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A 1-amp, 12 to 13.5 volt, well filtered and regulated power supply should be used with the receiver. If the voltage drops below 12 volts, some of the IR switching may not work. Total current draw of the receiver with no signal is 440 mA, but the audio amplifier (TDA2002) can draw instantaneous peaks of up to 700 mA. The first indication of a weak power supply will be distortion in the audio amplifier. Wall warts (plug-in wall transformers/power supplies) are not filtered enough and will cause hum in the audio. A 1 amp wall wart will work, but be aware that most of them will cause hum in the audio. Several wall warts were tested with the receiver and we never found one that did not cause hum in the audio. |
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The receiver is designed for use with Hamstick antennas on the inside of a condo/townhouse window. The RF amplifier and lots of mixer gain helps the receiver work in areas with poor reception. The best antenna for SWL reception will be a long wire antenna mounted outside as high as possible. The longer the better, but some lengths will do better than others. An antenna tuner used with the long wire antenna will help in receiving weak stations. For additional study on the use of long wire antennas, check out the following links: There is a 50 ohm pad at the output of the RF amplifier that can be changed to fit the receiver to your antenna farm. A 3dB 50 ohm pad comes stock with the receiver, but it can be increased to 6 dB, or even 12 dB, to reduce overloading the receiver when using very good outside antennas. See Circuit Details, RF Amplifier, "Adjusting Gain". A third method of adjusting the receiver for outside antennas is changing the Gate 1 resistors at the first mixer. Gate 1 is marked at the MOSFETs in the first mixer. The stock value is 100K, and they can be lowered to 470 ohm for maximum dynamic range and stability of the mixer in high level signal situations. |
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The VFO injection level can be adjusted with the black PC mount potentiometer located at the first VFO amplifier on the VFO board. The pot is labeled "Gain Adjust" and is located directly above the FT50A-75 ferrite core, the RF choke for the VFO circuit. The injection level can be seen visually with LEDs at two locations. The LED at the first VFO amplifier will vary in intensity as the pot is varied. A dim LED indicates lower VFO injection levels. The second place the VFO injection level can be seen is at the first mixer. Both LEDs at the first mixer will visually show the injection level of the VFO signal. When using a tuned outside antenna for 41 and 31 meters, it is often necessary to adjust the VFO injection level to where the LEDs at the first mixer are barely on. With nonresonant antennas, the LEDs at the first mixer are usually run at full brightness. The VFO "Gain Adjust" pot can be lifted off the board and run to a panel mounted pot on the front panel of the receiver. The ungrounded lead of the PC mounted pot is run to the panel mounted potentiometer, connected to one outside terminal and the center one, with the other end of the panel pot grounded. Add another .01 bypass capacitor to the ungrounded side of the panel pot. The gain of the mixer is changed with the VFO injection levels. Higher injection levels yield more gain. The gain and dynamic range of the mixer are inversely related. Higher dynamic range means less gain. More gain means less dynamic range. The VFO mixer injection level adjustment is very handy to adjust the receiver to varying band conditions and antenna combinations. Be sure to experiment with this pot when first using the receiver to become familiar with its advantages in receiver performance. Check out the article in the May/June 2002 QEX article, "HF Receiver Dynamic Range: How Much Do We Need", by Peter E. Chadwick, on just how useful this mixer injection level in controlling the dynamic range of the receiver can be. |
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There is so much difference in hearing sensitivities between individuals that being able to set the audio gain to different levels was considered very important in the design of this receiver. There are two places where the audio gain can be adjusted for individual preferences. There is a black PC mount potentiometer labeled "Gain Adjust" at the BFO amplifier directly above the MOSFET. This pot varies the injection level to the detector. The injection level is indicated by the LED brightness at the BFO amplifier. The second place for audio gain adjustment is the 455 kHz transformer at the output of the BFO amplifier. The adjustment of the tuning slug will make a small difference in the audio output. |
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Check out the article in the September/October 2002 QEX, "The DX Prowess of HF Receivers", and the design features of what he considers best for DX hunting. Most of those features are incorporated into this receiver. Some of them are the following, taken from the article, page 38: >Only single or double conversion is used instead of a chain of several mixers commonly used by other makers.>A relatively low first IF that allows installation of narrow SSB/CW crystal filters with good shape factors to greatly attenuate out-of-band IF signals just at the front of the IF amplifier. >The main IF selectivity of the crystal filters is very close to the receiver front end, which helps substantially to obtain high BDR and good IMD DR even for closely spaced strong signals. >Ham-band-only preselector filters that substantially suppress strong signals outside of the ham bands and prevent receiver front-end overload and IMD. >Narrow double-tuned preselector filters are switched by relays, so the receiver front end offers much better IMD response then when diode switching is used. >A switchable HF pre-amplifier and switchable attenuator increase the range of receiver sensitivity adjustments, which allow the operator to adjust the receiver to particular propagation conditions and the receiving antenna in use. Note: This concept is implemented in the Electroluminescent receiver with the 50 ohm pad at the tail end of the RF amplifier, and the adjustable VFO mixer injection level. >AGC is derived from the IF signal. >A sharp IF crystal filter is close to the mixer and because of the relatively low IF, the crystal filter greatly attenuates out-of-IF signals. That helps to prevent receiver overloading by strong signals from outside the IF-filter pass-band. Check out the article and compare! You are welcome to send an image of your completed project. Any comments you have, be sure to email. Enjoy and Have Fun - Don't forget your sunglasses! |
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