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Martyn McKinney
May 09, 2024
In Solid State Radios
A Google Image search for Direct Conversion receivers using the NE602 (NE612 SA612) double balanced mixer will reveal numerous, perhaps over 100 similar schematics designed for the 80 to 10 meter amateur bands. Kits used to be sold by Vectronics and Ramsey. I was never happy with their performance and for this reason I built a simple superhet using an LM386 audio amplifier as a regenerative IF detector-amplifier. Using 10 turns on a ferrite rod as a preselector enables the receiver with the values shown to use both high side and low side injection to tune from 3.4 MHz to 10.2 MHz in 2 bands. No external antenna is necessary for casual listening and selectivity and performance are comparable to my best commercial portable shortwave receivers using their built in whip antennas. I use it most evenings to listen to SSB on 80 and 40 meters up and down the East coast of the U.S. The IF transformer is a red medium wave local oscillator transformer which in my case is tuned to 1.7 MHz. A variable local oscillator frequency of 5.1 to 8.5 MHz permits 2 band coverage using both low side and high side injection by tuning the front end preselector variable capacitor. Image rejection is good and by properly arranging the IF and LO frequency values there is little chance of overlap. Unlike a pure regenerative receiver, a superhet using a regenerative IF permits tuning across the bands without having to vary the regeneration control. The LM386 audio amplifier is set up as a high gain regenerative RF envelope detector which when oscillating allows the reception of CW and SSB signals. The first photo shown below is that of a stock QRP Kits EASY Direct Conversion receiver which has a varactor fine tuning control and after modification uses the original audio volume control for regeneration. The second photo is a modified version of the stock receiver in which I have added a ferrite bar inductor, a PVC tuning capacitor, a red MW local oscillator transformer, a trimmer preselector variable capacitor and an audio jack and modified the wiring to match the schematic of the new receiver. The stock varactor fine tuning may be used in conjunction with the main PVC tuning capacitor, but for SSB the new regeneration control varies the frequency slightly and also may be used for finer tuning. https://www.qrpkits.com/ezseries.html#ezreceiver Link to EASY receiver construction and schematic: https://www.qrpkits.com/files/EZRX20170527.pdf STOCK QRP RECEIVER MODIFIED QRP RECEIVER The first schematic is the stock Direct Conversion QRP receiver. The second schematic is the superhet version with regenerative IF.
Simple Shortwave Superhet With Regenerative IF content media
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Martyn McKinney
May 06, 2024
In Solid State Radios
On the old Radioboard the circuit which I posted which had the largest response was a regenerative medium wave receiver using only an LM386 audio amplifier chip. The front end of the LM386 was configured as a Colpitts oscillator using the intrinsic Base-Emitter capacitance of the input transistors and an external 220pF capacitor. The tuning inductor used is a standard MW ferrite rod inductor and the tuning capacitor is a PVC MW 2 gang variable. Because the gain of the LM386 used in these configurations is so large care should be taken to use short lead lengths and a ground plane would be preferred. Using a 9v supply a speaker may be driven to a comfortable volume with the MW version. Regeneration could be controlled in a number of ways. In the simplest version reducing the value of the 1K regen variable resistor increases the audio gain, but reduces the regeneration. Increasing the value the opposite occurs. The three other methods of controlling the regeneration include a 1 mH choke which isolates the RF gain from the audio gain. Audio gain may be varied by adding a 100 Ohm variable resistor in series with the choke. Changing the DC operating point of the LM386 with the regen control inhibits the oscillation. These alternate methods can be tried if the technique used in the simplest versions of both the MW and SW receivers proves to be inadequate. The shortwave version is basically the same as the medium wave version except for the values in the tank circuit and the omission of the 220 pF capacitor. The intrinsic capacitances in the front end of the LM386 provide the proper ratios for Colpitts oscillation at SW frequencies. Using a 9V supply and chips from National or Samsung the SW version functioned well to over 8 MHz which includes the 80 and 40 meter amateur bands. Using a ferrite rod with 20 turns no external antenna is needed to receive a considerable amount of activity on the SW bands. The links below are videos posted by a radio amateur who built an earlier version of the MW receiver. youtube.com/watch?v=-UCACxbQS1U youtube.com/watch?v=PTWd82KI53s
MW and SW Regen Receiver Using  Only An LM386 Audio Amplifier content media
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Martyn McKinney
Apr 29, 2024
In Solid State Radios
I miss the old Radioboard but have found a nice replacement thanks to the talented contributors on this forum. I wanted to post largely to get feedback from others who have the capabilities of building some of these circuits. My first post is a medium wave regenerative receiver using a series regulator as the regenerative oscillator and a shunt regulator as an audio amplifier. The inductor is a standard medium wave ferrite rod tapped inductor from a transistor radio. An unusual attribute of the circuit is that it can function as an acceptable superregen receiver in the medium wave broadcast band. I would look forward to feedback from any others who might be interested in constructing this circuit and hearing of their results.
Medium Wave Regenerative and Superregenerative Receiver using Two Voltage Regulators
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Martyn McKinney

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