Ramsey Sx-20 Schematic


20m Transceiver

Ramsey Electronics 793 Canning Parkway Victor. NY 14564-8924 Price class: 5349,95 (Kit) S429.95 (wired and tested)

A QRP powerhouse.

Yes. that's right . , . another QRP rig for 20 meters. Soy, it s getting harder to flip through 73 these days without coming across an advertisement or review for transceivers like these. Buitd-it-yourself QRP rigs for 20 m. 40 m. and other bands are about as hot as pogs, 0. J. Simpson, and sport utility vehicles! But the SX-20 isn't ready ■just another QRP rig," what with features like direct digital synthesis (DOS), duat VFOs, RITP built-in iambic keyer, three different tuning speeds, WWV coverage, and a pretty versatile CW filter. And it's all yours for about $350 and a coupfe of weekends of your time.

When I first saw the SX-20 at Dayton 94h I was impressed with its compact size, one-piece board construction, and range of features. It was only a matter of lime before \ persuaded John Ramsey to send one down for a build-up and evaluation. And my timing was perfect White SX-20s had previously been shipped v.ired and tested. The kit versions were iust being released, meaning i d have a chance to debug the manual during construction,


The design behind the SX-20 is pretty clever packing a lot of features tnto a 9 5"W x 3.7MH x 9WD case weighing all of 5 pounds (see Figure 1). It makes extensive use of LSI chips for all active signal processing, and tuning adjustments are kept to a minimum Frequency coverage is from 14.0 to 14,5 MHz on receive and 14,0 to 14.35 MHz on transmit, using either 10-Hz, 100-Hz, or 1-kHz steps.

The output stage uses a pair of F3Q55E power MOSFETs, and maximum power output is spec'ed at 10 watts, though you can adjust this down to under 1 watt using the ALC control tor true-blue QRP operation. Receiver sensitivity is claimed to be less than .25 |iV for 10 dB S'N. and selectivity in standard SSB mode is -6 dB > 2 3 kHz and -60 dB < 4.0 kHz But enough numbers! Let's take a look inside (he SX-20 and see what makes it tick.

How It Works

The heart of the SX-20 is a Harris HSP45102 chip, working as a direct digital synthesizer. DOS circuits have become quite popular in contemporary transceiver designs, owing to a combination of reasonable cost, compact size, and accuracy. It outputs LO frequencies between 7.5 and 8.5 MHz. which are mixed with the incoming 20 meter signals in an NE6Q2A mixer chip to the first IF of 6.143 MHz. Additional filtering is provided by a six-po[e, 2.5-kHz crystal filter before the signal is sent to a CA1350P IF amplifier Another NE6G2A mixer works with the second LO at 6.1415 MH2 and a CW/SSB frequency offset circuit to provide signals for the LM324

audio amplifier.

In transmit mode, incoming audio signals are amplified a couple of times and fed to a MC1496 balanced modulator mixer circuit. The suppressed earner 6.1415 MHz signal from this chip is then sent through the same six-pole crystal filter from before and into an NE602 mixer, which combines this signal with the same 7,5- to 8,5MHz DDS frequencies to produce the finai 20 meter signal. Additional bandpass filtering Cleans up the RF before it gets to the 2N3866 pre-driver, P3055E driver, and Ihe final output stage. The ^brains" of the circuit (as Ramsey likes to call it) is a member of the popular Motorola 68HC05-series microcontroller family. Working with a handful of other components, including an inverter and shift register this processor controls front panel displays, interprets switch closures from the front pane1 membrane keyboard and selects the various TX RXr filter and mode states.

While there are quite a few bipolar transistors scattered throughout the circuit. they re ail either 228256 PNP or 2N3904 NPN devices that perform very simpfe functions—in fact most of the time they're just working as switches or low-level amplifiers. This is one of the reasons there are so few circuits to tune in the SX-20, and why the kit is so reasonably priced—none of the components used is that exotic and costly. Even the 68HC05 micro is available in abundant quantities and iterations,

Human interfaces to the SX-20 (let's not forgei ihose!) include a main tuning control, microphone gain control, volume control, a standard eight-pin microphone connector that works with Ramsey s mikes or any ICOM mi crophone, and membrane-button controls for TUNING SPEED, DIAL LOCK, RIT, MODE, KEYER, ATTENUATOR, AGO FAST/SLOW, VFO A/B, and WWV, The display is made up of eight-segment, red, alphanumeric LEDs (you were expecting LCDs?) that read to 10Hz resolution; an eight-step LED S-meter and headphone jack round out the front panel.

Rear-pane] connections are kept to a minimum: a standard SG-239 UHF jack for the antenna, stereo phone jack for your CW key, and a 2,5 mm power jack for connection to 12-14 voits. i should add that I'm not thrilled with the use of that particular power plug, as they slide out of the jacks quite easily. My suggestion woufd be to go to a Molex or other locking connector.

