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

[Craig Heaton]

Hi Alan,

Four hours and no one has touch it yet, here goes.

Shortly after the first R-390/A followed me home, the question of good vs.
bad arose. Next step was a Sprague TO-6A capacitor analyzer purchase &
downloaded the manual from BAMA. You might want to take a look at the
manual, good reading.

Short version: One end of the capacitor has to be disconnected in order to
test. In other words, it (the capacitor) is half way out. Would it just be
easier to replace? If your receiver has those BBOD's, it isn't worth the
time messing with them. They are bad, duds, served their country well;
replace them with something newer. 

Asking what type will start the capacitor wars here on this e-mail
reflector. The dead horse will be resurrected and beaten to death once more.
But the discussions are fun and sometimes a learning experience. Some type
of meat grease seems to have a calming effect afterwards.

Anyway, the BBOD's are way off in value and their insulation resistance
sucks wind. YMMV on other caps in the R-390/A. The 2MF oil filled paper &
foil cap, C551, in the IF deck has never tested good on my cap analyzer. The
insulation resistance is near zero! The blocking cap for the mechanical
filters should be replaced. Old electrolytic caps should be replaced.

After those are taken care of; get the darned thing back together, alignment
is next, work on bugs. In that order. Don't forget DeOxit on switches, etc. 

Could be more, but you are making progress!

Regards,
Craig 

-----Original Message-----
From: R-390 [mailto:r-390-bounces at mailman.qth.net] On Behalf Of Alan Victor
Sent: Friday, October 24, 2014 2:09 PM
To: 390 list
Subject: [R-390] Megger and Capacitor Measures ... Good vs. Bad ?

 I have been playing with a 500V Megger looking at questionable capacitors
in the R-390A. Somewhat of a challenge to determine is this really a bad cap
or borderline ok. 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 (infinity!).
Handy to know and this is about what I saw on the Megger.
Their applied V is in this work was ~ 200 V. Curious if any of the folks on
the list have established a neat pass-fail criteria.
Alan W4AMV
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