Monday, May 15, 2023

MFT-40 a double sideband TX, direct conversion RX kit from Spain

I build lots of little electronic projects, some work well, some work poorly, some just don't come to life. After a few failures I lose confidence and a good way to get my home made mojo back is to build a kit. A kit for a "My first transceiver" MFT-40 (20m also available) from Spain's QRPHamradiokits caught my eye recently.

The designer references Peter, VK3YE's "Beach-40" which I've also built but has improved on the design. Peter's simple circuit suffers from feedback during switching from transmit to receive and uses a diode ring mixer rather than an NE602.

"The MFT-40 incorporates a DC (direct conversion) receiver with a 3-stage front-end passband filter, followed by a balanced mixer, an audio preamplifier and filter using an operational amplifier, and an output amplifier for driving a loudspeaker. The local oscillator is based on a 7.2 MHz ceramic resonator element that allows coverage of a part of the 40m band.

The DSB (double sideband) transmitter uses a DSB generator with input from an economical electret condenser microphone and three stages of amplification which produce 3-4W to the antenna."

The kit is well presented with a top quality board that cleverly can be cut in half to make separate receiver and transmitter sub-boards if you wish.

The instructions are good (and you can see the schematic there) but the designer can't quite make up his mind about the best order to add components. Parts lists are presented both sorted by value and again but sorted by number (R1, R2, etc) which doesn't always group by where they are located in the circuit. Also there's the suggestion that new constructors might just build the receiver first - probably a good idea but maybe there could be a separate kit for a receiver only.

There's some minor errors with carrier suppression pot P4 in the text actually being P3 on the schematic and mic gain P3 actually being P2 on the schematic but it wasn't hard to figure this out. My kit had an incorrect component that was easily found in my junk box.

In use, the receiver works very well although, not-unexpectedly, tuning is very sensitive. The transmitter perhaps suffers from not enough mic gain - the supplied electret mic directly drives the input to the NE602 mixer. (I don't know where QRP Ham Radio Kits managed to source through hole NE602s - they are very hard to find these days).

Here it is in use:

I spotted the biscuit tin while shopping in Kyneton. The size is perfect for this project and others for sure. Drilling was easy. The biscuits are also very nice, if a bit sweet for my taste.

An enjoyable kit.

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