Topband: W1FB preamp failure

Tue Jan 4 07:52:09 PST 2011

Thanks for all the responses (many responses received).  Below is a  
summary of the responses I received, along with the actual fix I came up with  
that has solved the W1FB preamp failure problem I was having when I  
Summary of responses :
1) Use a "Front End Saver" as described in CQ FEB 1997.
2) Use I.C.E. model 196 which is advertised as a RF Limiter / Receiver  
Protector (note : this is a very interesting looking device that appears to  
use back to back diodes and core saturation techniques to limit the RF input, 
no  relays, etc.).
3) Add back to back diodes on the input of the preamp (note :  back to back 
diodes on the input is already part of the W1FB preamp design,  so this was 
not a satisfactory solution for my problem)
4) Add a relay to short the input of the preamp to ground when  
5) Add a relay to disconnect the receive antenna from the preamp when  
My Actual Solution :
Since the front end FET was not failing (and it already had back to back  
diodes on the input), I felt adding complexity to the front end might not be  
necessary and might not solve my problem (I really wanted to stay away from 
 relays, PTT control, etc).
I felt I should be able to directly protect the MC1350 IC and  below are 
the modifications made to the original W1FB preamp circuit  which has solved 
my problem.
1)  Added back to back diodes across the output of the preamp to  clip any 
strong RF that might back feed into the preamp via the preamp  output port 
when I transmit (this also provides additional protection to  the receiver).
2)  Added a 100uH RF choke to the +12 volt supply line inside the  preamp.
3)  Added back to back diode protection directly to the front end of  the 
MC1350 by adding a 0.1 uf cap between the existing 100pf cap (C5), and pin 4  
of the MC1350, and then installed back to back diodes from the junction of 
these  two caps to ground.
1) The only other mod I am using with the W1FB preamp is a series diode  
(1N4007) in line with the +12 volt supply for reverse polarity protection, and 
I  have been using this from the very beginning. 
2) All back to back diodes mentioned above are 1N914.
3)  I received lots of positive remarks about the W7IUV preamp that  uses 
the 2N5109 transistor.
4)  I modeled (using SPICE) the front end FET preamp section of  the W1FB 
preamp and used a 2K ohm load to simulate the input of the  MC1350.  I then 
compared gain and frequency of resonance of the tuned  circuit with and 
without the back to back diodes added between the FET and the  MC1350, and the 
differences were insignificant. 
Thanks to everyone that responded to my original question, the response was 
Don Kirk (wd8dsb)

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