Schematic and components
The schematic is shown below but you can also click here to download a PDF.
You really need to read the introduction first for this to make sense.
It's several separate diagram segments showing the pieces that I put together. Where component names appear (Q4, R11 etc) these refer to those on the Universal QRP Transmitter schematic.
I haven't shown all the bypass capacitors. Liberal use of 0.1 uF capacitors on the DC lines is recommended.
Some of the component values were governed by what I had in the junk box rather than any exact calculations. 10K resistors setting the base current of the switching transistors seem to work well but the value is probably not critical so long as the transistor is saturated.
Those components that I didn't have in the junk box came from Mouser.
Don't forget that the NE602 needs 5 volts. I used a three terminal regulator. I run the LM386 from 12 volts as does the Micromountaineer although the EMRFD circuit powers it from 5 volts.
You might not be able to buy an NE602. Luckily, the NE612 and SA612 are available and, for our purposes, are the same thing. See this post where Bill N2CQR of SolderSmoke podcast fame publishes a post from Paul NA5N with the whole story of this chip.
You might not be able to get an LM386 either. I used an equivalent from Mouser called a NJM386BD.
The EMRFD receiver design has slightly different component values for the tuned circuit at the receiver front end than the Micromountaineer 40 meter values here. The EMRFD design specifies the inductor as 20 turns of #26 wire on a T37-6 toroid. The trimmer is a 180 pf and the capacitor across it is 270pf. The exact combination is probably not critical but obviously the important thing is that it must resonate at the received frequency. Trimmer capacitors seem to be difficult to get these days. I used a 120pf trimmer from Mouser which is the biggest capacitance they stock. One setting easily covers the 40 meter band so a trimmer is fine for this. It doesn't need to be a front panel control.
If you use the typical type of trimmer capacitor which turns around and around, make sure the receiver noise peaks in two places. It's easy to be fooled and think it's peaking when it's really just getting closer to resonance when it's at maximum or minimum capacitance. If it's truly resonating then you should be able to turn the trimmer so that the other side of the movable plates are meshed with the same result. You may need to fiddle with the fixed capacitor value or the number turns on the coil if it's not resonating.
Since I didn't really know what was required, I used a trimpot to set the level of oscillator injection to the NE602. The level does not seem to be critical. I don't have anything equivalent to the 51 ohm resistor, r23 in the Micromountaineer circuit. I have nothing on the NE602 pin 6 except for the 3300pf (random value from the junk box) coupling capacitor.
The Micromountaineer design has its RF gain control / attenuator in what looks to me to be a unusual configuration. I have my RF gain control in the more typical voltage divider configuration as does the EMRFD design.