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
transmitted.
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
transmitting.
5) Add a relay to disconnect the receive antenna from the preamp when
transmitting.
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.
Notes
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
tremendous.
73's
Don Kirk (wd8dsb)
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UR RST IS ... ... ..9 QSB QSB - hw? BK
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