance value was shown.  When I check them with my HeathKit capacitor checker, all dented capacitors had high leakage current.  The only one of the group to escape a dent tested good.

Aaron Hunter
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