Going Retro on 75m AM – Small Wonder Labs Kit 20 - March - 2010Posted by k9zw in Amateur Radio, K9ZW, K9ZW Built, NE1RD.
Tags: K1SWL, K9ZW, KE9V, NE1RD, Retro-75 AM Transceiver, Small Wonder Labs
Based on write-ups and recommendations from Scott NE1RD and Jeff KE9V, I took at look at Dave K1SWL’s newest offering “The ‘Retro-75’ 75/80 Meter AM Transceiver board kit.”
Although AM has long been supplanted by more effective modes of communication, AM activity is largely channelized into ‘watering hole’ frequencies2. It’s therefore pretty easy to find activity on the air. Here in the Northeast, 3885 kHz is the ‘hot spot’, and 3880 kHz is similarly used on the West coast.
The receiver front end is entirely conventional. …. Rather than wrestle with more stages of filtering or the use of ceramic filters, I reverted to a tried-and-true crystal filter. ……
The IF amplifier uses the so-called ‘cascode’ configuration…..
The following stage is a peak detector which serves to recover the modulation envelope from the received signal. Q3 is an ‘infinite-impedance detector’- a JFET biased near cutoff. I’d always wanted to say I’d used one- the phrase has a nice ‘Hitchhiker’s Guide’ ring to it…..
The audio output waveform is also peak-detected … and applied back …. as a form of Automatic Gain Control (AGC). Without it, nearby ‘Big Gun’ stations are easily capable of saturating this amplifier stage, yielding harsh-sounding clipped audio. …. With this feedback path in place, even the strongest of stations sounds fairly clean in the receiver.
The RF strip consists of a crystal-controlled Colpitts oscillator, a FET-driver IC, and a MOSFET Power Amplifier stage. The PA stage runs class-C to keep the transmitter efficiency reasonably high. Generating an amplitude-modulated output is done using one of the oldest tricks in the book: the transmitter’s supply voltage is varied by superimposing audio on it through … an inexpensive 48Ω:3.2Ω audio output transformer. ….
In theory, this method is simplicity itself….. The transmitter runs about 2W of carrier power and 6-7W peak output. I ran key-down tests of up to a half-hour duration- they showed minimal transformer warming.
Receiver ‘Test’ function:
One of the impediments to building an L-C oscillator (here used as the receiver LO) is that component tolerances make the resulting operating frequency somewhat uncertain. CW/SSB projects can usually count on a borrowed signal from a ‘big rig’ in the shack to locate and adjust the local oscillator to frequency. With AM, though, there’s no ‘beat note’, and you’ll probably hear an ‘in-shack’ AM signal everywhere you tune. The circuit block labeled ‘Test’ provides a built-in frequency adjustment tool. …. (Quotes taken from Dave K1SWL’s Kit Instructions)
The Small Wonder Labs Retro-75 AM Transceiver Board Kit is so new it isn’t even linked from Small Wonder Labs Homepage and can be found at Small Wonder Labs Retro-75 AM Transceiver Web Page
It is listed in the ordering section of the main Small Wonders Lab Website.
I received a nice note from Dave K1SWL thanking me for my order. And I have to admit I am enthused to see it arrive; even more to do the build & get it on the air!