Topband: W1FB preamp failure
WD8DSB at aol.com
WD8DSB at aol.com
Sun Jan 2 11:33:20 PST 2011
Built a W1FB preamp (Aug 1988 QST) for use with my 4.5 square shielded loop
which is located in my shack, and the MC1350 IC gets permanently altered
when I transmit on 160 meters using my 53 foot base loaded vertical.
Originally the preamp gain pot causes the gain to continuously increase and level
off as I turn the gain pot CW, but after I transmit on 160 meters, the
gain only increase up to a certain point in the CW rotation, and if I continue
to turn the pot CW past a certain point the gain starts to reduce which
indicates something has permanently been damaged in the MC1350.
When I replace the MC1350 the gain pot works properly again until I
transmit on 160 meters, and I have duplicated this event 3 times within a few
hours time and am getting tired of replacing the MC1350. After the first
failure I switched to a separate power supply to feed the preamp, and this did
not fix the problem. After the second failure I added a 100 ufd cap to the
+ supply line in the preamp and this still did not fix the problem.
Wonder if anyone else has experienced this same problem, and it would not
surprise me that others have had this same problem and don't even recognize
it since the amp still appears to work but the gain adjustment no longer
acts as it did prior to exposure to high RF levels (and therefore wonder if
other properties of the amp are no longer optimum too).
I would appreciate having folks using the W1FB preamp to check and make
sure their gain does not start to decrease when turning their gain all the way
up to the maximum gain setting, and would also like to know how they have
the W1FB preamp wired into their transceiver (any kind of T/R switch added
which shorts the preamp input and output to ground, etc).
Right now I have my preamp output connected direct to the RX jack on my
old Kenwood TS-180s, and the loop is located in my shack on the second story
of the house and about 15 feet from my 53 foot base loaded vertical.
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