In my last post I had everything wired and simply needed to add power. Why on earth would I leave the devoted readers of this blog hanging on that moment? Well I’ll tell you: It didn’t work!
I said from the beginning I expected to fail, but learn through failure. Mission Accomplished!
If you read my last post you probably caught my mistake. I was mildly distracted while doing the final stages of the build with dad duty. I also did not ever complete an entire circuit in one setting. Even when getting power to the chips I failed to ground the IC ground pins. In retrospect it really was a recipe for disaster!
When I plugged the power in I got a pop. Good news, the LM386 is working correctly! That is the extent of what happened. I attempted to move the RF gain up and down. I tried to “peak” the capacitor in the front end. Nothing happened! I started putting my screwdriver down on different pads on the board. Pin 3 and 4 for the LM386 gave me a noticeable buzz. Seems right. Pins 4 and 5 of the NE602 had the same result. So far no concerns. I put my screwdriver down on the antenna input, no change. Seems strange. I tried adjusting the RF gain, no change. Hmmm. . . Look at the picture, do you see the problem? Well the truth is there were many problems, but the first was that I incorrectly connected the potentiometer for the RF gain. I didn’t realize that a pot is actually a voltage divider network. So your input goes to a resistor on pin 1, your signal feeds through at the voltage divide and you need a lead to ground for the second resistor. You don’t just connect to the next step in the circuit. Speaking of which. . . somehow in wiring all the front end I managed to put all the parts in, but failed to actually connect each of these components to each other to make a circuit. DUH! Even after adding a ground cable for the RF gain control and connecting all the steps for the front end, the receiver was still pretty deaf. Hmmm . . .
I broke out the schematic and started going through each stage and mapping it to my board making sure that I had connected everything correctly. Do you remember my drawing out the schematic. If you look at the original schematic and my sketched schematic your going to see it. Go ahead, compare pictures and tell me. . .
Pin 6 on the NE602 is the input of the local oscillator. Pin 7 is the feedback. I have them reversed. Despite having tried to avoid the crossing of wires by sketching it all out, I have to cross pins 6 and 7 because of how I ran the capacitors. The good news is that a hot soldering iron can help get your wires out and repositioned pretty quick. I managed to do it with out permanently harming myself or anyone else. For what its worth, it’s probably a good idea to wear glasses or safety glasses when hot solder and prying wire is involved. Not that I found out first hand. . . but I’m not going to have cat-like reflexes for my whole life. Ha!
OK. . . all pads connected properly. Time to connect an antenna and give it a try. What I experienced was magic. Initially I was receiving local 96 FM station. As I adjusted the input capacitor I lost the FM station and gained radio Havana Cuba. They are very concerned about our President. . . me too I guess. . . prayed for the last one, will keep praying for this one.
As I tweaked the capacitors I realized that I could here RTTY signals and the occasional CW signal. True to the write up, the front end is pretty wide on this thing. I think I sent an email to everyone I know to share with them my accomplishment. It’s a below average receiver, but I did it!
So what now?
Well, I’d like to study what is actually happening in this receiver. Take some measurements and attempt to optimize my LO tuning to be as selective as is possible with the capacitors I have. Then I am likely to start another receiver. Hopefully one that will be made entirely out of discrete components and also hopefully much more selective. Maybe work on a filter of some kind.
More to come!
I’m a relatively new ham. Got my license, joined a club and I’m having fun! I enjoy CW, pedestrian mobile, backpacking, and I dream of building radios at some point. At present I have a wife and daughters so my building time is limited.