Several weeks ago I posted about the failure of both ICE 419A filter boxes
during DXpedition to the Falkland Islands. Here is a follow-up:
The ICE filter boxes were affected only in the 10 meter section. It remains
a mystery to me why the other sections were not affected. The VSWR in the
10 meter section went up to about 5:1 for both boxes. This was caused by
capacitors that changed value, probably because of high current causing them
to get too hot. The 10m filter section consists of two tank circuits to
ground connected via a capacitor. I guess this could be called a pi-L
network with extra caps across L. The tank circuits use 540pf mica caps.
These are actually two 270pf 500 volt micas in parallel, yielding 540pf 500
volt. The connecting cap is 78pf, actually two 39pf 500 volt micas in
parallel.
A call to ICE resulted in new caps for both boxes (4 pairs 270 and 2 pairs
39) being sent out immediately and received the next day. The old caps were
removed and compared with new. One pair of 270s was about 50pf low for both
boxes. This caused the unbalanced circuit resulting in high VSWR. The
other caps checked okay. New caps were soldered in to one box. In the
other box the 4 original 270s were replaced with 16 new 270s. Each tank
circuit has 8 270s: two sets of 4 in parallel, in series to yield 540pf at
1000 volts with double the original current rating. These new caps were
purchased from Mouser. I got enough to do both boxes but will wait to
modify the second one if it goes bad again.
BTW, use of these boxes on both transceivers completely eliminated
interference during the VP8WWW operation. I highly recommend them.
Keith NM5G
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See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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