[R-390] My recapping measurements

[Tim Shoppa]

OK, I put orange drops in for every 0.01 - 0.1 uF cap in my yellow
striper.  I admit it.

It had a mix of makers (Stewart-Warner RF deck, EAC IF deck, Collins
AF deck) and a mix of capacitors in it when I found it.  The RF
deck had brown beauties, the IF deck was a mix of Aerovox yellow
caps, metal Westcaps, and brown beauties, and the AF deck was yellow
caps (Sprague and Aerovox) and Sprague Vitamin Q's and a few brown
beauties.  All looked to be original or very-consistently-wired
replacements done decades ago.

I made an impromptu leakage tester out of 20 9-volts in series and
a microammeter.

Every brown beauty had large leakage (milliamps in some cases, always
at least 50 or 100 microamps).

Every Vitamin Q and Westcap and Aerovox cap had zero to no measurable
leakage.  One of the Sprague yellow caps was a dead short.  I think
I would've been able to measure a microamp or two if it had been there
(the meter is 50microamp full scale).

The canned electrolytics on the AF deck did pretty well, leakages
in the hundreds of microamps.  I had expected them to do worse.  I
was surprised that between pin #1 and pin #7 on the 45mfd two-section
electolytic that there was several milliamps flowing there.  My
understanding is that there's no capacitor/no connection there!

My conclusion: replacing the brown beauties was absolutely the right
thing to do, even though they didn't have any obvious cracks or
similar.  The westcaps and vitamin Q's might have held up another
40 (or more!) years, but who knows.

Incidentally, every un-recapped IF deck I can recall seeing (hamfests
etc.) seems to have had a Vitamin Q or Westcap in the IF deck as the
coupling capacitor to the mecahnical filters.  Has anyone actually seen
a brown beauty in that position?  I get the impression that the makers
of the IF decks knew they needed high-quality caps in some positions
and that they could get away with lower-quality caps (brown beauties)
in others.  (How they would've known at the time that the brown beauties
would eventually develop leakage, I don't know.)  Of course it's possible
that the depots replaced all brown beauties in critical positions long
before I ever saw the guts of an IF deck.

Tim.