Troubleshooting continues apace on the new NE602-based direct-conversion receiver. As I mentioned in the previous post about it, the receiver develops and unfortunate, LOUD squeal whenever the 10K audio gain pot at sits between the NE602 and the LM386 is advanced past about the 20% position. This is a terrible impediment to reception, so I’ve been working on eliminating this problem.
TL;DR: The receiver is a whole object tied together by its power system. Good power and a good ground are important.
My suspicions centered on the LM386 chip – after all, an audio amplifier with a couple of feedback caps seems like a prime candidate to turn itself into an audio-frequency oscillator. This, as it turned out, was a red herring – I’d like to publicly apologize to the LM386 for ever doubting it.
The first step I took was to meter the potentiometer’s resistance at the setting just below squealing, and to replace the pot with two resistors that replicated the resistance at this setting. A little fiddling showed that the squeal could still be induced by varying these resistances slightly, which was encouraging – the problem was somewhere else in the circuit, and not a phenomenon of the pot itself.
So I took an entirely different tack to connect the NE602 and the LM386. The 0.01µF cap in series with a 10K pot was derived from a number of other designs, but it seemed worthwhile to utilize the complementary outputs from the NE602 as a means of input to the LM386’s complementary inputs. I stole this linkage directly from the EMRFD. The NE602’s pin 5 is connected to the LM386’s pin 3 through a 220nF series cap with a 10kΩ resistor to ground. Similarly, the mixer’s pin 4 was connected to the audio amp’s pin 2. Both lines were tied together with a 100nF cap.
Long story short – no help on the oscillation-front, but a little more stability in the gain, it seems?
Next, I set about fiddling with the feedback circuitry for the LM386. As the datasheet shows, there are gain configurations from 20x to 200x gain with a simple RC network between pins 1 and 8. My original configuration had a 10µF electrolytic cap in between these two pins. I tried replacing that with a 4.7µF cap plus a 2kΩ resistor, leaving those pins totally unconnected… in all configurations, it was possible to get the audio to oscillate. So no such luck there. I put the 10µF cap back in, and joined it to pin 8 with a small piece of female header, so that I could insert various resistors in that position to adjust the gain later, if need be.
Oddly, both reception gain and AM bleedthrough is increased whenever I touch the leads of the gain resistor. The AM bleedthrough sounds a lot like what happens when you touch one of the un-connected leads of the LM386, but why do I hear CW signals a lot clearer as well?
It was this oddity with the gain resistor that gave me the vital clue. “Well,” I thought, “maybe touching and fiddling with the other components will give me some clues.” I just played around on the board for 20 minutes with it powered on, tying this point to ground, putting that point high through a resistor… interesting things happened, but squealing was still very much a possibility. That is, until I happened to accidentally pull the oscillator crystal out of its holder, and the squealing stopped.
“Now that’s odd!” I thought. “I assumed this problem existed entirely in the audio half of this receiver. Why should removing the crystal, and thereby halting oscillation in the mixer, have any effect?”
A little poking around with a meter and a scope and I had my answer – the little (used!) 9V batteries I was using as a power supply for this receiver were woefully under-powered. Under load (i.e. with audio coming out of the headphones), the voltmeter read about 7.3V, and dropped by about 10 mV per second. Oscillation seems to occur when the voltage drops too low. (I’m measuring voltage as a proxy for available power, in this case, I think.)
So, after borrowing a nice big 13.8V, 8aH SLA battery from work and bodging together a quick-connect to battery-clip connector… wait for it…
No more motorboating! (And a bunch more audio output to boot.)
I still need to boast the overall gain of the system, since it takes a pretty strong signal to get into the receiver, but at least it’s not likely to HOWL in my ears!
Next thoughts: to increase gain, perhaps stealing the alternate LM386 gain configuration from AA7EE’s WBR Recevier, or one of the broadband, ~18dB IF amplifier designs from the original BITX or QRPKit’s variant.