Other controls are provided for calibrating the S-meter, setting the sidetone level when transmitting CW, setting the CW TX/RX cycle delay, and controlling power output. But you'll need to remove the cover to adjust them. Considering that CW delay and sidetone levels are often adjusted to suit the operating circumstances, I think it would be smart of Ramsey to relocate these controls and provide access to them either through the side or top of the transceiver

Putting It Together

Ramsey has always done a first-rate job of packaging up their kits, and the SX-20 is no exception. There are eight different subassemblies to put together, and individual bags with alf the parts for each are clearly identified. Good thing, too, as there are nearly 600 individual components in the kit (not including the optional CW filter)! But mistakes can happen; my particular kit was missing the microcontroller and DDS chips, which were promptly shipped via express mail after a call to Rochester.

If you read the manual carefully, you should have little difficulty in getting each stage of the SX-20 up and running. Each section opens with a discussion of the theory and operation of that circuit, followed by a detailed parts list and parts overlay. Three schematics and two large overlays are also provided with views of the main board and front panel, so locating parts is a quick job.

In addition, both the front panel and main PC boards have part numbers and locations screened light on them; thus, if you install something backwards, blame that guy in the mirror! (Hint: When first setting up my work area: I usually spread out sheets of white paper to sort the various parts on. It is considerably easier to spot small capacitors, diodes, and resistors this way.)

The SX-20 instruction manual contains the usual detailed instructions, check-your-work boxes, and off-the-wall Ramsey wit, {The jokes and puns are especially effective during a 3 am building session.) At the end of each chapter, you solder up a few wires, apply power, and check to see if that particular circuit is functional. If not, some troubleshooting hints are provided so you can back up and recheck your work.

Fun and Games

Probably the trickiest part of the project is the front panel assembly, which uses 21 different LEDs for indicators. Because the front panel itself is a membrane keyboard, you must take care must when soldering in the LEDs to avoid pressing too hard against the panel and damaging it What you actually do is insert all the LEDs in their holes, attach the panel, and tilt everything upside down. The LEDs will just touch the membrane and you can finish soldering and trimming leads, (Of course, I managed to put one LED in backwards!)

Another tricky job involves winding the various transformers between the predriver and driver stages, driver and finals, and final output. These are turns of #24 enamel on large ferrite cores which have been pre-drilled While winding the turns isn't hard in itself, there have been problems with sharp edges on the core holes that have actually nicked the wire. Since the cores are made of a conductive material, it's possible that the 13.8 volt supply could be shorted, or that turns could be shorted to themselves. (This happened to me,) Tom Hodge of Ramsey advised me that they are now checking these cores under magnifying glasses to make sure they're deburred.

More fun stuff; While testing the front panel, I inadvertently installed one of the Relliflex cables into the wrong side of the connector. After talking to the folks at Ramsey and realizing my error I inserted it the correct way— and nothing happened! When I first installed the cable incorrectly, it 'dimpled' the silver fingers enough so they wouldn't make contact. Rubbing them gently with a pencil eraser cured the problem.

Here's another change for the better: During testing of each stage of the SX-20: the manual requires you to solder up volume control, power, and speaker wires, then des-older them so you can proceed with the next assembly and be able to flip the board over and back. Current kit versions now use molex connectors to eliminate these steps and relieve wire stress.

While these may sound like an annoying series of minor problems, I can assure you they are quite common when debugging a kit and in ever^ case were easily iixed (especially problems caused by my careless assembly!); nor did they slow me down much, as I was still able to test all ot the stages of the rig to make sure things were working. When I hit a real stumper. Tom Hodge was able to come up with a good answer in short order.

By now, you may be wondering how long it will take to assemble an SX-20. I like to build at odd hours and work pretty quickly, but my conservative guess is probably four or five nights, with plenty of breaks to check your work and stretch. And you don't need much in the way of tools, either; a 25W to 40W soldering iron, diagonal cutters, small pliers, and wire strippers will suffice for board assembly. A magnifying worklamp is a real help.


Tuning up the SX-20 is a fairly quick procedure, and using some rudimentary test equipment makes it go a lot faster. The receiver can be aligned with a strong on-air signal, but I used my time-tested HP608F signal generator, since it isn't affected by sunspotsl Ramsey provides their universal "diddle stick" plastic alignment tool, which is used to peak up about nine different transformers for maximum signal. The calibrated generator also makes it easier to set the S-meter correctly, but you could also do this by comparing signals to a calibrated reading on another rig.

On transmit, a frequency counter is a must to set the local oscillators to 6.143 and 6.1415 MHz. I suggest letting the radio warm up and sit for about 15 minutes before making this adjustment. You'll need a VOM or FETVOM to set the idling current for the driver and finals, and this setting will change slightly as the FETs warm up. Make sure you hook up a good 50-ohm dummy load (or antenna, if you haven t anything else to use) during this step. If you have access to an accurate wattmeter, you can set the power output down as low as 1 watt and as high as 12 watts by adjusting the ALC control.

When you first turn on the SX-20, it signals UT in CW and the numbers "T and ^ march

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