Re-coating flybacks, what do you use?

[jshorva65]

I have to agree that replacing all paper capacitors is best unless the set is in unusually good condition and the originals are still in like-new condition, which is extremely rare. Disconnecting one side of each electrolytic capacitor and individually re-forming them with a variable DC power supply, 20 to 50K 5 watt series resistor and voltmeter will yield surprising success rates. As long as the capacitors are not open-circuited or shorted and the pressure seals on the cans are intact, not noticeably swollen or blown open, applying rated working voltage to each electrolytic through a 20 to 50K 5W series resistor shunted by a voltmeter normally re-forms the aluminum oxide dielectric inside the capacitors well after 6 to 8 hours of charge time. Voltage reading across the resistor multipled by the series resistor value will reflect the DC leakage current and ALL electrolytics (even new ones) have very small leakage current on the order of tens of microamperes. Anything above 50 to 100 uA (depending on capacitance value and working voltage) becomes questionable. Maximum acceptable leakage current in microamperes for any capacitor of known capacitance and working voltage may be calculated as one fifth of the sqare root of capacitance multiplied by working voltage. Multiply C in uF by V in volts, calculate square root of the result and divide by 5. As an example, a 100uF 450V electrolytic may draw maximum DC leakage current of 42uA. Voltage drop across a 20K series resistor with 450V applied across the circuit of 20K resistor and 100uF capacitor will be 0.84 volts.

The expression 1/5*(sqrt(CV)) is often cited as being a typical factory QA specification used by Sprague, Mallory, CDE and other manufacturers of electrolytic capacitors for the consumer electronics industry. Western Electric, however, used far superior materials and manufacturing techniques as the main source of electrolytics for the telephone industry and many defense contractors and their electrolytics typically had about 1/4 the maximum leakage currents of consumer products grade components. Taking that into consideration, 1/20*(sqrt(CV)) is likely the best that anyone can expect from any electrolytic capacitor of the "aluminum can" type.