W1FB preamp modifications by WD8DSB

Failure Mode Description

Built a W1FB preamp (Aug 1988 QST) for use with my 4.5 foot x 4.5 foot 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.

My 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.

Webpage created and maintained by Don Kirk (wd8dsb) Oct. 30, 2013