[R-390] Megger and Capacitor Measures ... Good vs. Bad ?

[Charles Steinmetz]

Alan wrote:

>After looking at a few known good caps, it became
>apparent that a baseline for good versus bad would be useful. As it turns
>out, this work was already done very nicely back in 1955 by a couple of
>folks at the Diamond Ordnance Fuze Labs in Washington, DC. They plot the
>MEGOHM x MICROFARD product for a variety of caps from the time, namely
>mica, ceramic, paper, glass, and some poly plastics. At room temp, all of
>these caps fall in the range of 4-6.2 (meg x uf ) product! Hence, a 47 uF
>looks like about 100k ohm while a .01 uF should look like 400 -500 meg ohms

Some data points (everything below assumes that the capacitor is 
being measured at or below its DC voltage rating):

At 200v, an apparent leakage resistance of 100k represents a leakage 
current of 2mA.  What kind of dielectric did the authors test at 47 
uF?  An ordinary aluminum electrolytic of that value (not even a 
low-leakage type), which should be the leakiest capacitor of that 
value you can find, is specified at around 250uA maximum leakage at 
room temperature (>35 megohm x uF).  Any plastic film capacitor is 
specified for leakage much, much lower than that (for example, WIMA 
FKP3 metallized PP caps are specified at 500,000 megohms minimum, 
while Series 225 Orange Drops are specified at 25,000 megohm x uF 
product minimum -- 5,000 times better than the spec you 
quote).  Typical ceramics are specified at ~10,000 megohms.

5 megohms x uF sounds awfully low to me, even for 1955.  It's also 
*very* suspicious that caps made for tuned RF circuits (glass, mica, 
ceramic), which need very low leakage to deliver high Q, didn't score 
much higher than caps intended just to block or bypass DC (paper, 
plastic).  I haven't read the study (do you have a link?), but I'm 
suspicious that there may have been systemic measurement errors.

In any case, it is my belief that a capacitor checker or megger is 
entirely unnecessary for working on tube radios.  Indeed, I'd go so 
far as to say it's usually counterproductive because it is very often 
used way too early in the troubleshooting process and focuses the 
tech on individual parts when (s)he should be keeping an open mind 
and looking at the circuit as a whole.

Note that most of the suspect caps in tube radios are (i) the main 
filter capacitors (electrolytic in all but the oldest BAs) and (ii) 
paper bypass caps on power supply lines and tube cathodes.  Bad 
bypass caps can almost always be found easily with a VTVM, since 
there are invariably decoupling resistors between the raw power bus 
and the bypass caps -- leaky caps will cause the local B+ at each bad 
cap to be low.  (If the radio no longer works (blows fuses or 
smokes), then any leaky B+ bypass caps can easily be found with an 
ohmmeter when the radio is unplugged -- they will generally read less 
than 1k to ground.)


Best regards,

Charles