Tag Archives: Retro-75 AM Transceiver

On the K9ZW Build Bench – Winter Projects while Ice has me Off-Air

Here I’m stuck with serious Ice still on all my antennas after a full week from the “big storm.” Mother Nature sure was quick in the issue of the Ice and now is so slow in providing any relief melt. Sun is too weak and the temperatures too low to loosen more than 1/3 of the ice load.

Standing underneath to work mechanisms hoping the ice clears without damage wouldn’t be very safe. Two-Three pound chunks of ice falling 40-to-70 feet could do some damage. Also fear causing more damage by horsing the ice off. Safety first and then the knowledge that active efforts could make a small project in patience into a huge project in repairs, make waiting the ice out the best idea.

So I’ve been inventorying by electronic “to-build” kits and thinking about which to tackle next. For the most part I had reserved the present kits for deep winter builds – during that long stretch between New Years & Easter. Guess I will just move at least one kit forward.

On the shelf are a full range of kits:

Pretty well decided to tackle the Bubble Dial Clock kit next:

Evil Mad Scientist "Bubble Dial Clock Kit"

Evil Mad Scientist "Bubble Dial Clock Kit"

Followed by the Retro-75 AM QRP Transceiver:

Small Wonder Labs "Retro-75 AM QRP Transceiver"

Small Wonder Labs "Retro-75 AM QRP Transceiver"

The HF Packer-Amp would seem a natural third build if winter holds on that long:

HF Packer-Amp Kit

HF Packer-Amp Kit

And the Tuna-Tin Kits (with the various Nixie Tube Clocks) will wait for another season:

QRPMe Tuna Tin Kits "in the Can"

QRPMe Tuna Tin Kits "in the Can"

Of course things might change…..

73

Steve
K9ZW

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Small Wonder Labs Retro-75 Kit Arrives and Project Start

The Retro-75 Board Kit has arrived this week. It came in two parts, everything but one component which arrived separately a few days latter (there was a note in the first package).

I’ve a satellite antenna project with my son Victor KC9NWB and a frequency reference for my shack to finish, both “on the bench” to complete first, putting the Retro-75 into a May build.

That will give me time to layout & source an enclosure, the second crystal socket & switch I will install and have those items on hand for the build.

My initial post on the Retro-75 is at: Going Retro on 75m AM – Small Wonder Labs Kit

I’m planning to shoot pictures and write something about all three projects as they go along.

73

Steve
K9ZW

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Going Retro on 75m AM – Small Wonder Labs Kit

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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