From:
        
        Steve A.   (steve_ahola@yahoo.com)
   Date:
        11/13/2000 4:55 AM
 Subject:
        Randall's findings on the outer foil terminal

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Mark:

The cap in question was a 2500pF@1000Vac(350vac), the purpose
of the post was for those who may have tried this exercise on a cap
similar to the one mentioned above, and wouldn't have known that it
may have been the cap at fault and not them.

While doing this exercise, I found IMB brand foil caps (they look
similar to Mallory 150's), have a outer shield going around the cap
after the capacitor plates, however this outer shield has no terminal
allowing it to be earthed.

    I'm quoting your post in full because it is ready to scroll off the BBS...
all that is left of Randall's great thread on the proper use of the outer foil
in some caps. In any case, my observations pertain only to the Sprague
Type 418P caps that I get from Torres Engineering...

    I have mentioned that my Blues Express amp is a bit noisy... you can
hear plenty of background noise even with the volume control set to "0".
So I tried reversing the .001uF coupling cap after the second stage and
guess what... it cut the background noise down considerably with the
volume set to "0".

    I then proceeded to see if I could figure out which lead corresponded
to the outer foil without using a scope. The "noise" test I suggested
earlier did not really work that well. Holding the cap between my fingers
I clipped the hot lead (going into my amps input jack) to first one and then
the other cap lead. I even tried measuring the ac voltage at the speaker
output to see if one lead was louder than the other. I noted a small
difference but nothing conclusive.

    Here is one observation, while holding the cap body between my
thumb and middle finger I would then tap the end opposite the lead with
the test clip on it and I could hear a noticeable difference between the
two different leads. I am not quite sure how to interpret that finding...   
    

    In any case a visual inspection of the ends of the 418P caps revealed
something very interesting. On the smaller caps (like a 0.022uF) you can
see the outline of a wire almost like a question mark. On larger caps like
0.1uF you can see a wire but it goes horizontally across the end of the
cap. I believe that the "question mark" wire on the smaller caps goes
directly to the outer foil and my tests in reversing the two 0.022uF tone
caps seems to confirm this as it further lowered the background noise in
my amp, this time with the volume set to "10" and the guitar pot set to
"0".

    By reversing these three caps, the noise level on this amp is 100%
better (at least in my Humble opinion). BTW in all of the 418P caps I
have inspected so far it appears that the outer foil connection is made to
the lead to the right of the lettering (with the lettering right side up, of
course).

    The "finger noise" test I had suggested earlier does not work that well
so perhaps a better idea would be to build a small test circuit that would
be connected to one of the caps, the output of which could be fed to a
scope or a DMM.

--Thanks!

Steve Ahola