Sunday, December 18, 2011

70Mhz receive converter

My versions of the converters. The top photo is the 50Mhz converter and the second one is the 70Mhz version. I currently use these as part of my transverters. I also made one for 144Mhz in a similar style. The 50 Mhz converter has a 3sk88 in the front end, while the 70Mhz uses 40673 mosfet. Note the different 28Mhz IF inductors, the 70Mhz uses a small torroid, while the 50Mhz uses a 1/4" coil former.

I ought to know the circuit of this converter by heart now, I have built so many of them for different frequencies.

My first attempts at a 144Mhz converter with an output frequency of 2 - 4Mhz (remember tuning high to low?) was not the best. It took ages to get things right with a FET front end and mixer. I could never seem to get it stable even after several rebuilds. Later, when I got the Yaesu FT101E I wanted to use this as a tunable IF of 28 - 30Mhz, so I decided on this design. It used two dual-gate MOSFETS, and also used feed-through decoupling capacitors and leadless discs. They were very easy to use and the circuit was very stable!

I first built a 144Mhz convertor which formed part of my transverter project, and then later built one for 70Mhz and also 50Mhz. All with the same layout, but different crystal and coils.

I still use all of these in my transverters, all are sensitive and stable

A great circuit, recommended for anyone wanting to build one of these as part of a project.

In the circuit I replaced the 3N140 with 40673 MOSFETS and then later used GASFETS like the 3SK88. My transverters for 144Mhz and 50Mhz have 3SK88's and for 70Mhz I used 40673's.

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