Thursday, 3 November 2011

Landwehr masthead 2m preamp. Barry Zarucki M0DGQ.

I will never have a harsh word about Mosfets again! Four of these preamplifiers were given to me for repair all with various faults. This one however was the worst of the bunch but amazingly the Mosfet survived the firestorm within. The print was cleaned and rebuilt, the two 47uF caps were replaced along with the resistors in the output tee pad ( a common fault ). The measured gain was slightly over 18dB and unconditionaly stable.
As a matter of interest, one of the 2m preamps was converted to 4m with great success; two turns were removed from the input tuned circuit, the tuning capacitor was replaced with a 60pf trimmer. The printed circuit output inductor was disconnected from circuit and replaced with a coil with the same dimensions as the new input coil. The output tuning capacitor was replaced with a 60pF trimmer. The drain isolating choke had an additional 10uH choke wired in series with it to increase the total inductance. Measured gain was 20dB and unconditionally stable.

Masthead Preamp.


Tee Pad Resistors.

Converted to 4m.

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