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Going Retro on 75m AM – Small Wonder Labs Kit 20 - March - 2010

Posted by k9zw in Amateur Radio, K9ZW, K9ZW Built, NE1RD.
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Small Wonder Labs Retro-75 AM Transceiver

Small Wonder Labs Retro-75 AM Transceiver

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.

Receiver:

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.

Transmitter:

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!

73

Steve
K9ZW

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Comments»

1. k9zw - 31 - March - 2010

I had a nice email from Rich N4ESS who supplies the Crystals for the Retro-75 and has available other Crystals to make use of the second “channel” that Dave K1SWL designed into the board. Within reason it could be a nice add-on to extend the frequency range with either a user-changeable crystal socket or with a selector picking from a small bank of crystals.

Here is Rich N4ESS’s note:

Steve,

I read your article on the Retro 75. The response seems to be great all the way around. I have read several small blurbs by people that are working with the Retro 75 kits now.

My company supplied the 3885 crystals to Dave Benson and we are adding 3880 to our list of crystals by mid April. I keep between 600 and 1000 of each frequency offered on the company website. I will be adding 1000 of the 3880 to the already 1000 crystals of 3885. Hopefully this will keep the new kits going strong and provide another frequency for that extra slot he has provided.

I think Dave has hit another homerun with this kit,

Rich Kennedy – N4ESS
Expanded Spectrum Systems
6645 Founders St.
Zephyrhills, FL. 33542-1828
USA

813-620-0062 Office

http://www.expandedspectrumsystems.com

n4ess@tampabay.rr.com

73

Steve
K9ZW


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