Foil patterns (typical foil arrangement shown below) and filter
coils (typical coil shown to the left) are designed to protect electronic circuits
from transient voltage spikes and surges. They act to fuse the primary side
of the low-voltage transformer (which, in most cases is mounted directly on
the printed circuit board) by quickly rupturing at the occurrence of abnormal
voltages. The coil shown on the left has been ruptured, as indicated by the
blacked area just to its right.
CAUTION:Static electricity in clothes
and other items can damage certain static-sensitive control panel components.
Therefore, handle the circuit board by its edges only; or ensure that you, the
soldering iron, and the work bench are properly grounded.
No, the illustration to the right is not the Pebble Beach Golf Course in California.
It is a picture of a typical foil pattern configuration on a printed circuit board.
The procedure to restore the foil pattern is as follows. If the foil is open at
point "A," solder in a jumper wire from point "B" to point "C" (the narrow foil
pattern to the left of point "B" now becomes the new "fuse"). If, in the future,
the foil pattern to the left of point "B" is ruptured, solder in a 22 microhenry
filter coil (Sharp part # RCILF2003YAZZ) between points "D" and "E." If, instead
of foil patterns, the board is equipped with one or two filter coils, they can
be checked for continuity with an ohmmeter. An open coil can be replaced with
the same 22 microhenry coil mentioned above. 
If a foil pattern or filter coil is open, the varistor
should also be checked for integrity. In some instances the varistor may
be cracked or split apart. A normal ohmmeter reading across an intact varistor
is near-infinity. In addition, the primary coil of the low-voltage transformer
should be checked. A typical normal resistance is about 250 to 300 ohms. If the
transformer coil is open, the entire circuit board may need to be replaced, depending
upon the availability of a replacement transformer.
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© 1996-2002 J. Carlton
Gallawa . All Rights Reserved Worldwide
Send correspondence to:
Microtech, P.O.
Box 940, Gonzalez, FL 32560
Ph: (850) 477-8885
Disclaimer: The author assumes no liability for any incidental, consequential or other liability from the use of this information. All risks and damages, incidental or otherwise, arising from the use or misuse of the information contained herein are entirely the responsibility of the user. Although careful precaution has been taken in the preparation of this material, we assume no responsibility for omissions or errors.