Topband: W1FB preamp failure

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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