The first attempt at an AM detector for the receiver was Duncan's as documented in the following quote from his email:
"3. Disconnect the BFO and put in a simple diode at the end of the IF strip (before the 455 kHz ceramic filter).
"Works reasonably well, but there is some problem with the AGC action. What I noticed is that there wasn't enough audio volume, and the output was a little distorted. I connected the diode detector to the output of board 1, and apart from lack of gain, the signal sounded okay.
"If I disconnect the AGC line and connect it to a 0 - 6 Volt pot, I can set the gain so that there is background noise between signals (though the receiver is really quiet!), and set the gain to give good undistorted volume when tuned in to an AM station.
"When the AGC line is connected to the AGC circuitry, the audio that comes out is low volume and sounds compressed and distorted. The AGC action does not seem to be right. I haven't had a chance to work out what is going on yet - have you used the receiver with AM stations much?
"It is as though the voltage drops too quickly when tuning in to a station, but won't drop off far enough on really strong stations."
This schematic is redrawn from the 1988 ARRL Handbook, page 18.11, Fig. 20, Chapter 18, Voice Communications.
"David - I'm pleased to report that an Infinite Impedance Detector, as per attached circuit (from ARRL handbook) gives very good results. I used an MPF102, with the input to a 455 kHz IF coming from the output of the 455 kHz ceramic filter (the BFO circuitry completely out of circuit at this stage of testing).
"The output (without the 50K gain control) going to the 1.5K resistor (R60). Works very well. I've been tuning across the 9 MHz - 10 MHz band and SW reception is quite good.
"The next problem is bandwidth for AM. I've removed two of the 4 MHz crystals and that is better, but for proper AM reception really need a 5 kHz wide filter with steep sides.
"I'm still deciding what to do here, and how to get good SW coverage. Currently I have the VFO frequency varying from about 12 - 14 MHz, and stability seems very good."
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